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8.5. Using Rules to Control Resource Options

Often some cluster nodes will be different from their peers. Sometimes, these differences — e.g. the location of a binary or the names of network interfaces — require resources to be configured differently depending on the machine they’re hosted on.
By defining multiple instance_attributes objects for the resource and adding a rule to each, we can easily handle these special cases.
In the example below, mySpecialRsc will use eth1 and port 9999 when run on node1, eth2 and port 8888 on node2 and default to eth0 and port 9999 for all other nodes.

Example 8.11. Defining different resource options based on the node name

<primitive id="mySpecialRsc" class="ocf" type="Special" provider="me">
   <instance_attributes id="special-node1" score="3">
    <rule id="node1-special-case" score="INFINITY" >
     <expression id="node1-special-case-expr" attribute="#uname"
       operation="eq" value="node1"/>
    <nvpair id="node1-interface" name="interface" value="eth1"/>
   <instance_attributes id="special-node2" score="2" >
    <rule id="node2-special-case" score="INFINITY">
     <expression id="node2-special-case-expr" attribute="#uname"
       operation="eq" value="node2"/>
    <nvpair id="node2-interface" name="interface" value="eth2"/>
    <nvpair id="node2-port" name="port" value="8888"/>
   <instance_attributes id="defaults" score="1" >
    <nvpair id="default-interface" name="interface" value="eth0"/>
    <nvpair id="default-port" name="port" value="9999"/>

The order in which instance_attributes objects are evaluated is determined by their score (highest to lowest). If not supplied, score defaults to zero, and objects with an equal score are processed in listed order. If the instance_attributes object has no rule or a rule that evaluates to true, then for any parameter the resource does not yet have a value for, the resource will use the parameter values defined by the instance_attributes.
For example, given the configuration above, if the resource is placed on node1:
  1. special-node1 has the highest score (3) and so is evaluated first; its rule evaluates to true, so interface is set to eth1.
  2. special-node2 is evaluated next with score 2, but its rule evaluates to false, so it is ignored.
  3. defaults is evaluated last with score 1, and has no rule, so its values are examined; interface is already defined, so the value here is not used, but port is not yet defined, so port is set to 9999.