Object Inheritance

Introduction

This documentation attempts to explain object inheritance and how it can be used in your object definitions.

If you are confused about how recursion and inheritance work after reading this, take a look at the sample object config files provided in the Shinken distribution. If that still doesn’t help, have a look to the shinken resources for help.

Basics

There are three variables affecting recursion and inheritance that are present in all object definitions. They are indicated in red as follows...

define someobjecttype{
       object-specific variables ...
       name            template_name
       use             name_of_template_to_use
       register        [0/1]
       }

The first variable is “name”. Its just a “template” name that can be referenced in other object definitions so they can inherit the objects properties/variables. Template names must be unique amongst objects of the same type, so you can’t have two or more host definitions that have “hosttemplate” as their template name.

The second variable is “use”. This is where you specify the name of the template object that you want to inherit properties/variables from. The name you specify for this variable must be defined as another object’s template named (using the name variable).

The third variable is “register”. This variable is used to indicate whether or not the object definition should be “registered” with Shinken. By default, all object definitions are registered. If you are using a partial object definition as a template, you would want to prevent it from being registered (an example of this is provided later). Values are as follows: 0 = do NOT register object definition, 1 = register object definition (this is the default). This variable is NOT inherited; every (partial) object definition used as a template must explicitly set the “register” directive to be 0. This prevents the need to override an inherited “register” directive with a value of 1 for every object that should be registered.

Local Variables vs. Inherited Variables

One important thing to understand with inheritance is that “local” object variables always take precedence over variables defined in the template object. Take a look at the following example of two host definitions (not all required variables have been supplied):

define host{
       host_name               bighost1
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      5
       name                    hosttemplate1
       }

define host{
       host_name               bighost2
       max_check_attempts      3
       use                     hosttemplate1
       }

You’ll note that the definition for host bighost1 has been defined as having hosttemplate1 as its template name. The definition for host bighost2 is using the definition of bighost1 as its template object. Once Shinken processes this data, the resulting definition of host bighost2 would be equivalent to this definition:

define host{
       host_name               bighost2
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      3
       }

You can see that the “check_command” and “notification_options” variables were inherited from the template object (where host bighost1 was defined). However, the host_name and max_check_attempts variables were not inherited from the template object because they were defined locally. Remember, locally defined variables override variables that would normally be inherited from a template object. That should be a fairly easy concept to understand.

If you would like local string variables to be appended to inherited string values, you can do so. Read more about how to accomplish this below.

Inheritance Chaining

Objects can inherit properties/variables from multiple levels of template objects. Take the following example:

define host{
       host_name               bighost1
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      5
       name                    hosttemplate1
       }

define host{
       host_name               bighost2
       max_check_attempts      3
       use                     hosttemplate1
       name                    hosttemplate2
       }

define host{
       host_name               bighost3
       use                     hosttemplate2
       }

You’ll notice that the definition of host bighost3 inherits variables from the definition of host bighost2, which in turn inherits variables from the definition of host bighost1. Once Shinken processes this configuration data, the resulting host definitions are equivalent to the following:

define host{
       host_name               bighost1
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      5
       }

define host{
       host_name               bighost2
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      3
       }

define host{
       host_name               bighost3
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      3
       }

There is no inherent limit on how “deep” inheritance can go, but you’ll probably want to limit yourself to at most a few levels in order to maintain sanity.

Using Incomplete Object Definitions as Templates

It is possible to use incomplete object definitions as templates for use by other object definitions. By “incomplete” definition, I mean that all required variables in the object have not been supplied in the object definition. It may sound odd to use incomplete definitions as templates, but it is in fact recommended that you use them. Why? Well, they can serve as a set of defaults for use in all other object definitions. Take the following example:

define host{
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      5
       name                    generichosttemplate
       register                        0
       }

define host{
       host_name               bighost1
       address                 192.168.1.3
       use                     generichosthosttemplate
       }

define host{
       host_name               bighost2
       address                 192.168.1.4
       use                     generichosthosttemplate
       }

Notice that the first host definition is incomplete because it is missing the required “host_name” variable. We don’t need to supply a host name because we just want to use this definition as a generic host template. In order to prevent this definition from being registered with Shinken as a normal host, we set the “register” variable to 0.

The definitions of hosts bighost1 and bighost2 inherit their values from the generic host definition. The only variable we’ve chosed to override is the “address” variable. This means that both hosts will have the exact same properties, except for their “host_name” and “address” variables. Once Shinken processes the config data in the example, the resulting host definitions would be equivalent to specifying the following:

define host{
       host_name               bighost1
       address                 192.168.1.3
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      5
       }

define host{
       host_name               bighost2
       address                 192.168.1.4
       check_command           check-host-alive
       notification_options    d,u,r
       max_check_attempts      5
       }

At the very least, using a template definition for default variables will save you a lot of typing. It’ll also save you a lot of headaches later if you want to change the default values of variables for a large number of hosts.

Custom Object Variables

Any custom object variables that you define in your host, service, or contact definition templates will be inherited just like other standard variables. Take the following example:

define host{
       _customvar1             somevalue  ; <-- Custom host variable
       _snmp_community         public  ; <-- Custom host variable
       name                    generichosttemplate
       register                        0
       }

define host{
       host_name               bighost1
       address                 192.168.1.3
       use                     generichosthosttemplate
       }

The host bighost1 will inherit the custom host variables “_customvar1” and “_snmp_community”, as well as their respective values, from the generichosttemplate definition. The effective result is a definition for bighost1 that looks like this:

define host{
       host_name               bighost1
       address                 192.168.1.3
       _customvar1             somevalue
       _snmp_community         public
       }

Cancelling Inheritance of String Values

In some cases you may not want your host, service, or contact definitions to inherit values of string variables from the templates they reference. If this is the case, you can specify “null” (without quotes) as the value of the variable that you do not want to inherit. Take the following example:

define host{
       event_handler           my-event-handler-command
       name                    generichosttemplate
       register                0
       }

define host{
       host_name               bighost1
       address                 192.168.1.3
       event_handler           null
       use                     generichosthosttemplate
       }

In this case, the host bighost1 will not inherit the value of the “event_handler” variable that is defined in the generichosttemplate. The resulting effective definition of bighost1 is the following:

define host{
       host_name               bighost1
       address                 192.168.1.3
       }

Additive Inheritance of String Values

Shinken gives preference to local variables instead of values inherited from templates. In most cases local variable values override those that are defined in templates. In some cases it makes sense to allow Shinken to use the values of inherited and local variables together.

This “additive inheritance” can be accomplished by prepending the local variable value with a plus sign (+). This features is only available for standard (non-custom) variables that contain string values. Take the following example:

define host{
       hostgroups              all-servers
       name                    generichosttemplate
       register                0
       }

define host{
       host_name              linuxserver1
       hostgroups             +linux-servers,web-servers
       use                    generichosthosttemplate
       }

In this case, the host linuxserver1 will append the value of its local “hostgroups” variable to that from generichosttemplate. The resulting effective definition of linuxserver1 is the following:

define host{
       host_name              linuxserver1
       hostgroups             all-servers,linux-servers,web-servers
       }

Important

If you use a field twice using several templates, the value of the field will be the first one found! In the example above, fields values in all-servers won’t we be replaced. Be careful with overlaping field!

Implied Inheritance

Normally you have to either explicitly specify the value of a required variable in an object definition or inherit it from a template. There are a few exceptions to this rule, where Shinken will assume that you want to use a value that instead comes from a related object. For example, the values of some service variables will be copied from the host the service is associated with if you don’t otherwise specify them.

The following table lists the object variables that will be implicitly inherited from related objects if you don’t explicitly specify their value in your object definition or inherit them from a template.

Object Type Object Variable Implied Source
Services contact_groups contact_groups in the associated host definition
notification_interval notification_interval in the associated host definition  
notification_period notification_period in the associated host definition  
check_period check_period in the associated host definition  
Host Escalations contact_groups contact_groups in the associated host definition
notification_interval notification_interval in the associated host definition  
escalation_period notification_period in the associated host definition  
Service Escalations contact_groups contact_groups in the associated service definition
notification_interval notification_interval in the associated service definition  
escalation_period notification_period in the associated service definition  

Implied/Additive Inheritance in Escalations

Service and host escalation definitions can make use of a special rule that combines the features of implied and additive inheritance. If escalations 1) do not inherit the values of their “contact_groups” or “contacts” directives from another escalation template and 2) their “contact_groups” or “contacts” directives begin with a plus sign (+), then the values of their corresponding host or service definition’s “contact_groups” or “contacts” directives will be used in the additive inheritance logic.

Confused? Here’s an example:

define host{
       name                    linux-server
       contact_groups          linux-admins
       ...
       }

define hostescalation{
       host_name               linux-server
       contact_groups          +management
       ...
       }

This is a much simpler equivalent to:

define hostescalation{
       host_name               linux-server
       contact_groups          linux-admins,management
       ...
       }

Multiple Inheritance Sources

Thus far, all examples of inheritance have shown object definitions inheriting variables/values from just a single source. You are also able to inherit variables/values from multiple sources for more complex configurations, as shown below.

# Generic host template

define host{
       name                    generic-host
       active_checks_enabled   1
       check_interval          10
       register                0
       }
# Development web server template
define host{
       name                    development-server
       check_interval          15
       notification_options    d,u,r
       ...
       register                0
       }
# Development web server
define host{
       use                    generic-host,development-server
       host_name              devweb1
       ...
       }
../_images/multiple-templates1.png

In the example above, devweb1 is inheriting variables/values from two sources: generic-host and development-server. You’ll notice that a check_interval variable is defined in both sources. Since generic-host was the first template specified in devweb1’s use directive, its value for the “check_interval” variable is inherited by the devweb1 host. After inheritance, the effective definition of devweb1 would be as follows:

# Development web serve
define host{
       host_name               devweb1
       active_checks_enabled   1
       check_interval          10
       notification_options    d,u,r
       ...
       }

Precedence With Multiple Inheritance Sources

When you use multiple inheritance sources, it is important to know how Shinken handles variables that are defined in multiple sources. In these cases Shinken will use the variable/value from the first source that is specified in the use directive. Since inheritance sources can themselves inherit variables/values from one or more other sources, it can get tricky to figure out what variable/value pairs take precedence.

Consider the following host definition that references three templates:

# Development web server
define host{
       use        1, 4, 8
       host_name  devweb1
       ...
}

If some of those referenced templates themselves inherit variables/values from one or more other templates, the precendence rules are shown below. Testing, trial, and error will help you better understand exactly how things work in complex inheritance situations like this. :-)

../_images/multiple-templates2.png

Inheritance overriding

Inheritance is a core feature allowing to factorize configuration. It is possible from a host or a service template to build a very large set of checks with relatively few lines. The drawback of this approach is that it requires all hosts or services to be consistent. But if it is easy to instanciate new hosts with their own definitions attributes sets, it is generally more complicated with services, because the order of magnitude is larger (hosts * services per host), and because few attributes may come from the host. This is is especially true for packs, which is a generalization of the inheritance usage.

If some hosts require special directives for the services they are hosting (values that are different from those defined at template level), it is generally necessary to define new service.

Imagine two web servers clusters, one for the frontend, the other for the backend, where the frontend servers should notify any HTTP service in CRITICAL and WARNING state, and backend servers should only notify on CRITICAL state.

To implement this configuration, we may define 2 different HTTP services with different notification options.

Example:

define service {
       service_description     HTTP Front
       hostgroup_name          front-web
       notification_options    c,w,r
            ...
}

define service {
       service_description     HTTP Back
       hostgroup_name          front-back
       notification_options    c,r
          ...
}

define host {
       host_name               web-front-01
       hostgroups              web-front
       ...
}
...

define host {
       host_name               web-back-01
       hostgroups              web-back
       ...
}
...

Another way is to inherit attributes on the service side directly from the host: some service attributes may be inherited directly from the host if not defined on the service template side (see Implied Inheritance), but not all. Our notification_options in our example cannot be picked up from the host.

If the attribute you want to be set a custom value cannot be inherited from the host, you may use the service_overrides host directive. Its role is to enforce a service directive directly from the host. This allows to define specific service instance attributes from a same generalized service definition.

Its syntax is:

service_overrides xxx,yyy zzz

It could be summarized as “For the service bound to me, named ``xxx``, I want the directive ``yyy`` set to ``zzz`` rather tran the inherited value

Example:

define service {
       service_description     HTTP
       hostgroup_name          web
       notification_options    c,w,r
       ...
}

define host {
       host_name               web-front-01
       hostgroups              web
       ...
}
...

define host {
       host_name               web-back-01
       hostgroups              web
       service_overrides       HTTP,notification_options c,r
       ...
}
...

In the previous example, we defined only one instance of the HTTP service, and we enforced the service notification_options for the web servers composing the backend. The final result is the same, but the second example is shorter, and does not require the second service definition.

Using packs allows an even shorter configuration.

Example:

define host {
       use                     http
       host_name               web-front-01
       ...
}
...

define host {
       use                     http
       host_name               web-back-01
       service_overrides       HTTP,notification_options c,r
       ...
}
...

In the packs example, the web server from the front-end cluster uses the value defined in the pack, and the one from the backend cluster has its HTTP service (inherited from the HTTP pack also) enforced its notification_options directive.

Important

The service_overrides attribute may himself be inherited from an upper host template. This is a multivalued attribute wchich syntax requires that each value is set on its own line. If you add a line on a host instance, it will not add it to the ones defined at template level, it will overlobad them. If some of the values on the template level are needed, they have to be explicitely copied.

Example:

define host {
       name                    web-front
       service_overrides       HTTP,notification_options c,r
       ...
       register                0
}
...

define host {
       use                     web-fromt
       host_name               web-back-01
       hostgroups              web
       service_overrides       HTTP,notification_options c,r
       service_overrides       HTTP,notification_interval 15
       ...
}
...

Inheritance exclusions

Packs and hostgroups allow de factorize the configuration and greatly reduce the amount of configuration to write to describe infrastructures. The drawback is that it forces hosts to be consistent, as the same configuration is applied to a possibly very large set of machines.

Imagine a web servers cluster. All machines except one should be checked its managenent interface (ILO, iDRAC). In the cluster, there is one virtual server that should be checked the exact same services than the others, except the management interface (as checking it on a virtual server has no meaning). The corresponding service comes from a pack.

In this situation, there is several ways to manage the situation:

  • create in intermadiary template on the pack level to have the management interface check attached to an upper level template
  • re define all the services for the specifed host.
  • use service overrides to set a dummy command on the corresponding service.

None of these options are satisfying.

There is a last solution that consists of excluding the corresponding service from the specified host. This may be done using the service_excludes directive.

Example:

define host {
       use                     web-fromt
       host_name               web-back-01
       ...
}

define host {
       use                     web-fromt
       host_name               web-back-02    ; The virtual server
       service_excludes        Management interface
       ...
}
...

In the case you want the opposite (exclude all except) you can use the service_includes directive

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