Docs: Core reload for technical concepts

doc/19-technical-concepts.md

@@ -176,6 +176,49 @@ The following signals are triggered in the stages:
 * [Flex](https://github.com/westes/flex)
 * [GNU Bison](https://www.gnu.org/software/bison/)
 
+## Core <a id="technical-concepts-core"></a>
+
+#:## Core: Reload Handling <a id="technical-concepts-core-reload"></a>
+
+The initial design of the reload state machine looks like this:
+
+* receive reload signal SIGHUP
+* fork a child process, start configuration validation in parallel work queues
+* parent process continues with old configuration objects and the event scheduling
+(doing checks, replicating cluster events, triggering alert notifications, etc.)
+* validation NOT ok: child process terminates, parent process continues with old configuration state
+* validation ok: child process signals parent process to terminate and save its current state (all events until now) into the icinga2 state file
+* parent process shuts down writing icinga2.state file
+* child process waits for parent process gone, reads the icinga2 state file and synchronizes all historical and status data
+* child becomes the new session leader
+
+Since Icinga 2.6, there are two processes when checked with `ps aux | grep icinga2` or `pidof icinga2`.
+This was to ensure that feature file descriptors don't leak into the plugin process (e.g. DB IDO MySQL sockets).
+
+Icinga 2.9 changed the reload handling a bit with SIGUSR2 signals
+and systemd notifies.
+
+With systemd, it could occur that the tree was broken thus resulting
+in killing all remaining processes on stop, instead of a clean exit.
+You can read the full story [here](https://github.com/Icinga/icinga2/issues/7309).
+
+With 2.11 you'll now see 3 processes:
+
+- The umbrella process which takes care about signal handling and process spawning/stopping
+- The main process with the check scheduler, notifications, etc.
+- The execution helper process
+
+During reload, the umbrella process spawns a new reload process which validates the configuration.
+Once successful, the new reload process signals the umbrella process that it is finished.
+The umbrella process forwards the signal and tells the old main process to shutdown.
+The old main process writes the icinga2.state file. The umbrella process signals
+the reload process that the main process terminated.
+
+The reload process was in idle wait before, and now continues to read the written
+state file and run the event loop (checks, notifications, "events", ...). The reload
+process itself also spawns the execution helper process again.
+
+
 ## Features <a id="technical-concepts-features"></a>
 
 Features are implemented in specific libraries and can be enabled