6.6. Ordered Set

Figure 6.1. Visual representation of the four resources' start order for the above constraints

To simplify this situation, there is an alternate format for ordering constraints:

Example 6.6. A chain of ordered resources expressed as a set

<constraints>
    <rsc_order id="order-1">
      <resource_set id="ordered-set-example" sequential="true">
        <resource_ref id="A"/>
        <resource_ref id="B"/>
        <resource_ref id="C"/>
        <resource_ref id="D"/>
      </resource_set>
    </rsc_order>
</constraints>

Note

Resource sets have the same ordering semantics as groups.

Example 6.7. A group resource with the equivalent ordering rules

<group id="dummy">
    <primitive id="A" .../>
    <primitive id="B" .../>
    <primitive id="C" .../>
    <primitive id="D" .../>
</group>

While the set-based format is not less verbose, it is significantly easier to get right and maintain. It can also be expanded to allow ordered sets of (un)ordered resources. In the example below, rscA and rscB can both start in parallel, as can rscC and rscD, however rscC and rscD can only start once both rscA and rscB are active.

Example 6.8. Ordered sets of unordered resources

<constraints>
    <rsc_order id="order-1">
      <resource_set id="ordered-set-1" sequential="false">
        <resource_ref id="A"/>
        <resource_ref id="B"/>
      </resource_set>
      <resource_set id="ordered-set-2" sequential="false">
        <resource_ref id="C"/>
        <resource_ref id="D"/>
      </resource_set>
    </rsc_order>
  </constraints>