## <a id="dependencies"></a> Dependencies

Icinga 2 uses host and service [Dependency](#objecttype-dependency) objects either directly
defined or as inline definition as `dependencies` dictionary. The `parent_host` and `parent_service`
attributes are mandatory, `child_host` and `child_service` attributes are obsolete within
inline definitions in an existing service object or service inline definition.

A service can depend on a host, and vice versa. A service has an implicit dependency (parent)
to its host. A host to host dependency acts implicit as host parent relation.
When dependencies are calculated, not only the immediate parent is taken into
account but all parents are inherited.

A common scenario is the Icinga 2 server behind a router. Checking internet
access by pinging the Google DNS server `google-dns` is a common method, but
will fail in case the `dsl-router` host is down. Therefore the example below
defines a host dependency which acts implicit as parent relation too.

Furthermore the host may be reachable but ping probes are dropped by the
router's firewall. In case the `dsl-router``ping4` service check fails, all
further checks for the `ping4` service on host `google-dns` service should
be suppressed. This is achieved by setting the `disable_checks` attribute to `true`.

    object Host "dsl-router" {
      macros.address = "192.168.1.1"
    }

    object Host "google-dns" {
      macros.address = "8.8.8.8"
    }

    apply Service "ping4" {
      import "generic-service"

      check_command = "ping4"

      assign where host.macros.address
    }

    apply Dependency "internet" {
      parent_host = "dsl-router"
      disable_checks = true

      assign where host.name != "dsl-router"
    }