Defining advanced service dependencies

First, the basics. You create service dependencies by adding service dependency definitions in your object definition. In each definition you specify the dependent service, the service you are depending on, and the criteria (if any) that causes the execution and notification dependencies to fail (these are described later).

You can create several dependencies for a given service, but you must add a separate service dependency definition for each dependency you create.

Service Dependencies example

The image below shows an example logical layout of service notification and execution dependencies. Different services are dependent on other services for notifications and check execution.

../_images/service-dependencies.png

In this example, the dependency definitions for Service F on Host C would be defined as follows:

define servicedependency{
  host_name                       Host B
  service_description             Service D
  dependent_host_name             Host C
  dependent_service_description   Service F
  execution_failure_criteria      o
  notification_failure_criteria   w,u
}

define servicedependency{
  host_name                       Host B
  service_description             Service E
  dependent_host_name             Host C
  dependent_service_description   Service F
  execution_failure_criteria      n
  notification_failure_criteria   w,u,c
}

define servicedependency{
  host_name                       Host B
  service_description             Service C
  dependent_host_name             Host C
  dependent_service_description   Service F
  execution_failure_criteria      w
  notification_failure_criteria   c
}

The other dependency definitions shown in the image above would be defined as follows:

define servicedependency{
  host_name                       Host A
  service_description             Service A
  dependent_host_name             Host B
  dependent_service_description   Service D
  execution_failure_criteria      u
  notification_failure_criteria   n
}

define servicedependency{
  host_name                       Host A
  service_description             Service B
  dependent_host_name             Host B
  dependent_service_description   Service E
  execution_failure_criteria      w,u
  notification_failure_criteria   c
}

define servicedependency{
  host_name                       Host B
  service_description             Service C
  dependent_host_name             Host B
  dependent_service_description   Service E
  execution_failure_criteria      n
  notification_failure_criteria   w,u,c
}

How Service Dependencies Are Tested

Before than Alignak executes a service check or sends notifications out for a service, it will check to see if the service has any dependencies. If it doesn’t have any dependencies, the check is executed or the notification is sent out as it normally would be.

If the service does have one or more dependencies, Alignak will check each dependency entry as follows:

  • Alignak gets the current state of the service that is being depended upon.
  • Alignak compares this state against either the execution or notification failure options in the dependency definition (whichever one is relevant at the time).
  • If this state matches one of the failure options, the dependency is said to have failed and Alignak will break out of the dependency check loop.
  • If this state does not match any of the failure options for the dependency entry, the dependency is said to have passed and Alignak will go on and check the next dependency entry.

This cycle continues until either all dependencies for the service have been checked or until one dependency check fails.

Warning

One important thing to note is that by default, Alignak will use the most current hard state of the service(s) that is/are being depended upon when it does the dependency checks. If you want Alignak to use the most current state of the services (regardless of whether its a soft or hard state), enable the soft_state_dependencies option.

Execution Dependencies

Execution dependencies are used to restrict when active checks of a service can be performed. Passive checks are not restricted by execution dependencies.

If all of the execution dependency tests for the service passed, Alignak will execute the check of the service as it normally would. If even just one of the execution dependencies for a service fails, Alignak will temporarily prevent the execution of checks for that (dependent) service.

At some point in the future the execution dependency tests for the service may all pass. If this happens, Alignak will start checking the service again as it normally would.

In the example above, Service E would have failed execution dependencies if Service B is in a WARNING or UNKNOWN state. If it was the case, the service check would not be performed and the check would be scheduled for (potential) execution at a later time.

Warning

Execution dependencies will limit the load due to useless checks, but can limit some correlation logics, and so should be used only if you trully need them.

Notification Dependencies

If all of the notification dependency tests for the service passed, Alignak will send notifications out for the service as it normally would. If even just one of the notification dependencies for a service fails, Alignak will temporarily repress notifications for that (dependent) service.

At some point in the future the notification dependency tests for the service may all pass. If this happens, Alignak will start sending out notifications again as it normally would for the service.

In the example above, Service F would have failed notification dependencies if Service C is in a CRITICAL state, //and/or* Service D is in a WARNING or UNKNOWN state, and/or// if **Service E* is in a WARNING, UNKNOWN, or CRITICAL state. If this were the case, notifications for the service would not be sent out.

Dependency Inheritance

As mentioned before, service dependencies are not inherited by default. In the example above you can see that Service F is dependent on Service E. However, it does not automatically inherit Service E’s dependencies on Service B and Service C. In order to make Service F dependent on Service C we had to add another service dependency definition. There is no dependency definition for Service B, so Service F is not dependent on Service B.

If you do wish to make service dependencies inheritable, you must use the inherits_parent directive in the service dependency definition. When this directive is enabled, it indicates that the dependency inherits dependencies of the service that is being depended upon (also referred to as the master service). In other words, if the master service is dependent upon other services and any one of those dependencies fail, this dependency will also fail.

In the example above, imagine that you want to add a new dependency for service F to make it dependent on service A. You could create a new dependency definition that specified service F as the dependent service and service A as being the master service (i.e. the service that is being dependend on). You could alternatively modify the dependency definition for services D and F to look like this:

define servicedependency{
  host_name                       Host B
  service_description             Service D
  dependent_host_name             Host C
  dependent_service_description   Service F
  execution_failure_criteria      o
  notification_failure_criteria   n
  inherits_parent                 1
}

Since the inherits_parent directive is enabled, the dependency between services A and D will be tested when the dependency between services F and D are being tested.

Dependencies can have multiple levels of inheritance. If the dependency definition between A and D had its inherits_parent directive enable and service A was dependent on some other service (let’s call it service G), the service F would be dependent on services D, A, and G (each with potentially different criteria).

Host Dependencies

As you’d probably expect, host dependencies work in a similar fashion to service dependencies. The difference is that they’re for hosts, not services.

Do not confuse host dependencies with parent/child host relationships. You should be using parent/child host relationships (defined with the parents directive in host definitions) for most cases, rather than host dependencies. A description of how parent/child host relationships work can be found in the documentation on network reachability.

Here are the basics about host dependencies:

  • A host can be dependent on one or more other host
  • Host dependencies are not inherited (unless specifically configured to)
  • Host dependencies can be used to cause host check execution and host notifications to be suppressed under different circumstances (UP, DOWN, and/or UNREACHABLE states)
  • Host dependencies might only be valid during specific time periods

Example Host Dependencies

The image below shows an example of the logical layout of host notification dependencies. Different hosts are dependent on other hosts for notifications.

../_images/host-dependencies.png

In the example above, the dependency definitions for Host C would be defined as follows:

define hostdependency{
  host_name                       Host A
  dependent_host_name             Host C
  notification_failure_criteria   d
}

define hostdependency{
  host_name                       Host B
  dependent_host_name             Host C
  notification_failure_criteria   d,u
}

As with service dependencies, host dependencies are not inherited. In the example image you can see that Host C does not inherit the host dependencies of Host B. In order for Host C to be dependent on Host A, a new host dependency definition must be defined.

Host notification dependencies work in a similar manner to service notification dependencies. If all of the notification dependency tests for the host pass, Alignak will send notifications out for the host as it normally would. If even just one of the notification dependencies for a host fails, Alignak will temporarily repress notifications for that (dependent) host.

At some point in the future the notification dependency tests for the host may all pass. If this happens, Alignak will start sending out notifications again as it normally would for the host.