Product SiteDocumentation Site

8.4. Using Rules to Determine Resource Location

A location constraint may contain rules. When the constraint’s outermost rule evaluates to false, the cluster treats the constraint as if it were not there. When the rule evaluates to true, the node’s preference for running the resource is updated with the score associated with the rule.
If this sounds familiar, it is because you have been using a simplified syntax for location constraint rules already. Consider the following location constraint:

Example 8.8. Prevent myApacheRsc from running on c001n03

<rsc_location id="dont-run-apache-on-c001n03" rsc="myApacheRsc"
              score="-INFINITY" node="c001n03"/>

This constraint can be more verbosely written as:

Example 8.9. Prevent myApacheRsc from running on c001n03 - expanded version

<rsc_location id="dont-run-apache-on-c001n03" rsc="myApacheRsc">
    <rule id="dont-run-apache-rule" score="-INFINITY">
      <expression id="dont-run-apache-expr" attribute="#uname"
        operation="eq" value="c00n03"/>

The advantage of using the expanded form is that one can then add extra clauses to the rule, such as limiting the rule such that it only applies during certain times of the day or days of the week.

8.4.1. Location Rules Based on Other Node Properties

The expanded form allows us to match on node properties other than its name. If we rated each machine’s CPU power such that the cluster had the following nodes section:

Example 8.10. A sample nodes section for use with score-attribute

   <node id="uuid1" uname="c001n01" type="normal">
      <instance_attributes id="uuid1-custom_attrs">
        <nvpair id="uuid1-cpu_mips" name="cpu_mips" value="1234"/>
   <node id="uuid2" uname="c001n02" type="normal">
      <instance_attributes id="uuid2-custom_attrs">
        <nvpair id="uuid2-cpu_mips" name="cpu_mips" value="5678"/>

then we could prevent resources from running on underpowered machines with this rule:
<rule id="need-more-power-rule" score="-INFINITY">
   <expression id="need-more-power-expr" attribute="cpu_mips"
               operation="lt" value="3000"/>