Merge pull request #10290 from Icinga/icingadb-dependencies-sync

Sync dependencies to Redis

lib/icinga/CMakeLists.txt

@@ -39,7 +39,7 @@ set(icinga_SOURCES
   comment.cpp comment.hpp comment-ti.hpp
   compatutility.cpp compatutility.hpp
   customvarobject.cpp customvarobject.hpp customvarobject-ti.hpp
-  dependency.cpp dependency.hpp dependency-ti.hpp dependency-apply.cpp
+  dependency.cpp dependency-group.cpp dependency.hpp dependency-ti.hpp dependency-apply.cpp
   downtime.cpp downtime.hpp downtime-ti.hpp
   envresolver.cpp envresolver.hpp
   eventcommand.cpp eventcommand.hpp eventcommand-ti.hpp

lib/icinga/checkable-check.cpp

@@ -154,6 +154,10 @@ Checkable::ProcessingResult Checkable::ProcessCheckResult(const CheckResult::Ptr
 	bool reachable = IsReachable();
 	bool notification_reachable = IsReachable(DependencyNotification);
 
+	// Cache whether the previous state of this Checkable affects its children before overwriting the last check result.
+	// This will be used to determine whether the on reachability changed event should be triggered.
+	bool affectsPreviousStateChildren(reachable && AffectsChildren());
+
 	ObjectLock olock(this);
 
 	CheckResult::Ptr old_cr = GetLastCheckResult();
@@ -533,7 +537,7 @@ Checkable::ProcessingResult Checkable::ProcessCheckResult(const CheckResult::Ptr
 	}
 
 	/* update reachability for child objects */
-	if ((stateChange || hardChange) && !children.empty())
+	if ((stateChange || hardChange) && !children.empty() && (affectsPreviousStateChildren || AffectsChildren()))
 		OnReachabilityChanged(this, cr, children, origin);
 
 	return Result::Ok;

lib/icinga/checkable-dependency.cpp

@@ -3,26 +3,169 @@
 #include "icinga/service.hpp"
 #include "icinga/dependency.hpp"
 #include "base/logger.hpp"
-#include <unordered_map>
 
 using namespace icinga;
 
-void Checkable::AddDependency(const Dependency::Ptr& dep)
+/**
+ * The maximum number of allowed dependency recursion levels.
+ *
+ * This is a subjective limit how deep the dependency tree should be allowed to go, as anything beyond this level
+ * is just madness and will likely result in a stack overflow or other undefined behavior.
+ */
+static constexpr int l_MaxDependencyRecursionLevel(256);
+
+/**
+ * Register all the dependency groups of the current Checkable to the global dependency group registry.
+ *
+ * Initially, each Checkable object tracks locally its own dependency groups on Icinga 2 startup, and once the start
+ * signal of that Checkable is emitted, it pushes all the local tracked dependency groups to the global registry.
+ * Once the global registry is populated with all the local dependency groups, this Checkable may not necessarily
+ * contain the exact same dependency groups as it did before, as identical groups are merged together in the registry,
+ * but it's guaranteed to have the same *number* of dependency groups as before.
+ */
+void Checkable::PushDependencyGroupsToRegistry()
 {
-	std::unique_lock<std::mutex> lock(m_DependencyMutex);
-	m_Dependencies.insert(dep);
+	std::lock_guard lock(m_DependencyMutex);
+	if (m_PendingDependencies != nullptr) {
+		for (const auto& [key, dependencies] : *m_PendingDependencies) {
+			String redundancyGroup = std::holds_alternative<String>(key) ? std::get<String>(key) : "";
+			m_DependencyGroups.emplace(key, DependencyGroup::Register(new DependencyGroup(redundancyGroup, dependencies)));
+		}
+		m_PendingDependencies.reset();
+	}
 }
 
-void Checkable::RemoveDependency(const Dependency::Ptr& dep)
+std::vector<DependencyGroup::Ptr> Checkable::GetDependencyGroups() const
 {
-	std::unique_lock<std::mutex> lock(m_DependencyMutex);
-	m_Dependencies.erase(dep);
+	std::lock_guard lock(m_DependencyMutex);
+
+	std::vector<DependencyGroup::Ptr> dependencyGroups;
+	for (const auto& [_, dependencyGroup] : m_DependencyGroups) {
+		dependencyGroups.emplace_back(dependencyGroup);
+	}
+	return dependencyGroups;
 }
 
-std::vector<Dependency::Ptr> Checkable::GetDependencies() const
+/**
+ * Get the key for the provided dependency group.
+ *
+ * The key is either the parent Checkable object or the redundancy group name of the dependency object.
+ * This is used to uniquely identify the dependency group within a given Checkable object.
+ *
+ * @param dependency The dependency to get the key for.
+ *
+ * @return - Returns the key for the provided dependency group.
+ */
+static std::variant<Checkable*, String> GetDependencyGroupKey(const Dependency::Ptr& dependency)
+{
+	if (auto redundancyGroup(dependency->GetRedundancyGroup()); !redundancyGroup.IsEmpty()) {
+		return std::move(redundancyGroup);
+	}
+
+	return dependency->GetParent().get();
+}
+
+/**
+ * Add the provided dependency to the current Checkable list of dependencies.
+ *
+ * @param dependency The dependency to add.
+ */
+void Checkable::AddDependency(const Dependency::Ptr& dependency)
+{
+	std::unique_lock lock(m_DependencyMutex);
+
+	auto dependencyGroupKey(GetDependencyGroupKey(dependency));
+	if (m_PendingDependencies != nullptr) {
+		(*m_PendingDependencies)[dependencyGroupKey].emplace(dependency);
+		return;
+	}
+
+	std::set<Dependency::Ptr> dependencies;
+	bool removeGroup(false);
+
+	DependencyGroup::Ptr existingGroup;
+	if (auto it(m_DependencyGroups.find(dependencyGroupKey)); it != m_DependencyGroups.end()) {
+		existingGroup = it->second;
+		std::tie(dependencies, removeGroup) = DependencyGroup::Unregister(existingGroup, this);
+		m_DependencyGroups.erase(it);
+	}
+
+	dependencies.emplace(dependency);
+
+	auto dependencyGroup(DependencyGroup::Register(new DependencyGroup(dependency->GetRedundancyGroup(), dependencies)));
+	m_DependencyGroups.emplace(dependencyGroupKey, dependencyGroup);
+
+	lock.unlock();
+
+	if (existingGroup) {
+		dependencies.erase(dependency);
+		DependencyGroup::OnChildRemoved(existingGroup, {dependencies.begin(), dependencies.end()}, removeGroup);
+	}
+	DependencyGroup::OnChildRegistered(this, dependencyGroup);
+}
+
+/**
+ * Remove the provided dependency from the current Checkable list of dependencies.
+ *
+ * @param dependency The dependency to remove.
+ * @param runtimeRemoved Whether the given dependency object is being removed at runtime.
+ */
+void Checkable::RemoveDependency(const Dependency::Ptr& dependency, bool runtimeRemoved)
+{
+	std::unique_lock lock(m_DependencyMutex);
+
+	auto dependencyGroupKey(GetDependencyGroupKey(dependency));
+	auto it = m_DependencyGroups.find(dependencyGroupKey);
+	if (it == m_DependencyGroups.end()) {
+		return;
+	}
+
+	DependencyGroup::Ptr existingGroup(it->second);
+	auto [dependencies, removeGroup] = DependencyGroup::Unregister(existingGroup, this);
+
+	m_DependencyGroups.erase(it);
+	dependencies.erase(dependency);
+
+	DependencyGroup::Ptr newDependencyGroup;
+	if (!dependencies.empty()) {
+		newDependencyGroup = DependencyGroup::Register(new DependencyGroup(dependency->GetRedundancyGroup(), dependencies));
+		m_DependencyGroups.emplace(dependencyGroupKey, newDependencyGroup);
+	}
+
+	lock.unlock();
+
+	if (runtimeRemoved) {
+		dependencies.emplace(dependency);
+		DependencyGroup::OnChildRemoved(existingGroup, {dependencies.begin(), dependencies.end()}, removeGroup);
+
+		if (newDependencyGroup) {
+			DependencyGroup::OnChildRegistered(this, newDependencyGroup);
+		}
+	}
+}
+
+std::vector<Dependency::Ptr> Checkable::GetDependencies(bool includePending) const
 {
 	std::unique_lock<std::mutex> lock(m_DependencyMutex);
-	return std::vector<Dependency::Ptr>(m_Dependencies.begin(), m_Dependencies.end());
+	std::vector<Dependency::Ptr> dependencies;
+
+	if (includePending && m_PendingDependencies != nullptr) {
+		for (const auto& [group, groupDeps] : *m_PendingDependencies) {
+			dependencies.insert(dependencies.end(), groupDeps.begin(), groupDeps.end());
+		}
+	}
+
+	for (const auto& [_, dependencyGroup] : m_DependencyGroups) {
+		auto tmpDependencies(dependencyGroup->GetDependenciesForChild(this));
+		dependencies.insert(dependencies.end(), tmpDependencies.begin(), tmpDependencies.end());
+	}
+	return dependencies;
+}
+
+bool Checkable::HasAnyDependencies() const
+{
+	std::unique_lock lock(m_DependencyMutex);
+	return !m_DependencyGroups.empty() || !m_ReverseDependencies.empty();
 }
 
 void Checkable::AddReverseDependency(const Dependency::Ptr& dep)
@@ -43,89 +186,72 @@ std::vector<Dependency::Ptr> Checkable::GetReverseDependencies() const
 	return std::vector<Dependency::Ptr>(m_ReverseDependencies.begin(), m_ReverseDependencies.end());
 }
 
-bool Checkable::IsReachable(DependencyType dt, Dependency::Ptr *failedDependency, int rstack) const
+bool Checkable::IsReachable(DependencyType dt, int rstack) const
 {
-	/* Anything greater than 256 causes recursion bus errors. */
-	int limit = 256;
-
-	if (rstack > limit) {
+	if (rstack > l_MaxDependencyRecursionLevel) {
 		Log(LogWarning, "Checkable")
-			<< "Too many nested dependencies (>" << limit << ") for checkable '" << GetName() << "': Dependency failed.";
+			<< "Too many nested dependencies (>" << l_MaxDependencyRecursionLevel << ") for checkable '" << GetName() << "': Dependency failed.";
 
 		return false;
 	}
 
-	for (const Checkable::Ptr& checkable : GetParents()) {
-		if (!checkable->IsReachable(dt, failedDependency, rstack + 1))
-			return false;
-	}
-
 	/* implicit dependency on host if this is a service */
 	const auto *service = dynamic_cast<const Service *>(this);
 	if (service && (dt == DependencyState || dt == DependencyNotification)) {
 		Host::Ptr host = service->GetHost();
 
 		if (host && host->GetState() != HostUp && host->GetStateType() == StateTypeHard) {
-			if (failedDependency)
-				*failedDependency = nullptr;
+			return false;
+		}
+	}
+
+	for (auto& dependencyGroup : GetDependencyGroups()) {
+		if (auto state(dependencyGroup->GetState(this, dt, rstack + 1)); state != DependencyGroup::State::Ok) {
+			Log(LogDebug, "Checkable")
+				<< "Dependency group '" << dependencyGroup->GetRedundancyGroupName() << "' have failed for checkable '"
+				<< GetName() << "': Marking as unreachable.";
 
 			return false;
 		}
 	}
 
-	auto deps = GetDependencies();
-
-	std::unordered_map<std::string, Dependency::Ptr> violated; // key: redundancy group, value: nullptr if satisfied, violating dependency otherwise
-
-	for (const Dependency::Ptr& dep : deps) {
-		std::string redundancy_group = dep->GetRedundancyGroup();
-
-		if (!dep->IsAvailable(dt)) {
-			if (redundancy_group.empty()) {
-				Log(LogDebug, "Checkable")
-					<< "Non-redundant dependency '" << dep->GetName() << "' failed for checkable '" << GetName() << "': Marking as unreachable.";
-
-				if (failedDependency)
-					*failedDependency = dep;
-
-				return false;
-			}
-
-			// tentatively mark this dependency group as failed unless it is already marked;
-			//  so it either passed before (don't overwrite) or already failed (so don't care)
-			// note that std::unordered_map::insert() will not overwrite an existing entry
-			violated.insert(std::make_pair(redundancy_group, dep));
-		} else if (!redundancy_group.empty()) {
-			violated[redundancy_group] = nullptr;
-		}
-	}
-
-	auto violator = std::find_if(violated.begin(), violated.end(), [](auto& v) { return v.second != nullptr; });
-	if (violator != violated.end()) {
-		Log(LogDebug, "Checkable")
-			<< "All dependencies in redundancy group '" << violator->first << "' have failed for checkable '" << GetName() << "': Marking as unreachable.";
-
-		if (failedDependency)
-			*failedDependency = violator->second;
+	return true;
+}
 
+/**
+ * Checks whether the last check result of this Checkable affects its child dependencies.
+ *
+ * A Checkable affects its child dependencies if it runs into a non-OK state and results in any of its child
+ * Checkables to become unreachable. Though, that unavailable dependency may not necessarily cause the child
+ * Checkable to be in unreachable state as it might have some other dependencies that are still reachable, instead
+ * it just indicates whether the edge/connection between this and the child Checkable is broken or not.
+ *
+ * @return bool - Returns true if the Checkable affects its child dependencies, otherwise false.
+ */
+bool Checkable::AffectsChildren() const
+{
+	if (!GetLastCheckResult() || !IsReachable()) {
+		// If there is no check result, or the Checkable is not reachable, we can't safely determine whether
+		// the Checkable affects its child dependencies.
 		return false;
 	}
 
-	if (failedDependency)
-		*failedDependency = nullptr;
+	for (auto& dep : GetReverseDependencies()) {
+		if (!dep->IsAvailable(DependencyState)) {
+			// If one of the child dependency is not available, then it's definitely due to the
+			// current Checkable state, so we don't need to verify the remaining ones.
+			return true;
+		}
+	}
 
-	return true;
+	return false;
 }
 
 std::set<Checkable::Ptr> Checkable::GetParents() const
 {
 	std::set<Checkable::Ptr> parents;
-
-	for (const Dependency::Ptr& dep : GetDependencies()) {
-		Checkable::Ptr parent = dep->GetParent();
-
-		if (parent && parent.get() != this)
-			parents.insert(parent);
+	for (auto& dependencyGroup : GetDependencyGroups()) {
+		dependencyGroup->LoadParents(parents);
 	}
 
 	return parents;
@@ -145,32 +271,51 @@ std::set<Checkable::Ptr> Checkable::GetChildren() const
 	return parents;
 }
 
+/**
+ * Retrieve the total number of all the children of the current Checkable.
+ *
+ * Note, due to the max recursion limit of 256, the returned number may not reflect
+ * the actual total number of children involved in the dependency chain.
+ *
+ * @return int - Returns the total number of all the children of the current Checkable.
+ */
+size_t Checkable::GetAllChildrenCount() const
+{
+	// Are you thinking in making this more efficient? Please, don't.
+	// In order not to count the same child multiple times, we need to maintain a separate set of visited children,
+	// which is basically the same as what GetAllChildren() does. So, we're using it here!
+	return GetAllChildren().size();
+}
+
 std::set<Checkable::Ptr> Checkable::GetAllChildren() const
 {
-	std::set<Checkable::Ptr> children = GetChildren();
+	std::set<Checkable::Ptr> children;
 
 	GetAllChildrenInternal(children, 0);
 
 	return children;
 }
 
-void Checkable::GetAllChildrenInternal(std::set<Checkable::Ptr>& children, int level) const
+/**
+ * Retrieve all direct and indirect children of the current Checkable.
+ *
+ * Note, this function performs a recursive call chain traversing all the children of the current Checkable
+ * up to a certain limit (256). When that limit is reached, it will log a warning message and abort the operation.
+ *
+ * @param seenChildren - A container to store all the traversed children into.
+ * @param level - The current level of recursion.
+ */
+void Checkable::GetAllChildrenInternal(std::set<Checkable::Ptr>& seenChildren, int level) const
 {
-	if (level > 32)
+	if (level > l_MaxDependencyRecursionLevel) {
+		Log(LogWarning, "Checkable")
+			<< "Too many nested dependencies (>" << l_MaxDependencyRecursionLevel << ") for checkable '" << GetName() << "': aborting traversal.";
 		return;
-
-	std::set<Checkable::Ptr> localChildren;
-
-	for (const Checkable::Ptr& checkable : children) {
-		std::set<Checkable::Ptr> cChildren = checkable->GetChildren();
-
-		if (!cChildren.empty()) {
-			GetAllChildrenInternal(cChildren, level + 1);
-			localChildren.insert(cChildren.begin(), cChildren.end());
-		}
-
-		localChildren.insert(checkable);
 	}
 
-	children.insert(localChildren.begin(), localChildren.end());
+	for (const Checkable::Ptr& checkable : GetChildren()) {
+		if (auto [_, inserted] = seenChildren.insert(checkable); inserted) {
+			checkable->GetAllChildrenInternal(seenChildren, level + 1);
+		}
+	}
 }

lib/icinga/checkable-notification.cpp

@@ -167,8 +167,7 @@ void Checkable::FireSuppressedNotifications()
 				}
 			}
 
-			for (auto& dep : GetDependencies()) {
-				auto parent (dep->GetParent());
+			for (auto& parent : GetParents()) {
 				ObjectLock oLock (parent);
 
 				if (!parent->GetProblem() && parent->GetLastStateChange() >= threshold) {

lib/icinga/checkable.cpp

@@ -80,6 +80,8 @@ void Checkable::OnAllConfigLoaded()
 
 void Checkable::Start(bool runtimeCreated)
 {
+	PushDependencyGroupsToRegistry();
+
 	double now = Utility::GetTime();
 
 	{

lib/icinga/checkable.hpp

@@ -18,6 +18,7 @@
 #include <cstdint>
 #include <functional>
 #include <limits>
+#include <variant>
 
 namespace icinga
 {
@@ -57,6 +58,7 @@ enum FlappingStateFilter
 class CheckCommand;
 class EventCommand;
 class Dependency;
+class DependencyGroup;
 
 /**
  * An Icinga service.
@@ -77,10 +79,12 @@ public:
 	std::set<Checkable::Ptr> GetParents() const;
 	std::set<Checkable::Ptr> GetChildren() const;
 	std::set<Checkable::Ptr> GetAllChildren() const;
+	size_t GetAllChildrenCount() const;
 
 	void AddGroup(const String& name);
 
-	bool IsReachable(DependencyType dt = DependencyState, intrusive_ptr<Dependency> *failedDependency = nullptr, int rstack = 0) const;
+	bool IsReachable(DependencyType dt = DependencyState, int rstack = 0) const;
+	bool AffectsChildren() const;
 
 	AcknowledgementType GetAcknowledgement();
 
@@ -182,9 +186,12 @@ public:
 	bool IsFlapping() const;
 
 	/* Dependencies */
-	void AddDependency(const intrusive_ptr<Dependency>& dep);
-	void RemoveDependency(const intrusive_ptr<Dependency>& dep);
-	std::vector<intrusive_ptr<Dependency> > GetDependencies() const;
+	void PushDependencyGroupsToRegistry();
+	std::vector<intrusive_ptr<DependencyGroup>> GetDependencyGroups() const;
+	void AddDependency(const intrusive_ptr<Dependency>& dependency);
+	void RemoveDependency(const intrusive_ptr<Dependency>& dependency, bool runtimeRemoved = false);
+	std::vector<intrusive_ptr<Dependency> > GetDependencies(bool includePending = false) const;
+	bool HasAnyDependencies() const;
 
 	void AddReverseDependency(const intrusive_ptr<Dependency>& dep);
 	void RemoveReverseDependency(const intrusive_ptr<Dependency>& dep);
@@ -244,10 +251,21 @@ private:
 
 	/* Dependencies */
 	mutable std::mutex m_DependencyMutex;
-	std::set<intrusive_ptr<Dependency> > m_Dependencies;
+	std::map<std::variant<Checkable*, String>, intrusive_ptr<DependencyGroup>> m_DependencyGroups;
 	std::set<intrusive_ptr<Dependency> > m_ReverseDependencies;
+	/**
+	 * Registering a checkable to its parent DependencyGroups is delayed during config loading until all dependencies
+	 * were registered on the checkable. m_PendingDependencies is used to temporarily store the dependencies until then.
+	 * It is a pointer type for two reasons:
+	 * 1. The field is no longer needed after the DependencyGroups were registered, having it as a pointer reduces the
+	 *    overhead from sizeof(std::map<>) to sizeof(std::map<>*).
+	 * 2. It allows the field to also be used as a flag: the delayed group registration is only done until it is reset
+	 *    to nullptr.
+	 */
+	std::unique_ptr<std::map<std::variant<Checkable*, String>, std::set<intrusive_ptr<Dependency>>>>
+		m_PendingDependencies {std::make_unique<decltype(m_PendingDependencies)::element_type>()};
 
-	void GetAllChildrenInternal(std::set<Checkable::Ptr>& children, int level = 0) const;
+	void GetAllChildrenInternal(std::set<Checkable::Ptr>& seenChildren, int level = 0) const;
 
 	/* Flapping */
 	static const std::map<String, int> m_FlappingStateFilterMap;

lib/icinga/dependency-group.cpp

@@ -0,0 +1,348 @@
+/* Icinga 2 | (c) 2024 Icinga GmbH | GPLv2+ */
+
+#include "icinga/dependency.hpp"
+#include "base/object-packer.hpp"
+
+using namespace icinga;
+
+boost::signals2::signal<void(const Checkable::Ptr&, const DependencyGroup::Ptr&)> DependencyGroup::OnChildRegistered;
+boost::signals2::signal<void(const DependencyGroup::Ptr&, const std::vector<Dependency::Ptr>&, bool)> DependencyGroup::OnChildRemoved;
+
+std::mutex DependencyGroup::m_RegistryMutex;
+DependencyGroup::RegistryType DependencyGroup::m_Registry;
+
+/**
+ * Register the provided dependency group to the global dependency group registry.
+ *
+ * In case there is already an identical dependency group in the registry, the provided dependency group is merged
+ * with the existing one, and that group is returned. Otherwise, the provided dependency group is registered as is,
+ * and it's returned.
+ *
+ * @param dependencyGroup The dependency group to register.
+ */
+DependencyGroup::Ptr DependencyGroup::Register(const DependencyGroup::Ptr& dependencyGroup)
+{
+	std::lock_guard lock(m_RegistryMutex);
+	if (auto [it, inserted] = m_Registry.insert(dependencyGroup); !inserted) {
+		dependencyGroup->CopyDependenciesTo(*it);
+		return *it;
+	}
+	return dependencyGroup;
+}
+
+/**
+ * Detach the provided child Checkable from the specified dependency group.
+ *
+ * Unregisters all the dependency objects the child Checkable depends on from the provided dependency group and
+ * removes the dependency group from the global registry if it becomes empty afterward.
+ *
+ * @param dependencyGroup The dependency group to unregister the child Checkable from.
+ * @param child The child Checkable to detach from the dependency group.
+ *
+ * @return - Returns the dependency objects of the child Checkable that were member of the provided dependency group
+ *           and a boolean indicating whether the dependency group has been erased from the global registry.
+ */
+std::pair<std::set<Dependency::Ptr>, bool> DependencyGroup::Unregister(const DependencyGroup::Ptr& dependencyGroup, const Checkable::Ptr& child)
+{
+	std::lock_guard lock(m_RegistryMutex);
+	if (auto it(m_Registry.find(dependencyGroup)); it != m_Registry.end()) {
+		auto& existingGroup(*it);
+		auto dependencies(existingGroup->GetDependenciesForChild(child.get()));
+
+		for (const auto& dependency : dependencies) {
+			existingGroup->RemoveDependency(dependency);
+		}
+
+		bool remove = !existingGroup->HasChildren();
+		if (remove) {
+			m_Registry.erase(it);
+		}
+		return {{dependencies.begin(), dependencies.end()}, remove};
+	}
+	return {{}, false};
+}
+
+/**
+ * Retrieve the size of the global dependency group registry.
+ *
+ * @return size_t - Returns the size of the global dependency groups registry.
+ */
+size_t DependencyGroup::GetRegistrySize()
+{
+	std::lock_guard lock(m_RegistryMutex);
+	return m_Registry.size();
+}
+
+DependencyGroup::DependencyGroup(String name, const std::set<Dependency::Ptr>& dependencies)
+	: m_RedundancyGroupName(std::move(name))
+{
+	for (const auto& dependency : dependencies) {
+		m_Members[MakeCompositeKeyFor(dependency)].emplace(dependency->GetChild().get(), dependency.get());
+	}
+}
+
+/**
+ * Create a composite key for the provided dependency.
+ *
+ * The composite key consists of all the properties of the provided dependency object that influence its availability.
+ *
+ * @param dependency The dependency object to create a composite key for.
+ *
+ * @return - Returns the composite key for the provided dependency.
+ */
+DependencyGroup::CompositeKeyType DependencyGroup::MakeCompositeKeyFor(const Dependency::Ptr& dependency)
+{
+	return std::make_tuple(
+		dependency->GetParent().get(),
+		dependency->GetPeriod().get(),
+		dependency->GetStateFilter(),
+		dependency->GetIgnoreSoftStates()
+	);
+}
+
+/**
+ * Check if the current dependency has any children.
+ *
+ * @return bool - Returns true if the current dependency group has children, otherwise false.
+ */
+bool DependencyGroup::HasChildren() const
+{
+	std::lock_guard lock(m_Mutex);
+	return std::any_of(m_Members.begin(), m_Members.end(), [](const auto& pair) { return !pair.second.empty(); });
+}
+
+/**
+ * Retrieve all dependency objects of the current dependency group the provided child Checkable depend on.
+ *
+ * @param child The child Checkable to get the dependencies for.
+ *
+ * @return - Returns all the dependencies of the provided child Checkable in the current dependency group.
+ */
+std::vector<Dependency::Ptr> DependencyGroup::GetDependenciesForChild(const Checkable* child) const
+{
+	std::lock_guard lock(m_Mutex);
+	std::vector<Dependency::Ptr> dependencies;
+	for (auto& [_, children] : m_Members) {
+		auto [begin, end] = children.equal_range(child);
+		std::transform(begin, end, std::back_inserter(dependencies), [](const auto& pair) {
+			return pair.second;
+		});
+	}
+	return dependencies;
+}
+
+/**
+ * Load all parent Checkables of the current dependency group.
+ *
+ * @param parents The set to load the parent Checkables into.
+ */
+void DependencyGroup::LoadParents(std::set<Checkable::Ptr>& parents) const
+{
+	for (auto& [compositeKey, children] : m_Members) {
+		parents.insert(std::get<0>(compositeKey));
+	}
+}
+
+/**
+ * Retrieve the number of dependency objects in the current dependency group.
+ *
+ * This function mainly exists for optimization purposes, i.e. instead of getting a copy of the members and
+ * counting them, we can directly query the number of dependencies in the group.
+ *
+ * @return size_t
+ */
+size_t DependencyGroup::GetDependenciesCount() const
+{
+	std::lock_guard lock(m_Mutex);
+	size_t count(0);
+	for (auto& [_, dependencies] : m_Members) {
+		count += dependencies.size();
+	}
+	return count;
+}
+
+/**
+ * Add a dependency object to the current dependency group.
+ *
+ * @param dependency The dependency to add to the dependency group.
+ */
+void DependencyGroup::AddDependency(const Dependency::Ptr& dependency)
+{
+	std::lock_guard lock(m_Mutex);
+	auto compositeKey(MakeCompositeKeyFor(dependency));
+	auto it = m_Members.find(compositeKey);
+
+	// The dependency must be compatible with the group, i.e. its parent config must be known in the group already.
+	VERIFY(it != m_Members.end());
+
+	it->second.emplace(dependency->GetChild().get(), dependency.get());
+}
+
+/**
+ * Remove a dependency object from the current dependency group.
+ *
+ * @param dependency The dependency to remove from the dependency group.
+ */
+void DependencyGroup::RemoveDependency(const Dependency::Ptr& dependency)
+{
+	std::lock_guard lock(m_Mutex);
+	if (auto it(m_Members.find(MakeCompositeKeyFor(dependency))); it != m_Members.end()) {
+		auto [begin, end] = it->second.equal_range(dependency->GetChild().get());
+		for (auto childrenIt(begin); childrenIt != end; ++childrenIt) {
+			if (childrenIt->second == dependency) {
+				// This will also remove the child Checkable from the multimap container
+				// entirely if this was the last child of it.
+				it->second.erase(childrenIt);
+				return;
+			}
+		}
+	}
+}
+
+/**
+ * Copy the dependency objects of the current dependency group to the provided dependency group (destination).
+ *
+ * @param dest The dependency group to copy the dependencies to.
+ */
+void DependencyGroup::CopyDependenciesTo(const DependencyGroup::Ptr& dest)
+{
+	VERIFY(this != dest); // Prevent from doing something stupid, i.e. deadlocking ourselves.
+
+	std::lock_guard lock(m_Mutex);
+	for (auto& [_, children] : m_Members) {
+		std::for_each(children.begin(), children.end(), [&dest](const auto& pair) {
+			dest->AddDependency(pair.second);
+		});
+	}
+}
+
+/**
+ * Set the Icinga DB identifier for the current dependency group.
+ *
+ * The only usage of this function is the Icinga DB feature used to cache the unique hash of this dependency groups.
+ *
+ * @param identifier The Icinga DB identifier to set.
+ */
+void DependencyGroup::SetIcingaDBIdentifier(const String& identifier)
+{
+	std::lock_guard lock(m_Mutex);
+	m_IcingaDBIdentifier = identifier;
+}
+
+/**
+ * Retrieve the Icinga DB identifier for the current dependency group.
+ *
+ * When the identifier is not already set by Icinga DB via the SetIcingaDBIdentifier method,
+ * this will just return an empty string.
+ *
+ * @return - Returns the Icinga DB identifier for the current dependency group.
+ */
+String DependencyGroup::GetIcingaDBIdentifier() const
+{
+	std::lock_guard lock(m_Mutex);
+	return m_IcingaDBIdentifier;
+}
+
+/**
+ * Retrieve the redundancy group name of the current dependency group.
+ *
+ * If the current dependency group doesn't represent a redundancy group, this will return an empty string.
+ *
+ * @return - Returns the name of the current dependency group.
+ */
+const String& DependencyGroup::GetRedundancyGroupName() const
+{
+	// We don't need to lock the mutex here, as the name is set once during
+	// the object construction and never changed afterwards.
+	return m_RedundancyGroupName;
+}
+
+/**
+ * Retrieve the unique composite key of the current dependency group.
+ *
+ * The composite key consists of some unique data of the group members, and should be used to generate
+ * a unique deterministic hash for the dependency group. Additionally, for explicitly configured redundancy
+ * groups, the non-unique dependency group name is also included on top of the composite keys.
+ *
+ * @return - Returns the composite key of the current dependency group.
+ */
+String DependencyGroup::GetCompositeKey()
+{
+	// This a copy of the CompositeKeyType definition but with the String type instead of Checkable* and TimePeriod*.
+	// This is because we need to produce a deterministic value from the composite key after each restart and that's
+	// not achievable using pointers.
+	using StringTuple = std::tuple<String, String, int, bool>;
+	std::vector<StringTuple> compositeKeys;
+	for (auto& [compositeKey, _] : m_Members) {
+		auto [parent, tp, stateFilter, ignoreSoftStates] = compositeKey;
+		compositeKeys.emplace_back(parent->GetName(), tp ? tp->GetName() : "", stateFilter, ignoreSoftStates);
+	}
+
+	// IMPORTANT: The order of the composite keys must be sorted to ensure the deterministic hash value.
+	std::sort(compositeKeys.begin(), compositeKeys.end());
+
+	Array::Ptr data(new Array{GetRedundancyGroupName()});
+	for (auto& compositeKey : compositeKeys) {
+		// std::apply is used to unpack the composite key tuple and add its elements to the data array.
+		// It's like manually expanding the tuple into x variables and then adding them one by one to the array.
+		// See https://en.cppreference.com/w/cpp/language/fold for more information.
+		std::apply([&data](auto&&... args) { (data->Add(std::move(args)), ...); }, std::move(compositeKey));
+	}
+
+	return PackObject(data);
+}
+
+/**
+ * Retrieve the state of the current dependency group.
+ *
+ * The state of the dependency group is determined based on the state of the parent Checkables and dependency objects
+ * of the group. A dependency group is considered unreachable when none of the parent Checkables is reachable. However,
+ * a dependency group may still be marked as failed even when it has reachable parent Checkables, but an unreachable
+ * group has always a failed state.
+ *
+ * @param child The child Checkable to evaluate the state for.
+ * @param dt The dependency type to evaluate the state for, defaults to DependencyState.
+ * @param rstack The recursion stack level to prevent infinite recursion, defaults to 0.
+ *
+ * @return - Returns the state of the current dependency group.
+ */
+DependencyGroup::State DependencyGroup::GetState(const Checkable* child, DependencyType dt, int rstack) const
+{
+	auto dependencies(GetDependenciesForChild(child));
+	size_t reachable = 0, available = 0;
+
+	for (const auto& dependency : dependencies) {
+		if (dependency->GetParent()->IsReachable(dt, rstack)) {
+			reachable++;
+
+			// Only reachable parents are considered for availability. If they are unreachable and checks are
+			// disabled, they could be incorrectly treated as available otherwise.
+			if (dependency->IsAvailable(dt)) {
+				available++;
+			}
+		}
+	}
+
+	if (IsRedundancyGroup()) {
+		// The state of a redundancy group is determined by the best state of any parent. If any parent ist reachable,
+		// the redundancy group is reachable, analogously for availability.
+		if (reachable == 0) {
+			return State::Unreachable;
+		} else if (available == 0) {
+			return State::Failed;
+		} else {
+			return State::Ok;
+		}
+	} else {
+		// For dependencies without a redundancy group, dependencies.size() will be 1 in almost all cases. It will only
+		// contain more elements if there are duplicate dependency config objects between two checkables. In this case,
+		// all of them have to be reachable/available as they don't provide redundancy.
+		if (reachable < dependencies.size()) {
+			return State::Unreachable;
+		} else if (available < dependencies.size()) {
+			return State::Failed;
+		} else {
+			return State::Ok;
+		}
+	}
+}

lib/icinga/dependency.cpp

@@ -123,7 +123,7 @@ public:
 		}
 
 		// Explicitly configured dependency objects
-		for (const auto& dep : checkable->GetDependencies()) {
+		for (const auto& dep : checkable->GetDependencies(/* includePending = */ true)) {
 			m_Stack.emplace_back(dep);
 			AssertNoCycle(dep->GetParent());
 			m_Stack.pop_back();
@@ -251,16 +251,24 @@ void Dependency::OnAllConfigLoaded()
 	// InitChildParentReferences() has to be called before.
 	VERIFY(m_Child && m_Parent);
 
-	m_Child->AddDependency(this);
+	// Icinga DB will implicitly send config updates for the parent Checkable to refresh its affects_children and
+	// affected_children columns when registering the dependency from the child Checkable. So, we need to register
+	// the dependency from the parent Checkable first, otherwise the config update of the parent Checkable will change
+	// nothing at all.
 	m_Parent->AddReverseDependency(this);
+	m_Child->AddDependency(this);
 }
 
 void Dependency::Stop(bool runtimeRemoved)
 {
 	ObjectImpl<Dependency>::Stop(runtimeRemoved);
 
-	GetChild()->RemoveDependency(this);
+	// Icinga DB will implicitly send config updates for the parent Checkable to refresh its affects_children and
+	// affected_children columns when removing the dependency from the child Checkable. So, we need to remove the
+	// dependency from the parent Checkable first, otherwise the config update of the parent Checkable will change
+	// nothing at all.
 	GetParent()->RemoveReverseDependency(this);
+	GetChild()->RemoveDependency(this, runtimeRemoved);
 }
 
 bool Dependency::IsAvailable(DependencyType dt) const

lib/icinga/dependency.hpp

@@ -3,9 +3,14 @@
 #ifndef DEPENDENCY_H
 #define DEPENDENCY_H
 
+#include "base/shared-object.hpp"
+#include "config/configitem.hpp"
 #include "icinga/i2-icinga.hpp"
 #include "icinga/dependency-ti.hpp"
-#include "config/configitem.hpp"
+#include "icinga/timeperiod.hpp"
+#include <map>
+#include <tuple>
+#include <unordered_map>
 
 namespace icinga
 {
@@ -60,6 +65,163 @@ private:
 	static void BeforeOnAllConfigLoadedHandler(const ConfigItems& items);
 };
 
+/**
+ * A DependencyGroup represents a set of dependencies that are somehow related to each other.
+ *
+ * Specifically, a DependencyGroup is a container for Dependency objects of different Checkables that share the same
+ * child -> parent relationship config, thus forming a group of dependencies. All dependencies of a Checkable that
+ * have the same "redundancy_group" attribute value set are guaranteed to be part of the same DependencyGroup object,
+ * and another Checkable will join that group if and only if it has identical set of dependencies, that is, the same
+ * parent(s), same redundancy group name and all other dependency attributes required to form a composite key.
+ *
+ * More specifically, let's say we have a dependency graph like this:
+ * @verbatim
+ *                      PP1              PP2
+ *                      /\               /\
+ *                      ||               ||
+ *                  ––––||–––––––––––––––||–––––
+ *                      P1 - ( "RG1" ) - P2
+ *                  ––––––––––––––––––––––––––––
+ *                          /\      /\
+ *                          ||      ||
+ *                          C1      C2
+ * @endverbatim
+ * The arrows represent a dependency relationship from bottom to top, i.e. both "C1" and "C2" depend on
+ * their "RG1" redundancy group, and "P1" and "P2" depend each on their respective parents (PP1, PP2 - no group).
+ * Now, as one can see, both "C1" and "C2" have identical dependencies, that is, they both depend on the same
+ * redundancy group "RG1" (these could e.g. be constructed through some Apply Rules).
+ *
+ * So, instead of having to maintain two separate copies of that graph, we can bring that imaginary redundancy group
+ * into reality by putting both "P1" and "P2" into an actual DependencyGroup object. However, we don't really put "P1"
+ * and "P2" objects into that group, but rather the actual Dependency objects of both child Checkables. Therefore, the
+ * group wouldn't just contain 2 dependencies, but 4 in total, i.e. 2 for each child Checkable (C1 -> {P1, P2} and
+ * C2 -> {P1, P2}). This way, both child Checkables can just refer to that very same DependencyGroup object.
+ *
+ * However, since not all dependencies are part of a redundancy group, we also have to consider the case where
+ * a Checkable has dependencies that are not part of any redundancy group, like P1 -> PP1. In such situations,
+ * each of the child Checkables (e.g. P1, P2) will have their own (sharable) DependencyGroup object just like for RGs.
+ * This allows us to keep the implementation simple and treat redundant and non-redundant dependencies in the same
+ * way, without having to introduce any special cases everywhere. So, in the end, we'd have 3 dependency groups in
+ * total, i.e. one for the redundancy group "RG1" (shared by C1 and C2), and two distinct groups for P1 and P2.
+ *
+ * @ingroup icinga
+ */
+class DependencyGroup final : public SharedObject
+{
+public:
+	DECLARE_PTR_TYPEDEFS(DependencyGroup);
+
+	/**
+	 * Defines the key type of each dependency group members.
+	 *
+	 * This tuple consists of the dependency parent Checkable, the dependency time period (nullptr if not configured),
+	 * the state filter, and the ignore soft states flag. Each of these values influences the availability of the
+	 * dependency object, and thus used to group similar dependencies from different Checkables together.
+	 */
+	using CompositeKeyType = std::tuple<Checkable*, TimePeriod*, int, bool>;
+
+	/**
+	 * Represents the value type of each dependency group members.
+	 *
+	 * It stores the dependency objects of any given Checkable that produce the same composite key (CompositeKeyType).
+	 * In other words, when looking at the dependency graph from the class description, the two dependency objects
+	 * {C1, C2} -> P1 produce the same composite key, thus they are mapped to the same MemberValueType container with
+	 * "C1" and "C2" as their keys respectively. Since Icinga 2 allows to construct different identical dependencies
+	 * (duplicates), we're using a multimap instead of a simple map here.
+	 */
+	using MemberValueType = std::unordered_multimap<const Checkable*, Dependency*>;
+	using MembersMap = std::map<CompositeKeyType, MemberValueType>;
+
+	DependencyGroup(String name, const std::set<Dependency::Ptr>& dependencies);
+
+	static DependencyGroup::Ptr Register(const DependencyGroup::Ptr& dependencyGroup);
+	static std::pair<std::set<Dependency::Ptr>, bool> Unregister(const DependencyGroup::Ptr& dependencyGroup, const Checkable::Ptr& child);
+	static size_t GetRegistrySize();
+
+	static CompositeKeyType MakeCompositeKeyFor(const Dependency::Ptr& dependency);
+
+	/**
+	 * Check whether the current dependency group represents an explicitly configured redundancy group.
+	 *
+	 * @return bool - Returns true if it's a redundancy group, false otherwise.
+	 */
+	inline bool IsRedundancyGroup() const
+	{
+		return !m_RedundancyGroupName.IsEmpty();
+	}
+
+	bool HasChildren() const;
+	void AddDependency(const Dependency::Ptr& dependency);
+	void RemoveDependency(const Dependency::Ptr& dependency);
+	std::vector<Dependency::Ptr> GetDependenciesForChild(const Checkable* child) const;
+	void LoadParents(std::set<Checkable::Ptr>& parents) const;
+	size_t GetDependenciesCount() const;
+
+	void SetIcingaDBIdentifier(const String& identifier);
+	String GetIcingaDBIdentifier() const;
+
+	const String& GetRedundancyGroupName() const;
+	String GetCompositeKey();
+
+	enum class State { Ok, Failed, Unreachable };
+	State GetState(const Checkable* child, DependencyType dt = DependencyState, int rstack = 0) const;
+
+	static boost::signals2::signal<void(const Checkable::Ptr&, const DependencyGroup::Ptr&)> OnChildRegistered;
+	static boost::signals2::signal<void(const DependencyGroup::Ptr&, const std::vector<Dependency::Ptr>&, bool)> OnChildRemoved;
+
+private:
+	void CopyDependenciesTo(const DependencyGroup::Ptr& dest);
+
+	struct Hash
+	{
+		size_t operator()(const DependencyGroup::Ptr& dependencyGroup) const
+		{
+			size_t hash = std::hash<String>{}(dependencyGroup->GetRedundancyGroupName());
+			for (const auto& [key, group] : dependencyGroup->m_Members) {
+				boost::hash_combine(hash, key);
+			}
+			return hash;
+		}
+	};
+
+	struct Equal
+	{
+		bool operator()(const DependencyGroup::Ptr& lhs, const DependencyGroup::Ptr& rhs) const
+		{
+			if (lhs->GetRedundancyGroupName() != rhs->GetRedundancyGroupName()) {
+				return false;
+			}
+
+			return std::equal(
+				lhs->m_Members.begin(), lhs->m_Members.end(),
+				rhs->m_Members.begin(), rhs->m_Members.end(),
+				[](const auto& l, const auto& r) { return l.first == r.first; }
+			);
+		}
+	};
+
+private:
+	mutable std::mutex m_Mutex;
+	/**
+	 * This identifier is used by Icinga DB to cache the unique hash of this dependency group.
+	 *
+	 * For redundancy groups, once Icinga DB sets this identifier, it will never change again for the lifetime
+	 * of the object. For non-redundant dependency groups, this identifier is (mis)used to cache the shared edge
+	 * state ID of the group. Specifically, non-redundant dependency groups are irrelevant for Icinga DB, so since
+	 * this field isn't going to be used for anything else, we use it to cache the computed shared edge state ID.
+	 * Likewise, if that gets set, it will never change again for the lifetime of the object as well.
+	 */
+	String m_IcingaDBIdentifier;
+	String m_RedundancyGroupName;
+	MembersMap m_Members;
+
+	using RegistryType = std::unordered_set<DependencyGroup::Ptr, Hash, Equal>;
+
+	// The global registry of dependency groups.
+	static std::mutex m_RegistryMutex;
+	static RegistryType m_Registry;
+};
+
 }
 
 #endif /* DEPENDENCY_H */

lib/icinga/dependency.ti

@@ -20,6 +20,8 @@ public:
 
 class Dependency : CustomVarObject < DependencyNameComposer
 {
+	activation_priority -10;
+
 	load_after Host;
 	load_after Service;
 
@@ -77,18 +79,18 @@ class Dependency : CustomVarObject < DependencyNameComposer
 		}}}
 	};
 
-	[config] String redundancy_group;
+	[config, no_user_modify] String redundancy_group;
 
-	[config, navigation] name(TimePeriod) period (PeriodRaw) {
+	[config, no_user_modify, navigation] name(TimePeriod) period (PeriodRaw) {
 		navigate {{{
 			return TimePeriod::GetByName(GetPeriodRaw());
 		}}}
 	};
 
-	[config] array(Value) states;
+	[config, no_user_modify] array(Value) states;
 	[no_user_view, no_user_modify] int state_filter_real (StateFilter);
 
-	[config] bool ignore_soft_states {
+	[config, no_user_modify] bool ignore_soft_states {
 		default {{{ return true; }}}
 	};
 

lib/icingadb/icingadb-objects.cpp

@@ -19,6 +19,7 @@
 #include "icinga/command.hpp"
 #include "icinga/compatutility.hpp"
 #include "icinga/customvarobject.hpp"
+#include "icinga/dependency.hpp"
 #include "icinga/host.hpp"
 #include "icinga/service.hpp"
 #include "icinga/hostgroup.hpp"
@@ -94,8 +95,11 @@ void IcingaDB::ConfigStaticInitialize()
 		AcknowledgementClearedHandler(checkable, removedBy, changeTime);
 	});
 
-	Checkable::OnReachabilityChanged.connect([](const Checkable::Ptr&, const CheckResult::Ptr&, std::set<Checkable::Ptr> children, const MessageOrigin::Ptr&) {
-		IcingaDB::ReachabilityChangeHandler(children);
+	Checkable::OnReachabilityChanged.connect([](const Checkable::Ptr& parent, const CheckResult::Ptr&, std::set<Checkable::Ptr>, const MessageOrigin::Ptr&) {
+		// Icinga DB Web needs to know about the reachability of all children, not just the direct ones.
+		// These might get updated with their next check result anyway, but we can't rely on that, since
+		// they might not be actively checked or have a very high check interval.
+		IcingaDB::ReachabilityChangeHandler(parent->GetAllChildren());
 	});
 
 	/* triggered on create, update and delete objects */
@@ -106,6 +110,9 @@ void IcingaDB::ConfigStaticInitialize()
 		IcingaDB::VersionChangedHandler(object);
 	});
 
+	DependencyGroup::OnChildRegistered.connect(&IcingaDB::DependencyGroupChildRegisteredHandler);
+	DependencyGroup::OnChildRemoved.connect(&IcingaDB::DependencyGroupChildRemovedHandler);
+
 	/* downtime start */
 	Downtime::OnDowntimeTriggered.connect(&IcingaDB::DowntimeStartedHandler);
 	/* fixed/flexible downtime end or remove */
@@ -174,7 +181,7 @@ void IcingaDB::ConfigStaticInitialize()
 void IcingaDB::UpdateAllConfigObjects()
 {
 	m_Rcon->Sync();
-	m_Rcon->FireAndForgetQuery({"XADD", "icinga:schema", "MAXLEN", "1", "*", "version", "5"}, Prio::Heartbeat);
+	m_Rcon->FireAndForgetQuery({"XADD", "icinga:schema", "MAXLEN", "1", "*", "version", "6"}, Prio::Heartbeat);
 
 	Log(LogInformation, "IcingaDB") << "Starting initial config/status dump";
 	double startTime = Utility::GetTime();
@@ -203,10 +210,19 @@ void IcingaDB::UpdateAllConfigObjects()
 	m_Rcon->FireAndForgetQuery({"XADD", "icinga:dump", "MAXLEN", "1", "*", "key", "*", "state", "wip"}, Prio::Config);
 
 	const std::vector<String> globalKeys = {
-			m_PrefixConfigObject + "customvar",
-			m_PrefixConfigObject + "action:url",
-			m_PrefixConfigObject + "notes:url",
-			m_PrefixConfigObject + "icon:image",
+		m_PrefixConfigObject + "customvar",
+		m_PrefixConfigObject + "action:url",
+		m_PrefixConfigObject + "notes:url",
+		m_PrefixConfigObject + "icon:image",
+
+		// These keys aren't tied to a specific Checkable type but apply to both "Host" and "Service" types,
+		// and as such we've to make sure to clear them before we actually start dumping the actual objects.
+		// This allows us to wait on both types to be dumped before we send a config dump done signal for those keys.
+		m_PrefixConfigObject + "dependency:node",
+		m_PrefixConfigObject + "dependency:edge",
+		m_PrefixConfigObject + "dependency:edge:state",
+		m_PrefixConfigObject + "redundancygroup",
+		m_PrefixConfigObject + "redundancygroup:state",
 	};
 	DeleteKeys(m_Rcon, globalKeys, Prio::Config);
 	DeleteKeys(m_Rcon, {"icinga:nextupdate:host", "icinga:nextupdate:service"}, Prio::Config);
@@ -217,6 +233,7 @@ void IcingaDB::UpdateAllConfigObjects()
 		m_DumpedGlobals.ActionUrl.Reset();
 		m_DumpedGlobals.NotesUrl.Reset();
 		m_DumpedGlobals.IconImage.Reset();
+		m_DumpedGlobals.DependencyGroup.Reset();
 	});
 
 	upq.ParallelFor(types, false, [this](const Type::Ptr& type) {
@@ -259,11 +276,6 @@ void IcingaDB::UpdateAllConfigObjects()
 
 		upqObjectType.ParallelFor(objectChunks, [&](decltype(objectChunks)::const_reference chunk) {
 			std::map<String, std::vector<String>> hMSets;
-			// Two values are appended per object: Object ID (Hash encoded) and Object State (IcingaDB::SerializeState() -> JSON encoded)
-			std::vector<String> states = {"HMSET", m_PrefixConfigObject + lcType + ":state"};
-			// Two values are appended per object: Object ID (Hash encoded) and State Checksum ({ "checksum": checksum } -> JSON encoded)
-			std::vector<String> statesChksms = {"HMSET", m_PrefixConfigCheckSum + lcType + ":state"};
-			std::vector<std::vector<String> > transaction = {{"MULTI"}};
 			std::vector<String> hostZAdds = {"ZADD", "icinga:nextupdate:host"}, serviceZAdds = {"ZADD", "icinga:nextupdate:service"};
 
 			auto skimObjects ([&]() {
@@ -303,9 +315,11 @@ void IcingaDB::UpdateAllConfigObjects()
 					String objectKey = GetObjectIdentifier(object);
 					Dictionary::Ptr state = SerializeState(dynamic_pointer_cast<Checkable>(object));
 
+					auto& states = hMSets[m_PrefixConfigObject + lcType + ":state"];
 					states.emplace_back(objectKey);
 					states.emplace_back(JsonEncode(state));
 
+					auto& statesChksms = hMSets[m_PrefixConfigCheckSum + lcType + ":state"];
 					statesChksms.emplace_back(objectKey);
 					statesChksms.emplace_back(JsonEncode(new Dictionary({{"checksum", HashValue(state)}})));
 				}
@@ -314,27 +328,9 @@ void IcingaDB::UpdateAllConfigObjects()
 				if (!(bulkCounter % 100)) {
 					skimObjects();
 
-					for (auto& kv : hMSets) {
-						if (!kv.second.empty()) {
-							kv.second.insert(kv.second.begin(), {"HMSET", kv.first});
-							transaction.emplace_back(std::move(kv.second));
-						}
-					}
-
-					if (states.size() > 2) {
-						transaction.emplace_back(std::move(states));
-						transaction.emplace_back(std::move(statesChksms));
-						states = {"HMSET", m_PrefixConfigObject + lcType + ":state"};
-						statesChksms = {"HMSET", m_PrefixConfigCheckSum + lcType + ":state"};
-					}
+					ExecuteRedisTransaction(rcon, hMSets, {});
 
 					hMSets = decltype(hMSets)();
-
-					if (transaction.size() > 1) {
-						transaction.push_back({"EXEC"});
-						rcon->FireAndForgetQueries(std::move(transaction), Prio::Config);
-						transaction = {{"MULTI"}};
-					}
 				}
 
 				auto checkable (dynamic_pointer_cast<Checkable>(object));
@@ -357,22 +353,7 @@ void IcingaDB::UpdateAllConfigObjects()
 
 			skimObjects();
 
-			for (auto& kv : hMSets) {
-				if (!kv.second.empty()) {
-					kv.second.insert(kv.second.begin(), {"HMSET", kv.first});
-					transaction.emplace_back(std::move(kv.second));
-				}
-			}
-
-			if (states.size() > 2) {
-				transaction.emplace_back(std::move(states));
-				transaction.emplace_back(std::move(statesChksms));
-			}
-
-			if (transaction.size() > 1) {
-				transaction.push_back({"EXEC"});
-				rcon->FireAndForgetQueries(std::move(transaction), Prio::Config);
-			}
+			ExecuteRedisTransaction(rcon, hMSets, {});
 
 			for (auto zAdds : {&hostZAdds, &serviceZAdds}) {
 				if (zAdds->size() > 2u) {
@@ -788,6 +769,8 @@ void IcingaDB::InsertObjectDependencies(const ConfigObject::Ptr& object, const S
 			}
 		}
 
+		InsertCheckableDependencies(checkable, hMSets, runtimeUpdate ? &runtimeUpdates : nullptr);
+
 		return;
 	}
 
@@ -1121,6 +1104,195 @@ void IcingaDB::InsertObjectDependencies(const ConfigObject::Ptr& object, const S
 	}
 }
 
+/**
+ * Inserts the dependency data for a Checkable object into the given Redis HMSETs and runtime updates.
+ *
+ * This function is responsible for serializing the in memory representation Checkable dependencies into
+ * Redis HMSETs and runtime updates (if any) according to the Icinga DB schema. The serialized data consists
+ * of the following Redis HMSETs:
+ * - RedisKey::DependencyNode: Contains dependency node data representing each host, service, and redundancy group
+ *   in the dependency graph.
+ * - RedisKey::DependencyEdge: Dependency edge information representing all connections between the nodes.
+ * - RedisKey::RedundancyGroup: Redundancy group data representing all redundancy groups in the graph.
+ * - RedisKey::RedundancyGroupState: State information for redundancy groups.
+ * - RedisKey::DependencyEdgeState: State information for (each) dependency edge. Multiple edges may share the
+ *	 same state.
+ *
+ * If the `onlyDependencyGroup` parameter is set, only dependencies from this group are processed. This is useful
+ * when only a specific dependency group should be processed, e.g. during runtime updates. For initial config dumps,
+ * it shouldn't be necessary to set the `runtimeUpdates` and `onlyDependencyGroup` parameters.
+ *
+ * @param checkable The checkable object to extract dependencies from.
+ * @param hMSets The map of Redis HMSETs to insert the dependency data into.
+ * @param runtimeUpdates If set, runtime updates are additionally added to this vector.
+ * @param onlyDependencyGroup If set, only process dependency objects from this group.
+ */
+void IcingaDB::InsertCheckableDependencies(
+	const Checkable::Ptr& checkable,
+	std::map<String, RedisConnection::Query>& hMSets,
+	std::vector<Dictionary::Ptr>* runtimeUpdates,
+	const DependencyGroup::Ptr& onlyDependencyGroup
+)
+{
+	// Only generate a dependency node event if the Checkable is actually part of some dependency graph.
+	// That's, it either depends on other Checkables or others depend on it, and in both cases, we have
+	// to at least generate a dependency node entry for it.
+	if (!checkable->HasAnyDependencies()) {
+		return;
+	}
+
+	// First and foremost, generate a dependency node entry for the provided Checkable object and insert it into
+	// the HMSETs map and if set, the `runtimeUpdates` vector.
+	auto [host, service] = GetHostService(checkable);
+	auto checkableId(GetObjectIdentifier(checkable));
+	{
+		Dictionary::Ptr data(new Dictionary{{"environment_id", m_EnvironmentId}, {"host_id", GetObjectIdentifier(host)}});
+		if (service) {
+			data->Set("service_id", checkableId);
+		}
+
+		AddDataToHmSets(hMSets, RedisKey::DependencyNode, checkableId, data);
+		if (runtimeUpdates) {
+			AddObjectDataToRuntimeUpdates(*runtimeUpdates, checkableId, m_PrefixConfigObject + "dependency:node", data);
+		}
+	}
+
+	// If `onlyDependencyGroup` is provided, process the dependencies only from that group; otherwise,
+	// retrieve all the dependency groups that the Checkable object is part of.
+	std::vector<DependencyGroup::Ptr> dependencyGroups{onlyDependencyGroup};
+	if (!onlyDependencyGroup) {
+		dependencyGroups = checkable->GetDependencyGroups();
+	}
+
+	for (auto& dependencyGroup : dependencyGroups) {
+		String edgeFromNodeId(checkableId);
+		bool syncSharedEdgeState(false);
+
+		if (!dependencyGroup->IsRedundancyGroup()) {
+			// Non-redundant dependency groups are just placeholders and never get synced to Redis, thus just figure
+			// out whether we have to sync the shared edge state. For runtime updates the states are sent via the
+			// UpdateDependenciesState() method, thus we don't have to sync them here.
+			syncSharedEdgeState = !runtimeUpdates && m_DumpedGlobals.DependencyGroup.IsNew(dependencyGroup->GetCompositeKey());
+		} else {
+			auto redundancyGroupId(HashValue(new Array{m_EnvironmentId, dependencyGroup->GetCompositeKey()}));
+			dependencyGroup->SetIcingaDBIdentifier(redundancyGroupId);
+
+			edgeFromNodeId = redundancyGroupId;
+
+			// During the initial config sync, multiple children can depend on the same redundancy group, sync it only
+			// the first time it is encountered. Though, if this is a runtime update, we have to re-serialize and sync
+			// the redundancy group unconditionally, as we don't know whether it was already synced or the context that
+			// triggered this update.
+			if (runtimeUpdates || m_DumpedGlobals.DependencyGroup.IsNew(redundancyGroupId)) {
+				Dictionary::Ptr groupData(new Dictionary{
+					{"environment_id", m_EnvironmentId},
+					{"display_name", dependencyGroup->GetRedundancyGroupName()},
+				});
+				// Set/refresh the redundancy group data in the Redis HMSETs (redundancy_group database table).
+				AddDataToHmSets(hMSets, RedisKey::RedundancyGroup, redundancyGroupId, groupData);
+
+				Dictionary::Ptr nodeData(new Dictionary{
+					{"environment_id", m_EnvironmentId},
+					{"redundancy_group_id", redundancyGroupId},
+				});
+				// Obviously, the redundancy group is part of some dependency chain, thus we have to generate
+				// dependency node entry for it as well.
+				AddDataToHmSets(hMSets, RedisKey::DependencyNode, redundancyGroupId, nodeData);
+
+				if (runtimeUpdates) {
+					// Send the same data sent to the Redis HMSETs to the runtime updates stream as well.
+					AddObjectDataToRuntimeUpdates(*runtimeUpdates, redundancyGroupId, m_PrefixConfigObject + "redundancygroup", groupData);
+					AddObjectDataToRuntimeUpdates(*runtimeUpdates, redundancyGroupId, m_PrefixConfigObject + "dependency:node", nodeData);
+				} else {
+					syncSharedEdgeState = true;
+
+					// Serialize and sync the redundancy group state information a) to the RedundancyGroupState and b)
+					// to the DependencyEdgeState HMSETs. The latter is shared by all child Checkables of the current
+					// redundancy group, and since they all depend on the redundancy group, the state of that group is
+					// basically the state of the dependency edges between the children and the redundancy group.
+					auto stateAttrs(SerializeRedundancyGroupState(checkable, dependencyGroup));
+					AddDataToHmSets(hMSets, RedisKey::RedundancyGroupState, redundancyGroupId, stateAttrs);
+					AddDataToHmSets(hMSets, RedisKey::DependencyEdgeState, redundancyGroupId, Dictionary::Ptr(new Dictionary{
+						{"id", redundancyGroupId},
+						{"environment_id", m_EnvironmentId},
+						{"failed", stateAttrs->Get("failed")},
+					}));
+				}
+			}
+
+			Dictionary::Ptr data(new Dictionary{
+				{"environment_id", m_EnvironmentId},
+				{"from_node_id", checkableId},
+				{"to_node_id", redundancyGroupId},
+				// All redundancy group members share the same state, thus use the group ID as a reference.
+				{"dependency_edge_state_id", redundancyGroupId},
+				{"display_name", dependencyGroup->GetRedundancyGroupName()},
+			});
+
+			// Generate a dependency edge entry representing the connection between the Checkable and the redundancy
+			// group. This Checkable dependes on the redundancy group (is a child of it), thus the "dependency_edge_state_id"
+			// is set to the redundancy group ID. Note that if this group has multiple children, they all will have the
+			// same "dependency_edge_state_id" value.
+			auto edgeId(HashValue(new Array{checkableId, redundancyGroupId}));
+			AddDataToHmSets(hMSets, RedisKey::DependencyEdge, edgeId, data);
+
+			if (runtimeUpdates) {
+				AddObjectDataToRuntimeUpdates(*runtimeUpdates, edgeId, m_PrefixConfigObject + "dependency:edge", data);
+			}
+		}
+
+		auto dependencies(dependencyGroup->GetDependenciesForChild(checkable.get()));
+		// Sort the dependencies by their parent Checkable object to ensure that all dependency objects that share the
+		// same parent Checkable are placed next to each other in the container. See the while loop below for more info!
+		std::sort(dependencies.begin(), dependencies.end(), [](const Dependency::Ptr& lhs, const Dependency::Ptr& rhs) {
+			return lhs->GetParent() < rhs->GetParent();
+		});
+
+		// Traverse through each dependency objects within the current dependency group the provided Checkable depend
+		// on and generate a dependency edge entry. The generated dependency edge "from_node_id" may vary depending on
+		// whether the dependency group is a redundancy group or not. If it's a redundancy group, the "from_node_id"
+		// will be the redundancy group ID; otherwise, it will be the current Checkable ID. However, the "to_node_id"
+		// value will always be the parent Checkable ID of the dependency object.
+		for (auto it(dependencies.begin()); it != dependencies.end(); /* no increment */) {
+			auto dependency(*it);
+			auto parent(dependency->GetParent());
+			auto displayName(dependency->GetShortName());
+
+			Dictionary::Ptr edgeStateAttrs(SerializeDependencyEdgeState(dependencyGroup, dependency));
+
+			// In case there are multiple Dependency objects with the same parent, these are merged into a single edge
+			// to prevent duplicate edges in the resulting graph. All objects with the same parent were placed next to
+			// each other by the sort function above.
+			//
+			// Additionally, the iterator for the surrounding loop is incremented by this loop: after it has finished,
+			// "it" will either point to the next dependency with a different parent or to the end of the container.
+			while (++it != dependencies.end() && (*it)->GetParent() == parent) {
+				displayName += ", " + (*it)->GetShortName();
+				if (syncSharedEdgeState && edgeStateAttrs->Get("failed") == false) {
+					edgeStateAttrs = SerializeDependencyEdgeState(dependencyGroup, *it);
+				}
+			}
+
+			Dictionary::Ptr data(new Dictionary{
+				{"environment_id", m_EnvironmentId},
+				{"from_node_id", edgeFromNodeId},
+				{"to_node_id", GetObjectIdentifier(parent)},
+				{"dependency_edge_state_id", edgeStateAttrs->Get("id")},
+				{"display_name", std::move(displayName)},
+			});
+
+			auto edgeId(HashValue(new Array{data->Get("from_node_id"), data->Get("to_node_id")}));
+			AddDataToHmSets(hMSets, RedisKey::DependencyEdge, edgeId, data);
+
+			if (runtimeUpdates) {
+				AddObjectDataToRuntimeUpdates(*runtimeUpdates, edgeId, m_PrefixConfigObject + "dependency:edge", data);
+			} else if (syncSharedEdgeState) {
+				AddDataToHmSets(hMSets, RedisKey::DependencyEdgeState, edgeStateAttrs->Get("id"), edgeStateAttrs);
+			}
+		}
+	}
+}
+
 /**
  * Update the state information of a checkable in Redis.
  *
@@ -1177,6 +1349,115 @@ void IcingaDB::UpdateState(const Checkable::Ptr& checkable, StateUpdate mode)
 	}
 }
 
+/**
+ * Send dependencies state information of the given Checkable to Redis.
+ *
+ * If the dependencyGroup parameter is set, only the dependencies state of that group are sent. Otherwise, all
+ * dependency groups of the provided Checkable are processed.
+ *
+ * @param checkable The Checkable you want to send the dependencies state update for
+ * @param onlyDependencyGroup If set, send state updates only for this dependency group and its dependencies.
+ * @param seenGroups A container to track already processed DependencyGroups to avoid duplicate state updates.
+ */
+void IcingaDB::UpdateDependenciesState(const Checkable::Ptr& checkable, const DependencyGroup::Ptr& onlyDependencyGroup,
+	std::set<DependencyGroup*>* seenGroups) const
+{
+	if (!m_Rcon || !m_Rcon->IsConnected()) {
+		return;
+	}
+
+	std::vector<DependencyGroup::Ptr> dependencyGroups{onlyDependencyGroup};
+	if (!onlyDependencyGroup) {
+		dependencyGroups = checkable->GetDependencyGroups();
+		if (dependencyGroups.empty()) {
+			return;
+		}
+	}
+
+	RedisConnection::Queries streamStates;
+	auto addDependencyStateToStream([this, &streamStates](const String& redisKey, const Dictionary::Ptr& stateAttrs) {
+		RedisConnection::Query xAdd{
+			"XADD", "icinga:runtime:state", "MAXLEN", "~", "1000000", "*", "runtime_type", "upsert",
+			"redis_key", redisKey
+		};
+		ObjectLock olock(stateAttrs);
+		for (auto& [key, value] : stateAttrs) {
+			xAdd.emplace_back(key);
+			xAdd.emplace_back(IcingaToStreamValue(value));
+		}
+		streamStates.emplace_back(std::move(xAdd));
+	});
+
+	std::map<String, RedisConnection::Query> hMSets;
+	for (auto& dependencyGroup : dependencyGroups) {
+		bool isRedundancyGroup(dependencyGroup->IsRedundancyGroup());
+		if (isRedundancyGroup && dependencyGroup->GetIcingaDBIdentifier().IsEmpty()) {
+			// Way too soon! The Icinga DB hash will be set during the initial config dump, but this state
+			// update seems to occur way too early. So, we've to skip it for now and wait for the next one.
+			// The m_ConfigDumpInProgress flag is probably still set to true at this point!
+			continue;
+		}
+
+		if (seenGroups && !seenGroups->insert(dependencyGroup.get()).second) {
+			// Usually, if the seenGroups set is provided, IcingaDB is triggering a runtime state update for ALL
+			// children of a given initiator Checkable (parent). In such cases, we may end up with lots of useless
+			// state updates as all the children of a non-redundant group a) share the same entry in the database b)
+			// it doesn't matter which child triggers the state update first all the subsequent updates are just useless.
+			//
+			// Likewise, for redundancy groups, all children of a redundancy group share the same set of parents
+			// and thus the resulting state information would be the same from each child Checkable perspective.
+			// So, serializing the redundancy group state information only once is sufficient.
+			continue;
+		}
+
+		auto dependencies(dependencyGroup->GetDependenciesForChild(checkable.get()));
+		std::sort(dependencies.begin(), dependencies.end(), [](const Dependency::Ptr& lhs, const Dependency::Ptr& rhs) {
+			return lhs->GetParent() < rhs->GetParent();
+		});
+		for (auto it(dependencies.begin()); it != dependencies.end(); /* no increment */) {
+			const auto& dependency(*it);
+
+			Dictionary::Ptr stateAttrs;
+			// Note: The following loop is intended to cover some possible special cases but may not occur in practice
+			// that often. That is, having two or more dependency objects that point to the same parent Checkable.
+			// So, traverse all those duplicates and merge their relevant state information into a single edge.
+			for (; it != dependencies.end() && (*it)->GetParent() == dependency->GetParent(); ++it) {
+				if (!stateAttrs || stateAttrs->Get("failed") == false) {
+					stateAttrs = SerializeDependencyEdgeState(dependencyGroup, *it);
+				}
+			}
+
+			addDependencyStateToStream(m_PrefixConfigObject + "dependency:edge:state", stateAttrs);
+			AddDataToHmSets(hMSets, RedisKey::DependencyEdgeState, stateAttrs->Get("id"), stateAttrs);
+		}
+
+		if (isRedundancyGroup) {
+			Dictionary::Ptr stateAttrs(SerializeRedundancyGroupState(checkable, dependencyGroup));
+
+			Dictionary::Ptr sharedGroupState(stateAttrs->ShallowClone());
+			sharedGroupState->Remove("redundancy_group_id");
+			sharedGroupState->Remove("is_reachable");
+			sharedGroupState->Remove("last_state_change");
+
+			addDependencyStateToStream(m_PrefixConfigObject + "redundancygroup:state", stateAttrs);
+			addDependencyStateToStream(m_PrefixConfigObject + "dependency:edge:state", sharedGroupState);
+			AddDataToHmSets(hMSets, RedisKey::RedundancyGroupState, dependencyGroup->GetIcingaDBIdentifier(), stateAttrs);
+			AddDataToHmSets(hMSets, RedisKey::DependencyEdgeState, dependencyGroup->GetIcingaDBIdentifier(), sharedGroupState);
+		}
+	}
+
+	if (!streamStates.empty()) {
+		RedisConnection::Queries queries;
+		for (auto& [redisKey, query] : hMSets) {
+			query.insert(query.begin(), {"HSET", redisKey});
+			queries.emplace_back(std::move(query));
+		}
+
+		m_Rcon->FireAndForgetQueries(std::move(queries), Prio::RuntimeStateSync);
+		m_Rcon->FireAndForgetQueries(std::move(streamStates), Prio::RuntimeStateStream, {0, 1});
+	}
+}
+
 // Used to update a single object, used for runtime updates
 void IcingaDB::SendConfigUpdate(const ConfigObject::Ptr& object, bool runtimeUpdate)
 {
@@ -1194,34 +1475,7 @@ void IcingaDB::SendConfigUpdate(const ConfigObject::Ptr& object, bool runtimeUpd
 		UpdateState(checkable, runtimeUpdate ? StateUpdate::Full : StateUpdate::Volatile);
 	}
 
-	std::vector<std::vector<String> > transaction = {{"MULTI"}};
-
-	for (auto& kv : hMSets) {
-		if (!kv.second.empty()) {
-			kv.second.insert(kv.second.begin(), {"HMSET", kv.first});
-			transaction.emplace_back(std::move(kv.second));
-		}
-	}
-
-	for (auto& objectAttributes : runtimeUpdates) {
-		std::vector<String> xAdd({"XADD", "icinga:runtime", "MAXLEN", "~", "1000000", "*"});
-		ObjectLock olock(objectAttributes);
-
-		for (const Dictionary::Pair& kv : objectAttributes) {
-			String value = IcingaToStreamValue(kv.second);
-			if (!value.IsEmpty()) {
-				xAdd.emplace_back(kv.first);
-				xAdd.emplace_back(value);
-			}
-		}
-
-		transaction.emplace_back(std::move(xAdd));
-	}
-
-	if (transaction.size() > 1) {
-		transaction.push_back({"EXEC"});
-		m_Rcon->FireAndForgetQueries(std::move(transaction), Prio::Config, {1});
-	}
+	ExecuteRedisTransaction(m_Rcon, hMSets, runtimeUpdates);
 
 	if (checkable) {
 		SendNextUpdate(checkable);
@@ -1308,6 +1562,11 @@ bool IcingaDB::PrepareObject(const ConfigObject::Ptr& object, Dictionary::Ptr& a
 		attributes->Set("notes", checkable->GetNotes());
 		attributes->Set("icon_image_alt", checkable->GetIconImageAlt());
 
+		if (size_t totalChildren (checkable->GetAllChildrenCount()); totalChildren > 0) {
+			// Only set the Redis key if the Checkable has actually some child dependencies.
+			attributes->Set("total_children", totalChildren);
+		}
+
 		attributes->Set("checkcommand_id", GetObjectIdentifier(checkable->GetCheckCommand()));
 
 		Endpoint::Ptr commandEndpoint = checkable->GetCommandEndpoint();
@@ -2580,6 +2839,98 @@ void IcingaDB::SendCustomVarsChanged(const ConfigObject::Ptr& object, const Dict
 	}
 }
 
+void IcingaDB::SendDependencyGroupChildRegistered(const Checkable::Ptr& child, const DependencyGroup::Ptr& dependencyGroup)
+{
+	if (!m_Rcon || !m_Rcon->IsConnected()) {
+		return;
+	}
+
+	std::vector<Dictionary::Ptr> runtimeUpdates;
+	std::map<String, RedisConnection::Query> hMSets;
+	InsertCheckableDependencies(child, hMSets, &runtimeUpdates, dependencyGroup);
+	ExecuteRedisTransaction(m_Rcon, hMSets, runtimeUpdates);
+
+	UpdateState(child, StateUpdate::Full);
+	UpdateDependenciesState(child, dependencyGroup);
+
+	std::set<Checkable::Ptr> parents;
+	dependencyGroup->LoadParents(parents);
+	for (const auto& parent : parents) {
+		// The total_children and affects_children columns might now have different outcome, so update the parent
+		// Checkable as well. The grandparent Checkable may still have wrong numbers of total children, though it's not
+		// worth traversing the whole tree way up and sending config updates for each one of them, as the next Redis
+		// config dump is going to fix it anyway.
+		SendConfigUpdate(parent, true);
+	}
+}
+
+void IcingaDB::SendDependencyGroupChildRemoved(
+	const DependencyGroup::Ptr& dependencyGroup,
+	const std::vector<Dependency::Ptr>& dependencies,
+	bool removeGroup
+)
+{
+	if (!m_Rcon || !m_Rcon->IsConnected() || dependencies.empty()) {
+		return;
+	}
+
+	Checkable::Ptr child;
+	std::set<Checkable*> detachedParents;
+	for (const auto& dependency : dependencies) {
+		child = dependency->GetChild(); // All dependencies have the same child.
+		const auto& parent(dependency->GetParent());
+		if (auto [_, inserted] = detachedParents.insert(dependency->GetParent().get()); inserted) {
+			String edgeId;
+			if (dependencyGroup->IsRedundancyGroup()) {
+				// If the redundancy group has no members left, it's going to be removed as well, so we need to
+				// delete dependency edges from that group to the parent Checkables.
+				if (removeGroup) {
+					auto id(HashValue(new Array{dependencyGroup->GetIcingaDBIdentifier(), GetObjectIdentifier(parent)}));
+					DeleteRelationship(id, RedisKey::DependencyEdge);
+					DeleteState(id, RedisKey::DependencyEdgeState);
+				}
+
+				// Remove the connection from the child Checkable to the redundancy group.
+				edgeId = HashValue(new Array{GetObjectIdentifier(child), dependencyGroup->GetIcingaDBIdentifier()});
+			} else {
+				// Remove the edge between the parent and child Checkable linked through the removed dependency.
+				edgeId = HashValue(new Array{GetObjectIdentifier(child), GetObjectIdentifier(parent)});
+			}
+
+			DeleteRelationship(edgeId, RedisKey::DependencyEdge);
+
+			// The total_children and affects_children columns might now have different outcome, so update the parent
+			// Checkable as well. The grandparent Checkable may still have wrong numbers of total children, though it's
+			// not worth traversing the whole tree way up and sending config updates for each one of them, as the next
+			// Redis config dump is going to fix it anyway.
+			SendConfigUpdate(parent, true);
+
+			if (!parent->HasAnyDependencies()) {
+				// If the parent Checkable isn't part of any other dependency chain anymore, drop its dependency node entry.
+				DeleteRelationship(GetObjectIdentifier(parent), RedisKey::DependencyNode);
+			}
+		}
+	}
+
+	if (removeGroup && dependencyGroup->IsRedundancyGroup()) {
+		String redundancyGroupId(dependencyGroup->GetIcingaDBIdentifier());
+		DeleteRelationship(redundancyGroupId, RedisKey::DependencyNode);
+		DeleteRelationship(redundancyGroupId, RedisKey::RedundancyGroup);
+
+		DeleteState(redundancyGroupId, RedisKey::RedundancyGroupState);
+		DeleteState(redundancyGroupId, RedisKey::DependencyEdgeState);
+	} else if (removeGroup) {
+		// Note: The Icinga DB identifier of a non-redundant dependency group is used as the edge state ID
+		// and shared by all of its dependency objects. See also SerializeDependencyEdgeState() for details.
+		DeleteState(dependencyGroup->GetIcingaDBIdentifier(), RedisKey::DependencyEdgeState);
+	}
+
+	if (!child->HasAnyDependencies()) {
+		// If the child Checkable has no parent and reverse dependencies, we can safely remove the dependency node.
+		DeleteRelationship(GetObjectIdentifier(child), RedisKey::DependencyNode);
+	}
+}
+
 Dictionary::Ptr IcingaDB::SerializeState(const Checkable::Ptr& checkable)
 {
 	Dictionary::Ptr attrs = new Dictionary();
@@ -2623,6 +2974,7 @@ Dictionary::Ptr IcingaDB::SerializeState(const Checkable::Ptr& checkable)
 	attrs->Set("check_attempt", checkable->GetCheckAttempt());
 
 	attrs->Set("is_active", checkable->IsActive());
+	attrs->Set("affects_children", checkable->AffectsChildren());
 
 	CheckResult::Ptr cr = checkable->GetLastCheckResult();
 
@@ -2758,8 +3110,10 @@ void IcingaDB::StateChangeHandler(const ConfigObject::Ptr& object, const CheckRe
 void IcingaDB::ReachabilityChangeHandler(const std::set<Checkable::Ptr>& children)
 {
 	for (const IcingaDB::Ptr& rw : ConfigType::GetObjectsByType<IcingaDB>()) {
+		std::set<DependencyGroup*> seenGroups;
 		for (auto& checkable : children) {
 			rw->UpdateState(checkable, StateUpdate::Full);
+			rw->UpdateDependenciesState(checkable, nullptr, &seenGroups);
 		}
 	}
 }
@@ -2856,6 +3210,20 @@ void IcingaDB::NextCheckUpdatedHandler(const Checkable::Ptr& checkable)
 	}
 }
 
+void IcingaDB::DependencyGroupChildRegisteredHandler(const Checkable::Ptr& child, const DependencyGroup::Ptr& dependencyGroup)
+{
+	for (const auto& rw : ConfigType::GetObjectsByType<IcingaDB>()) {
+		rw->SendDependencyGroupChildRegistered(child, dependencyGroup);
+	}
+}
+
+void IcingaDB::DependencyGroupChildRemovedHandler(const DependencyGroup::Ptr& dependencyGroup, const std::vector<Dependency::Ptr>& dependencies, bool removeGroup)
+{
+	for (const auto& rw : ConfigType::GetObjectsByType<IcingaDB>()) {
+		rw->SendDependencyGroupChildRemoved(dependencyGroup, dependencies, removeGroup);
+	}
+}
+
 void IcingaDB::HostProblemChangedHandler(const Service::Ptr& service) {
 	for (auto& rw : ConfigType::GetObjectsByType<IcingaDB>()) {
 		/* Host state changes affect is_handled and severity of services. */
@@ -2973,3 +3341,139 @@ void IcingaDB::DeleteRelationship(const String& id, const String& redisKeyWithou
 
 	m_Rcon->FireAndForgetQueries(queries, Prio::Config);
 }
+
+void IcingaDB::DeleteRelationship(const String& id, RedisKey redisKey, bool hasChecksum)
+{
+	switch (redisKey) {
+		case RedisKey::RedundancyGroup:
+			DeleteRelationship(id, "redundancygroup", hasChecksum);
+			break;
+		case RedisKey::DependencyNode:
+			DeleteRelationship(id, "dependency:node", hasChecksum);
+			break;
+		case RedisKey::DependencyEdge:
+			DeleteRelationship(id, "dependency:edge", hasChecksum);
+			break;
+		default:
+			BOOST_THROW_EXCEPTION(std::invalid_argument("Invalid RedisKey provided"));
+	}
+}
+
+void IcingaDB::DeleteState(const String& id, RedisKey redisKey, bool hasChecksum) const
+{
+	String redisKeyWithoutPrefix;
+	switch (redisKey) {
+		case RedisKey::RedundancyGroupState:
+			redisKeyWithoutPrefix = "redundancygroup:state";
+			break;
+		case RedisKey::DependencyEdgeState:
+			redisKeyWithoutPrefix = "dependency:edge:state";
+			break;
+		default:
+			BOOST_THROW_EXCEPTION(std::invalid_argument("Invalid state RedisKey provided"));
+	}
+
+	Log(LogNotice, "IcingaDB")
+		<< "Deleting state " << std::quoted(redisKeyWithoutPrefix.CStr()) << " -> " << std::quoted(id.CStr());
+
+	RedisConnection::Queries hdels;
+	if (hasChecksum) {
+		hdels.emplace_back(RedisConnection::Query{"HDEL", m_PrefixConfigCheckSum + redisKeyWithoutPrefix, id});
+	}
+	hdels.emplace_back(RedisConnection::Query{"HDEL", m_PrefixConfigObject + redisKeyWithoutPrefix, id});
+
+	m_Rcon->FireAndForgetQueries(std::move(hdels), Prio::RuntimeStateSync);
+	// TODO: This is currently purposefully commented out due to how Icinga DB (Go) handles runtime state
+	//       upsert and delete events. See https://github.com/Icinga/icingadb/pull/894 for more details.
+	/*m_Rcon->FireAndForgetQueries({{
+		"XADD", "icinga:runtime:state", "MAXLEN", "~", "1000000", "*",
+		"redis_key", m_PrefixConfigObject + redisKeyWithoutPrefix, "id", id, "runtime_type", "delete"
+	}}, Prio::RuntimeStateStream, {0, 1});*/
+}
+
+/**
+ * Add the provided data to the Redis HMSETs map.
+ *
+ * Adds the provided data to the Redis HMSETs map for the provided Redis key. The actual Redis key is determined by
+ * the provided RedisKey enum. The data will be json encoded before being added to the Redis HMSETs map.
+ *
+ * @param hMSets The map of RedisConnection::Query you want to add the data to.
+ * @param redisKey The key of the Redis object you want to add the data to.
+ * @param id Unique Redis identifier for the provided data.
+ * @param data The actual data you want to add the Redis HMSETs map.
+ */
+void IcingaDB::AddDataToHmSets(std::map<String, RedisConnection::Query>& hMSets, RedisKey redisKey, const String& id, const Dictionary::Ptr& data) const
+{
+	RedisConnection::Query* query;
+	switch (redisKey) {
+		case RedisKey::RedundancyGroup:
+			query = &hMSets[m_PrefixConfigObject + "redundancygroup"];
+			break;
+		case RedisKey::DependencyNode:
+			query = &hMSets[m_PrefixConfigObject + "dependency:node"];
+			break;
+		case RedisKey::DependencyEdge:
+			query = &hMSets[m_PrefixConfigObject + "dependency:edge"];
+			break;
+		case RedisKey::RedundancyGroupState:
+			query = &hMSets[m_PrefixConfigObject + "redundancygroup:state"];
+			break;
+		case RedisKey::DependencyEdgeState:
+			query = &hMSets[m_PrefixConfigObject + "dependency:edge:state"];
+			break;
+		default:
+			BOOST_THROW_EXCEPTION(std::invalid_argument("Invalid RedisKey provided"));
+	}
+
+	query->emplace_back(id);
+	query->emplace_back(JsonEncode(data));
+}
+
+/**
+ * Execute the provided HMSET values and runtime updates in a single Redis transaction on the provided Redis connection.
+ *
+ * The HMSETs should just contain the necessary key value pairs to be set in Redis, i.e, without the HMSET command
+ * itself. This function will then go through each of the map keys and prepend the HMSET command when transforming the
+ * map into valid Redis queries. Likewise, the runtime updates should just contain the key value pairs to be streamed
+ * to the icinga:runtime pipeline, and this function will generate a XADD query for each one of the vector elements.
+ *
+ * @param rcon The Redis connection to execute the transaction on.
+ * @param hMSets A map of Redis keys and their respective HMSET values.
+ * @param runtimeUpdates A list of dictionaries to be sent to the icinga:runtime stream.
+ */
+void IcingaDB::ExecuteRedisTransaction(const RedisConnection::Ptr& rcon, std::map<String, RedisConnection::Query>& hMSets,
+	const std::vector<Dictionary::Ptr>& runtimeUpdates)
+{
+	RedisConnection::Queries transaction{{"MULTI"}};
+	for (auto& [redisKey, query] : hMSets) {
+		if (!query.empty()) {
+			query.insert(query.begin(), {"HSET", redisKey});
+			transaction.emplace_back(std::move(query));
+		}
+	}
+
+	for (auto& attrs : runtimeUpdates) {
+		RedisConnection::Query xAdd{"XADD", "icinga:runtime", "MAXLEN", "~", "1000000", "*"};
+
+		ObjectLock olock(attrs);
+		for (auto& [key, value] : attrs) {
+			if (auto streamVal(IcingaToStreamValue(value)); !streamVal.IsEmpty()) {
+				xAdd.emplace_back(key);
+				xAdd.emplace_back(std::move(streamVal));
+			}
+		}
+
+		transaction.emplace_back(std::move(xAdd));
+	}
+
+	if (transaction.size() > 1) {
+		transaction.emplace_back(RedisConnection::Query{"EXEC"});
+		if (!runtimeUpdates.empty()) {
+			rcon->FireAndForgetQueries(std::move(transaction), Prio::Config, {1});
+		} else {
+			// This is likely triggered by the initial Redis config dump, so a) we don't need to record the number of
+			// affected objects and b) we don't really know how many objects are going to be affected by this tx.
+			rcon->FireAndForgetQueries(std::move(transaction), Prio::Config);
+		}
+	}
+}

lib/icingadb/icingadb-utility.cpp

@@ -159,6 +159,66 @@ Dictionary::Ptr IcingaDB::SerializeVars(const Dictionary::Ptr& vars)
 	return res;
 }
 
+/**
+ * Serialize a dependency edge state for Icinga DB
+ *
+ * @param dependencyGroup The state of the group the dependency is part of.
+ * @param dep The dependency object to serialize.
+ *
+ * @return A dictionary with the serialized state.
+ */
+Dictionary::Ptr IcingaDB::SerializeDependencyEdgeState(const DependencyGroup::Ptr& dependencyGroup, const Dependency::Ptr& dep)
+{
+	String edgeStateId;
+	// The edge state ID is computed a bit differently depending on whether this is for a redundancy group or not.
+	// For redundancy groups, the state ID is supposed to represent the connection state between the redundancy group
+	// and the parent Checkable of the given dependency. Hence, the outcome will always be different for each parent
+	// Checkable of the redundancy group.
+	if (dependencyGroup->IsRedundancyGroup()) {
+		edgeStateId = HashValue(new Array{
+			dependencyGroup->GetIcingaDBIdentifier(),
+			GetObjectIdentifier(dep->GetParent()),
+		});
+	} else if (dependencyGroup->GetIcingaDBIdentifier().IsEmpty()) {
+		// For non-redundant dependency groups, on the other hand, all dependency objects within that group will
+		// always have the same parent Checkable. Likewise, the state ID will be always the same as well it doesn't
+		// matter which dependency object is used to compute it. Therefore, it's sufficient to compute it only once
+		// and all the other dependency objects can reuse the cached state ID.
+		edgeStateId = HashValue(new Array{dependencyGroup->GetCompositeKey(), GetObjectIdentifier(dep->GetParent())});
+		dependencyGroup->SetIcingaDBIdentifier(edgeStateId);
+	} else {
+		// Use the already computed state ID for the dependency group.
+		edgeStateId = dependencyGroup->GetIcingaDBIdentifier();
+	}
+
+	return new Dictionary{
+		{"id", std::move(edgeStateId)},
+		{"environment_id", m_EnvironmentId},
+		{"failed", !dep->IsAvailable(DependencyState) || !dep->GetParent()->IsReachable()}
+	};
+}
+
+/**
+ * Serialize the provided redundancy group state attributes.
+ *
+ * @param child The child checkable object to serialize the state for.
+ * @param redundancyGroup The redundancy group object to serialize the state for.
+ *
+ * @return A dictionary with the serialized redundancy group state.
+ */
+Dictionary::Ptr IcingaDB::SerializeRedundancyGroupState(const Checkable::Ptr& child, const DependencyGroup::Ptr& redundancyGroup)
+{
+	auto state(redundancyGroup->GetState(child.get()));
+	return new Dictionary{
+		{"id", redundancyGroup->GetIcingaDBIdentifier()},
+		{"environment_id", m_EnvironmentId},
+		{"redundancy_group_id", redundancyGroup->GetIcingaDBIdentifier()},
+		{"failed", state != DependencyGroup::State::Ok},
+		{"is_reachable", state != DependencyGroup::State::Unreachable},
+		{"last_state_change", TimestampToMilliseconds(Utility::GetTime())},
+	};
+}
+
 const char* IcingaDB::GetNotificationTypeByEnum(NotificationType type)
 {
 	switch (type) {

lib/icingadb/icingadb.hpp

@@ -90,6 +90,15 @@ private:
 		Full        = Volatile | RuntimeOnly,
 	};
 
+	enum class RedisKey : uint8_t
+	{
+		RedundancyGroup,
+		DependencyNode,
+		DependencyEdge,
+		RedundancyGroupState,
+		DependencyEdgeState,
+	};
+
 	void OnConnectedHandler();
 
 	void PublishStatsTimerHandler();
@@ -101,8 +110,12 @@ private:
 	void DeleteKeys(const RedisConnection::Ptr& conn, const std::vector<String>& keys, RedisConnection::QueryPriority priority);
 	std::vector<String> GetTypeOverwriteKeys(const String& type);
 	std::vector<String> GetTypeDumpSignalKeys(const Type::Ptr& type);
+	void InsertCheckableDependencies(const Checkable::Ptr& checkable, std::map<String, RedisConnection::Query>& hMSets,
+		std::vector<Dictionary::Ptr>* runtimeUpdates, const DependencyGroup::Ptr& onlyDependencyGroup = nullptr);
 	void InsertObjectDependencies(const ConfigObject::Ptr& object, const String typeName, std::map<String, std::vector<String>>& hMSets,
 			std::vector<Dictionary::Ptr>& runtimeUpdates, bool runtimeUpdate);
+	void UpdateDependenciesState(const Checkable::Ptr& checkable, const DependencyGroup::Ptr& onlyDependencyGroup = nullptr,
+		std::set<DependencyGroup*>* seenGroups = nullptr) const;
 	void UpdateState(const Checkable::Ptr& checkable, StateUpdate mode);
 	void SendConfigUpdate(const ConfigObject::Ptr& object, bool runtimeUpdate);
 	void CreateConfigUpdate(const ConfigObject::Ptr& object, const String type, std::map<String, std::vector<String>>& hMSets,
@@ -112,6 +125,9 @@ private:
 	void AddObjectDataToRuntimeUpdates(std::vector<Dictionary::Ptr>& runtimeUpdates, const String& objectKey,
 			const String& redisKey, const Dictionary::Ptr& data);
 	void DeleteRelationship(const String& id, const String& redisKeyWithoutPrefix, bool hasChecksum = false);
+	void DeleteRelationship(const String& id, RedisKey redisKey, bool hasChecksum = false);
+	void DeleteState(const String& id, RedisKey redisKey, bool hasChecksum = false) const;
+	void AddDataToHmSets(std::map<String, RedisConnection::Query>& hMSets, RedisKey redisKey, const String& id, const Dictionary::Ptr& data) const;
 
 	void SendSentNotification(
 		const Notification::Ptr& notification, const Checkable::Ptr& checkable, const std::set<User::Ptr>& users,
@@ -136,6 +152,8 @@ private:
 	void SendCommandEnvChanged(const ConfigObject::Ptr& command, const Dictionary::Ptr& oldValues, const Dictionary::Ptr& newValues);
 	void SendCommandArgumentsChanged(const ConfigObject::Ptr& command, const Dictionary::Ptr& oldValues, const Dictionary::Ptr& newValues);
 	void SendCustomVarsChanged(const ConfigObject::Ptr& object, const Dictionary::Ptr& oldValues, const Dictionary::Ptr& newValues);
+	void SendDependencyGroupChildRegistered(const Checkable::Ptr& child, const DependencyGroup::Ptr& dependencyGroup);
+	void SendDependencyGroupChildRemoved(const DependencyGroup::Ptr& dependencyGroup, const std::vector<Dependency::Ptr>& dependencies, bool removeGroup);
 
 	void ForwardHistoryEntries();
 
@@ -157,6 +175,8 @@ private:
 	static String CalcEventID(const char* eventType, const ConfigObject::Ptr& object, double eventTime = 0, NotificationType nt = NotificationType(0));
 	static const char* GetNotificationTypeByEnum(NotificationType type);
 	static Dictionary::Ptr SerializeVars(const Dictionary::Ptr& vars);
+	static Dictionary::Ptr SerializeDependencyEdgeState(const DependencyGroup::Ptr& dependencyGroup, const Dependency::Ptr& dep);
+	static Dictionary::Ptr SerializeRedundancyGroupState(const Checkable::Ptr& child, const DependencyGroup::Ptr& redundancyGroup);
 
 	static String HashValue(const Value& value);
 	static String HashValue(const Value& value, const std::set<String>& propertiesBlacklist, bool propertiesWhitelist = false);
@@ -180,6 +200,8 @@ private:
 	static void FlappingChangeHandler(const Checkable::Ptr& checkable, double changeTime);
 	static void NewCheckResultHandler(const Checkable::Ptr& checkable);
 	static void NextCheckUpdatedHandler(const Checkable::Ptr& checkable);
+	static void DependencyGroupChildRegisteredHandler(const Checkable::Ptr& child, const DependencyGroup::Ptr& dependencyGroup);
+	static void DependencyGroupChildRemovedHandler(const DependencyGroup::Ptr& dependencyGroup, const std::vector<Dependency::Ptr>& dependencies, bool removeGroup);
 	static void HostProblemChangedHandler(const Service::Ptr& service);
 	static void AcknowledgementSetHandler(const Checkable::Ptr& checkable, const String& author, const String& comment, AcknowledgementType type, bool persistent, double changeTime, double expiry);
 	static void AcknowledgementClearedHandler(const Checkable::Ptr& checkable, const String& removedBy, double changeTime);
@@ -195,6 +217,9 @@ private:
 	static void CommandArgumentsChangedHandler(const ConfigObject::Ptr& command, const Dictionary::Ptr& oldValues, const Dictionary::Ptr& newValues);
 	static void CustomVarsChangedHandler(const ConfigObject::Ptr& object, const Dictionary::Ptr& oldValues, const Dictionary::Ptr& newValues);
 
+	static void ExecuteRedisTransaction(const RedisConnection::Ptr& rcon, std::map<String, RedisConnection::Query>& hMSets,
+		const std::vector<Dictionary::Ptr>& runtimeUpdates);
+
 	void AssertOnWorkQueue();
 
 	void ExceptionHandler(boost::exception_ptr exp);
@@ -225,7 +250,7 @@ private:
 	std::atomic_size_t m_PendingRcons;
 
 	struct {
-		DumpedGlobals CustomVar, ActionUrl, NotesUrl, IconImage;
+		DumpedGlobals CustomVar, ActionUrl, NotesUrl, IconImage, DependencyGroup;
 	} m_DumpedGlobals;
 
 	// m_EnvironmentId is shared across all IcingaDB objects (typically there is at most one, but it is perfectly fine

test/CMakeLists.txt

@@ -232,6 +232,10 @@ add_boost_test(base
     icinga_checkresult/service_flapping_notification
     icinga_checkresult/suppressed_notification
     icinga_dependencies/multi_parent
+    icinga_dependencies/push_dependency_groups_to_registry
+    icinga_dependencies/default_redundancy_group_registration_unregistration
+    icinga_dependencies/simple_redundancy_group_registration_unregistration
+    icinga_dependencies/mixed_redundancy_group_registration_unregsitration
     icinga_notification/strings
     icinga_notification/state_filter
     icinga_notification/type_filter

test/icinga-dependencies.cpp

@@ -9,6 +9,67 @@ using namespace icinga;
 
 BOOST_AUTO_TEST_SUITE(icinga_dependencies)
 
+static Host::Ptr CreateHost(const std::string& name, bool pushDependencyGroupsToRegistry = true)
+{
+	Host::Ptr host = new Host();
+	host->SetName(name);
+	if (pushDependencyGroupsToRegistry) {
+		host->PushDependencyGroupsToRegistry();
+	}
+	return host;
+}
+
+static Dependency::Ptr CreateDependency(Checkable::Ptr parent, Checkable::Ptr child, const String& name)
+{
+	Dependency::Ptr dep = new Dependency();
+	dep->SetParent(parent);
+	dep->SetChild(child);
+	dep->SetName(name + "!" + child->GetName());
+	return dep;
+}
+
+static void RegisterDependency(Dependency::Ptr dep, const String& redundancyGroup)
+{
+	dep->SetRedundancyGroup(redundancyGroup);
+	dep->GetChild()->AddDependency(dep);
+	dep->GetParent()->AddReverseDependency(dep);
+}
+
+static void AssertCheckableRedundancyGroup(Checkable::Ptr checkable, int dependencyCount, int groupCount, int totalDependenciesCount)
+{
+	BOOST_CHECK_MESSAGE(
+		dependencyCount == checkable->GetDependencies().size(),
+		"Dependency count mismatch for '" << checkable->GetName() << "' - expected=" << dependencyCount << "; got="
+			<< checkable->GetDependencies().size()
+	);
+	auto dependencyGroups(checkable->GetDependencyGroups());
+	BOOST_CHECK_MESSAGE(
+		groupCount == dependencyGroups.size(),
+		"Dependency group count mismatch for '" << checkable->GetName() << "' - expected=" << groupCount
+			<< "; got=" << dependencyGroups.size()
+	);
+
+	for (auto& dependencyGroup : dependencyGroups) {
+		BOOST_CHECK_MESSAGE(
+			totalDependenciesCount == dependencyGroup->GetDependenciesCount(),
+			"Dependency group '" << dependencyGroup->GetRedundancyGroupName() << "' and Checkable '" << checkable->GetName()
+				<< "' total dependencies count mismatch - expected=" << totalDependenciesCount << "; got="
+				<< dependencyGroup->GetDependenciesCount()
+		);
+	}
+
+	if (groupCount > 0) {
+		BOOST_REQUIRE_MESSAGE(!dependencyGroups.empty(), "Checkable '" << checkable->GetName() << "' should have at least one dependency group.");
+	}
+}
+
+static std::vector<DependencyGroup::Ptr> ExtractGroups(const Checkable::Ptr& checkable)
+{
+	auto dependencyGroups(checkable->GetDependencyGroups());
+	std::sort(dependencyGroups.begin(), dependencyGroups.end());
+	return dependencyGroups;
+}
+
 BOOST_AUTO_TEST_CASE(multi_parent)
 {
 	/* One child host, two parent hosts. Simulate multi-parent dependencies. */
@@ -20,53 +81,28 @@ BOOST_AUTO_TEST_CASE(multi_parent)
 	 * - Parent objects need a CheckResult object
 	 * - Dependencies need a StateFilter
 	 */
-	Host::Ptr parentHost1 = new Host();
-	parentHost1->SetActive(true);
-	parentHost1->SetMaxCheckAttempts(1);
-	parentHost1->Activate();
-	parentHost1->SetAuthority(true);
+	Host::Ptr parentHost1 = CreateHost("parentHost1");
 	parentHost1->SetStateRaw(ServiceCritical);
 	parentHost1->SetStateType(StateTypeHard);
 	parentHost1->SetLastCheckResult(new CheckResult());
 
-	Host::Ptr parentHost2 = new Host();
-	parentHost2->SetActive(true);
-	parentHost2->SetMaxCheckAttempts(1);
-	parentHost2->Activate();
-	parentHost2->SetAuthority(true);
+	Host::Ptr parentHost2 = CreateHost("parentHost2");
 	parentHost2->SetStateRaw(ServiceOK);
 	parentHost2->SetStateType(StateTypeHard);
 	parentHost2->SetLastCheckResult(new CheckResult());
 
-	Host::Ptr childHost = new Host();
-	childHost->SetActive(true);
-	childHost->SetMaxCheckAttempts(1);
-	childHost->Activate();
-	childHost->SetAuthority(true);
+	Host::Ptr childHost = CreateHost("childHost");
 	childHost->SetStateRaw(ServiceOK);
 	childHost->SetStateType(StateTypeHard);
 
 	/* Build the dependency tree. */
-	Dependency::Ptr dep1 = new Dependency();
-
-	dep1->SetParent(parentHost1);
-	dep1->SetChild(childHost);
+	Dependency::Ptr dep1 (CreateDependency(parentHost1, childHost, "dep1"));
 	dep1->SetStateFilter(StateFilterUp);
+	RegisterDependency(dep1, "");
 
-	// Reverse dependencies
-        childHost->AddDependency(dep1);
-        parentHost1->AddReverseDependency(dep1);
-
-	Dependency::Ptr dep2 = new Dependency();
-
-	dep2->SetParent(parentHost2);
-	dep2->SetChild(childHost);
+	Dependency::Ptr dep2 (CreateDependency(parentHost2, childHost, "dep2"));
 	dep2->SetStateFilter(StateFilterUp);
-
-	// Reverse dependencies
-        childHost->AddDependency(dep2);
-        parentHost2->AddReverseDependency(dep2);
-
+	RegisterDependency(dep2, "");
 
 	/* Test the reachability from this point.
 	 * parentHost1 is DOWN, parentHost2 is UP.
@@ -77,18 +113,42 @@ BOOST_AUTO_TEST_CASE(multi_parent)
 
 	BOOST_CHECK(childHost->IsReachable() == false);
 
+	Dependency::Ptr duplicateDep (CreateDependency(parentHost1, childHost, "dep4"));
+	duplicateDep->SetIgnoreSoftStates(false, true);
+	RegisterDependency(duplicateDep, "");
+	parentHost1->SetStateType(StateTypeSoft);
+
+	// It should still be unreachable, due to the duplicated dependency object above with ignore_soft_states set to false.
+	BOOST_CHECK(childHost->IsReachable() == false);
+	parentHost1->SetStateType(StateTypeHard);
+	childHost->RemoveDependency(duplicateDep);
+
 	/* The only DNS server is DOWN.
 	 * Expected result: childHost is unreachable.
 	 */
-	dep1->SetRedundancyGroup("DNS");
+	childHost->RemoveDependency(dep1); // Remove the dep and re-add it with a configured redundancy group.
+	RegisterDependency(dep1, "DNS");
 	BOOST_CHECK(childHost->IsReachable() == false);
 
 	/* 1/2 DNS servers is DOWN.
 	 * Expected result: childHost is reachable.
 	 */
-	dep2->SetRedundancyGroup("DNS");
+	childHost->RemoveDependency(dep2);
+	RegisterDependency(dep2, "DNS");
 	BOOST_CHECK(childHost->IsReachable() == true);
 
+	auto grandParentHost(CreateHost("GrandParentHost"));
+	grandParentHost->SetLastCheckResult(new CheckResult());
+	grandParentHost->SetStateRaw(ServiceCritical);
+	grandParentHost->SetStateType(StateTypeHard);
+
+	Dependency::Ptr dep3 (CreateDependency(grandParentHost, parentHost1, "dep3"));
+	dep3->SetStateFilter(StateFilterUp);
+	RegisterDependency(dep3, "");
+	// The grandparent is DOWN but the DNS redundancy group has to be still reachable.
+	BOOST_CHECK_EQUAL(true, childHost->IsReachable());
+	childHost->RemoveDependency(dep3);
+
 	/* Both DNS servers are DOWN.
 	 * Expected result: childHost is unreachable.
 	 */
@@ -98,4 +158,278 @@ BOOST_AUTO_TEST_CASE(multi_parent)
 	BOOST_CHECK(childHost->IsReachable() == false);
 }
 
+BOOST_AUTO_TEST_CASE(push_dependency_groups_to_registry)
+{
+	Checkable::Ptr childHostC(CreateHost("C", false));
+	Checkable::Ptr childHostD(CreateHost("D", false));
+	std::set<Dependency::Ptr> dependencies; // Keep track of all dependencies to avoid unexpected deletions.
+	for (auto& parent : {String("A"), String("B"), String("E")}) {
+		Dependency::Ptr depC(CreateDependency(CreateHost(parent), childHostC, "depC" + parent));
+		Dependency::Ptr depD(CreateDependency(depC->GetParent(), childHostD, "depD" + parent));
+		if (parent == "A") {
+			Dependency::Ptr depCA2(CreateDependency(depC->GetParent(), childHostC, "depCA2"));
+			childHostC->AddDependency(depCA2);
+			dependencies.emplace(depCA2);
+		} else {
+			depC->SetRedundancyGroup("redundant", true);
+			depD->SetRedundancyGroup("redundant", true);
+
+			if (parent == "B") { // Create an exact duplicate of depC, but with a different name.
+				Dependency::Ptr depCB2(CreateDependency(depC->GetParent(), childHostC, "depCB2"));
+				depCB2->SetRedundancyGroup("redundant", true);
+				childHostC->AddDependency(depCB2);
+				dependencies.emplace(depCB2);
+			}
+		}
+		childHostC->AddDependency(depC);
+		childHostD->AddDependency(depD);
+		dependencies.insert({depC, depD});
+	}
+
+	childHostC->PushDependencyGroupsToRegistry();
+	childHostD->PushDependencyGroupsToRegistry();
+
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+	for (auto& checkable : {childHostC, childHostD}) {
+		BOOST_CHECK_EQUAL(2, checkable->GetDependencyGroups().size());
+		for (auto& dependencyGroup : checkable->GetDependencyGroups()) {
+			if (dependencyGroup->IsRedundancyGroup()) {
+				BOOST_CHECK_EQUAL(5, dependencyGroup->GetDependenciesCount());
+				BOOST_CHECK_EQUAL(checkable == childHostC ? 5 : 3, checkable->GetDependencies().size());
+			} else {
+				BOOST_CHECK_EQUAL(3, dependencyGroup->GetDependenciesCount());
+				BOOST_CHECK_EQUAL(checkable == childHostC ? 5 : 3, checkable->GetDependencies().size());
+			}
+		}
+	}
+}
+
+BOOST_AUTO_TEST_CASE(default_redundancy_group_registration_unregistration)
+{
+	Checkable::Ptr childHostC(CreateHost("C"));
+	Dependency::Ptr depCA(CreateDependency(CreateHost("A"), childHostC, "depCA"));
+	RegisterDependency(depCA, "");
+	AssertCheckableRedundancyGroup(childHostC, 1, 1, 1);
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	Dependency::Ptr depCB(CreateDependency(CreateHost("B"), childHostC, "depCB"));
+	RegisterDependency(depCB, "");
+	AssertCheckableRedundancyGroup(childHostC, 2, 2, 1);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	Checkable::Ptr childHostD(CreateHost("D"));
+	Dependency::Ptr depDA(CreateDependency(depCA->GetParent(), childHostD, "depDA"));
+	RegisterDependency(depDA, "");
+	AssertCheckableRedundancyGroup(childHostD, 1, 1, 2);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	Dependency::Ptr depDB(CreateDependency(depCB->GetParent(), childHostD, "depDB"));
+	RegisterDependency(depDB, "");
+	AssertCheckableRedundancyGroup(childHostD, 2, 2, 2);
+	AssertCheckableRedundancyGroup(childHostC, 2, 2, 2);
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	// This is an exact duplicate of depCA, but with a different dependency name.
+	Dependency::Ptr depCA2(CreateDependency(depCA->GetParent(), childHostC, "depCA2"));
+	// This is a duplicate of depCA, but with a different state filter.
+	Dependency::Ptr depCA3(CreateDependency(depCA->GetParent(), childHostC, "depCA3"));
+	depCA3->SetStateFilter(StateFilterUp, true);
+	// This is a duplicate of depCA, but with a different ignore_soft_states flag.
+	Dependency::Ptr depCA4(CreateDependency(depCA->GetParent(), childHostC, "depCA4"));
+	depCA4->SetIgnoreSoftStates(false, true);
+
+	for (auto& dependency : {depCA2, depCA3, depCA4}) {
+		bool isAnExactDuplicate = dependency == depCA2;
+		RegisterDependency(dependency, "");
+
+		if (isAnExactDuplicate) {
+			BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+		}
+
+		for (auto& dependencyGroup : childHostD->GetDependencyGroups()) {
+			if (dependency->GetParent() == dependencyGroup->GetDependenciesForChild(childHostD.get()).front()->GetParent()) {
+				BOOST_CHECK_EQUAL(isAnExactDuplicate ? 3 : 1, dependencyGroup->GetDependenciesCount());
+			} else {
+				BOOST_CHECK_EQUAL(2, dependencyGroup->GetDependenciesCount());
+			}
+			BOOST_CHECK_EQUAL(2, childHostD->GetDependencies().size());
+		}
+
+		for (auto& dependencyGroup : childHostC->GetDependencyGroups()) {
+			if (dependency->GetParent() == dependencyGroup->GetDependenciesForChild(childHostC.get()).front()->GetParent()) {
+				// If depCA2 is currently being processed, then the group should have 3 dependencies, that's because
+				// depCA2 is an exact duplicate of depCA, and depCA shares the same group with depDA.
+				BOOST_CHECK_EQUAL(isAnExactDuplicate ? 3 : 2, dependencyGroup->GetDependenciesCount());
+			} else {
+				BOOST_CHECK_EQUAL(2, dependencyGroup->GetDependenciesCount());
+			}
+			// The 3 dependencies are depCA, depCB, and the current one from the loop.
+			BOOST_CHECK_EQUAL(3, childHostC->GetDependencies().size());
+		}
+		BOOST_CHECK_EQUAL(isAnExactDuplicate ? 2 : 3, DependencyGroup::GetRegistrySize());
+		childHostC->RemoveDependency(dependency);
+	}
+
+	childHostC->RemoveDependency(depCA);
+	childHostD->RemoveDependency(depDA);
+	AssertCheckableRedundancyGroup(childHostC, 1, 1, 2);
+	AssertCheckableRedundancyGroup(childHostD, 1, 1, 2);
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	childHostC->RemoveDependency(depCB);
+	childHostD->RemoveDependency(depDB);
+	AssertCheckableRedundancyGroup(childHostC, 0, 0, 0);
+	AssertCheckableRedundancyGroup(childHostD, 0, 0, 0);
+	BOOST_CHECK_EQUAL(0, DependencyGroup::GetRegistrySize());
+}
+
+BOOST_AUTO_TEST_CASE(simple_redundancy_group_registration_unregistration)
+{
+	Checkable::Ptr childHostC(CreateHost("childC"));
+
+	Dependency::Ptr depCA(CreateDependency(CreateHost("A"), childHostC, "depCA"));
+	RegisterDependency(depCA, "redundant");
+	AssertCheckableRedundancyGroup(childHostC, 1, 1, 1);
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	Dependency::Ptr depCB(CreateDependency(CreateHost("B"), childHostC, "depCB"));
+	RegisterDependency(depCB, "redundant");
+	AssertCheckableRedundancyGroup(childHostC, 2, 1, 2);
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	Checkable::Ptr childHostD(CreateHost("childD"));
+	Dependency::Ptr depDA (CreateDependency(depCA->GetParent(), childHostD, "depDA"));
+	RegisterDependency(depDA, "redundant");
+	AssertCheckableRedundancyGroup(childHostD, 1, 1, 1);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	Dependency::Ptr depDB(CreateDependency(depCB->GetParent(), childHostD, "depDB"));
+	RegisterDependency(depDB, "redundant");
+	// Still 1 redundancy group, but there should be 4 dependencies now, i.e. 2 for each child Checkable.
+	AssertCheckableRedundancyGroup(childHostC, 2, 1, 4);
+	AssertCheckableRedundancyGroup(childHostD, 2, 1, 4);
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	childHostC->RemoveDependency(depCA);
+	// After unregistering depCA, childHostC should have a new redundancy group with only depCB as dependency, and...
+	AssertCheckableRedundancyGroup(childHostC, 1, 1, 1);
+	// ...childHostD should still have the same redundancy group as before but also with only two dependencies.
+	AssertCheckableRedundancyGroup(childHostD, 2, 1, 2);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	childHostD->RemoveDependency(depDA);
+	// Nothing should have changed for childHostC, but childHostD should now have a fewer group dependency, i.e.
+	// both child hosts should have the same redundancy group with only depCB and depDB as dependency.
+	AssertCheckableRedundancyGroup(childHostC, 1, 1, 2);
+	AssertCheckableRedundancyGroup(childHostD, 1, 1, 2);
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	RegisterDependency(depDA, depDA->GetRedundancyGroup());
+	childHostD->RemoveDependency(depDB);
+	// Nothing should have changed for childHostC, but both should now have a separate group with only depCB and depDA as dependency.
+	AssertCheckableRedundancyGroup(childHostC, 1, 1, 1);
+	AssertCheckableRedundancyGroup(childHostD, 1, 1, 1);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	childHostC->RemoveDependency(depCB);
+	childHostD->RemoveDependency(depDA);
+	AssertCheckableRedundancyGroup(childHostC, 0, 0, 0);
+	AssertCheckableRedundancyGroup(childHostD, 0, 0, 0);
+	BOOST_CHECK_EQUAL(0, DependencyGroup::GetRegistrySize());
+}
+
+BOOST_AUTO_TEST_CASE(mixed_redundancy_group_registration_unregsitration)
+{
+	Checkable::Ptr childHostC(CreateHost("childC"));
+	Dependency::Ptr depCA(CreateDependency(CreateHost("A"), childHostC, "depCA"));
+	RegisterDependency(depCA, "redundant");
+	AssertCheckableRedundancyGroup(childHostC, 1, 1, 1);
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	Checkable::Ptr childHostD(CreateHost("childD"));
+	Dependency::Ptr depDA(CreateDependency(depCA->GetParent(), childHostD, "depDA"));
+	RegisterDependency(depDA, "redundant");
+	AssertCheckableRedundancyGroup(childHostD, 1, 1, 2);
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	Dependency::Ptr depCB(CreateDependency(CreateHost("B"), childHostC, "depCB"));
+	RegisterDependency(depCB, "redundant");
+	AssertCheckableRedundancyGroup(childHostC, 2, 1, 2);
+	AssertCheckableRedundancyGroup(childHostD, 1, 1, 1);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	Dependency::Ptr depDB(CreateDependency(depCB->GetParent(), childHostD, "depDB"));
+	RegisterDependency(depDB, "redundant");
+	AssertCheckableRedundancyGroup(childHostC, 2, 1, 4);
+	AssertCheckableRedundancyGroup(childHostD, 2, 1, 4);
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	Checkable::Ptr childHostE(CreateHost("childE"));
+	Dependency::Ptr depEA(CreateDependency(depCA->GetParent(), childHostE, "depEA"));
+	RegisterDependency(depEA, "redundant");
+	AssertCheckableRedundancyGroup(childHostE, 1, 1, 1);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	Dependency::Ptr depEB(CreateDependency(depCB->GetParent(), childHostE, "depEB"));
+	RegisterDependency(depEB, "redundant");
+	// All 3 hosts share the same group, and each host has 2 dependencies, thus 6 dependencies in total.
+	AssertCheckableRedundancyGroup(childHostC, 2, 1, 6);
+	AssertCheckableRedundancyGroup(childHostD, 2, 1, 6);
+	AssertCheckableRedundancyGroup(childHostE, 2, 1, 6);
+	auto childHostCGroups(ExtractGroups(childHostC));
+	BOOST_TEST((childHostCGroups == ExtractGroups(childHostD) && childHostCGroups == ExtractGroups(childHostE)));
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	Dependency::Ptr depEZ(CreateDependency(CreateHost("Z"), childHostE, "depEZ"));
+	RegisterDependency(depEZ, "redundant");
+	// Child host E should have a new redundancy group with 3 dependencies and the other two should still share the same group.
+	AssertCheckableRedundancyGroup(childHostC, 2, 1, 4);
+	AssertCheckableRedundancyGroup(childHostD, 2, 1, 4);
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	AssertCheckableRedundancyGroup(childHostE, 3, 1, 3);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	childHostE->RemoveDependency(depEA);
+	AssertCheckableRedundancyGroup(childHostC, 2, 1, 4);
+	AssertCheckableRedundancyGroup(childHostD, 2, 1, 4);
+	BOOST_TEST(ExtractGroups(childHostC) == ExtractGroups(childHostD), boost::test_tools::per_element());
+	AssertCheckableRedundancyGroup(childHostE, 2, 1, 2);
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	RegisterDependency(depEA, depEA->GetRedundancyGroup()); // Re-register depEA and instead...
+	childHostE->RemoveDependency(depEZ); // ...unregister depEZ and check if all the hosts share the same group again.
+	// All 3 hosts share the same group again, and each host has 2 dependencies, thus 6 dependencies in total.
+	AssertCheckableRedundancyGroup(childHostC, 2, 1, 6);
+	AssertCheckableRedundancyGroup(childHostD, 2, 1, 6);
+	AssertCheckableRedundancyGroup(childHostE, 2, 1, 6);
+	childHostCGroups = ExtractGroups(childHostC);
+	BOOST_TEST((childHostCGroups == ExtractGroups(childHostD) && childHostCGroups == ExtractGroups(childHostE)));
+	BOOST_CHECK_EQUAL(1, DependencyGroup::GetRegistrySize());
+
+	childHostC->RemoveDependency(depCA);
+	childHostD->RemoveDependency(depDB);
+	childHostE->RemoveDependency(depEB);
+	// Child host C has now a separate group with only depCB as dependency, and child hosts D and E share the same group.
+	AssertCheckableRedundancyGroup(childHostC, 1, 1, 1);
+	AssertCheckableRedundancyGroup(childHostD, 1, 1, 2);
+	AssertCheckableRedundancyGroup(childHostE, 1, 1, 2);
+	BOOST_TEST(ExtractGroups(childHostD) == ExtractGroups(childHostE), boost::test_tools::per_element());
+	BOOST_CHECK_EQUAL(2, DependencyGroup::GetRegistrySize());
+
+	childHostC->RemoveDependency(depCB);
+	childHostD->RemoveDependency(depDA);
+	childHostE->RemoveDependency(depEA);
+	AssertCheckableRedundancyGroup(childHostC, 0, 0, 0);
+	AssertCheckableRedundancyGroup(childHostD, 0, 0, 0);
+	AssertCheckableRedundancyGroup(childHostE, 0, 0, 0);
+	BOOST_CHECK_EQUAL(0, DependencyGroup::GetRegistrySize());
+}
+
 BOOST_AUTO_TEST_SUITE_END()