Let's start with host dependency definitions. These are objects that have several attributes, and each dependency can actually describe one or more dependencies, for example, it is possible to tell Nagios that 20 machines rely on a particular host in a single dependency definition.

Here is an example of a dependency specifying that during maintenance, a Windows backup storage server in another branch depends upon a VPN server:

  define hostdependency 
  { 
    dependent_host_name            backupstorage-branch2 
    host_name                      vpnserver-branch1 
    dependency_period              maintenancewindows 
  } 

The following table describes all of the available directives for defining a host dependency. Items in bold are required when specifying a dependency:

The question is where to store such dependency files. As for service definitions, it is recommended that you store dependencies specific to a particular host in the file containing the definition of the dependent host. For the previous example, we would put it in the hosts/backupstorage-branch2.cfg file.

When defining a dependency that will describe a relationship between more than one master or dependent host, it's best to put these into a generic file for dependencies—for example, we can put it in hosts/dependencies.cfg. Another good option is to put the dependency definitions that only affect a single master host in the master host's definition.

If you are defining a dependency that covers more than one master or dependent host, it is best to use host groups to manage the list of hosts that should be included in the dependency's definition. This can be one or more host group names, and very often, these groups will also be the same as for the service definitions.

Service dependencies work in a similar way to host dependencies. For hosts, you need to specify one or more master hosts and one or more dependent hosts; for services, you need to define a master service and a dependent service.

Service dependencies can be defined only for a single service, but on multiple hosts. For example, you can tell Nagios that IMAP services on the emailservers host group depend on the LDAP service on the ldapserver host.

Here is an example of how to define such a service dependency:

  define servicedependency 
  { 
    host_name                      ldapserver 
    service_description            LDAP 
    dependent_hostgroup_name       emailservers 
    dependent_service_description  IMAP 
    execution_failure_criteria     c,u 
    notification_failure_criteria  c,u,w 
  } 

The following table describes all available directives for defining a service dependency. Items in bold are required when specifying a dependency:

As is the case for host dependencies, there is a question of where to store the service dependency definitions. A good answer to this is to store dependencies in the same files that the dependent service definitions are kept in. If you are following the previous suggestions regarding how to keep services in the file structure, then for a service bound to a single host, both service and the related dependencies should be kept in the same file as the host definition itself. If a service is used by more than one host, it is kept in a separate file. In this case, dependencies related to that service should also be kept in the same file as the service.