14. Reusing Parts of the Configuration
Pacemaker provides multiple ways to simplify the configuration XML by reusing parts of it in multiple places.
Besides simplifying the XML, this also allows you to manipulate multiple configuration elements with a single reference.
14.1. Reusing Resource Definitions
If you want to create lots of resources with similar configurations, defining a resource template simplifies the task. Once defined, it can be referenced in primitives or in certain types of constraints.
14.1.1. Configuring Resources with Templates
The primitives referencing the template will inherit all meta-attributes, instance attributes, utilization attributes and operations defined in the template. And you can define specific attributes and operations for any of the primitives. If any of these are defined in both the template and the primitive, the values defined in the primitive will take precedence over the ones defined in the template.
Hence, resource templates help to reduce the amount of configuration work. If any changes are needed, they can be done to the template definition and will take effect globally in all resource definitions referencing that template.
Resource templates have a syntax similar to that of primitives.
Once you define a resource template, you can use it in primitives by specifying the
template property.
In the example above, the new primitive vm1 will inherit everything from vm-template. For
example, the equivalent of the above two examples would be:
If you want to overwrite some attributes or operations, add them to the particular primitive’s definition.
In the example above, the new primitive vm2 has special attribute values.
Its monitor operation has a longer timeout and interval, and
the primitive has an additional stop operation.
To see the resulting definition of a resource, run:
# crm_resource --query-xml --resource vm2
To see the raw definition of a resource in the CIB, run:
# crm_resource --query-xml-raw --resource vm2
14.1.2. Using Templates in Constraints
A resource template can be referenced in the following types of constraints:
orderconstraints (see Specifying the Order in which Resources Should Start/Stop)colocationconstraints (see Placing Resources Relative to other Resources)rsc_ticketconstraints (for multi-site clusters as described in Configuring Ticket Dependencies)
Resource templates referenced in constraints stand for all primitives which are derived from that template. This means, the constraint applies to all primitive resources referencing the resource template. Referencing resource templates in constraints is an alternative to resource sets and can simplify the cluster configuration considerably.
For example, given the example templates earlier in this chapter:
<rsc_colocation id="vm-template-colo-base-rsc" rsc="vm-template" rsc-role="Started" with-rsc="base-rsc" score="INFINITY"/>
would colocate all VMs with base-rsc and is the equivalent of the following constraint configuration:
<rsc_colocation id="vm-colo-base-rsc" score="INFINITY">
<resource_set id="vm-colo-base-rsc-0" sequential="false" role="Started">
<resource_ref id="vm1"/>
<resource_ref id="vm2"/>
</resource_set>
<resource_set id="vm-colo-base-rsc-1">
<resource_ref id="base-rsc"/>
</resource_set>
</rsc_colocation>
Note
In a colocation constraint, only one template may be referenced from either
rsc or with-rsc; the other reference must be a regular resource.
14.1.3. Using Templates in Resource Sets
Resource templates can also be referenced in resource sets.
For example, given the example templates earlier in this section, then:
<rsc_order id="order1" score="INFINITY">
<resource_set id="order1-0">
<resource_ref id="base-rsc"/>
<resource_ref id="vm-template"/>
<resource_ref id="top-rsc"/>
</resource_set>
</rsc_order>
is the equivalent of the following constraint using a sequential resource set:
<rsc_order id="order1" score="INFINITY">
<resource_set id="order1-0">
<resource_ref id="base-rsc"/>
<resource_ref id="vm1"/>
<resource_ref id="vm2"/>
<resource_ref id="top-rsc"/>
</resource_set>
</rsc_order>
Or, if the resources referencing the template can run in parallel, then:
<rsc_order id="order2" score="INFINITY">
<resource_set id="order2-0">
<resource_ref id="base-rsc"/>
</resource_set>
<resource_set id="order2-1" sequential="false">
<resource_ref id="vm-template"/>
</resource_set>
<resource_set id="order2-2">
<resource_ref id="top-rsc"/>
</resource_set>
</rsc_order>
is the equivalent of the following constraint configuration:
<rsc_order id="order2" score="INFINITY">
<resource_set id="order2-0">
<resource_ref id="base-rsc"/>
</resource_set>
<resource_set id="order2-1" sequential="false">
<resource_ref id="vm1"/>
<resource_ref id="vm2"/>
</resource_set>
<resource_set id="order2-2">
<resource_ref id="top-rsc"/>
</resource_set>
</rsc_order>
14.2. Reusing Rules, Options and Sets of Operations
Sometimes a number of constraints need to use the same set of rules,
and resources need to set the same options and parameters. To
simplify this situation, you can refer to an existing object using an
id-ref instead of an id.
So if for one resource you have
<rsc_location id="WebServer-connectivity" rsc="Webserver">
<rule id="ping-prefer-rule" score-attribute="pingd" >
<expression id="ping-prefer" attribute="pingd" operation="defined"/>
</rule>
</rsc_location>
Then instead of duplicating the rule for all your other resources, you can instead specify:
Important
The cluster will insist that the rule exists somewhere. Attempting
to add a reference to a nonexistent id will cause a validation failure,
as will attempting to remove a rule with an id that is referenced
elsewhere.
Some rule syntax is allowed only in
certain contexts. Validation cannot ensure that the
referenced rule is allowed in the context of the rule containing id-ref,
so such errors will be caught (and logged) only after the new configuration
is accepted. It is the administrator’s reponsibility to check for these.
The same principle applies for meta_attributes and
instance_attributes as illustrated in the example below:
id-ref can similarly be used with resource_set (in any constraint type),
nvpair, and operations.
14.3. Tagging Configuration Elements
Pacemaker allows you to tag any configuration element that has an XML ID.
The main purpose of tagging is to support higher-level user interface tools; Pacemaker itself only uses tags within constraints. Therefore, what you can do with tags mostly depends on the tools you use.