5.6. Ordering Sets of Resources

⁠5.6. Ordering Sets of Resources

A common situation is for an administrator to create a chain of ordered resources, such as:

Example 5.9. A chain of ordered resources

<constraints>
    <rsc_order id="order-1" first="A" then="B" />
    <rsc_order id="order-2" first="B" then="C" />
    <rsc_order id="order-3" first="C" then="D" />
</constraints>

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Figure 5.1. Visual representation of the four resources' start order for the above constraints

⁠5.6.1. Ordered Set

To simplify this situation, resource sets (see Section 5.5, “Resource Sets”) can be used within ordering constraints:

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Example 5.10. 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>

While the set-based format is not less verbose, it is significantly easier to get right and maintain.

Important

If you use a higher-level tool, pay attention to how it exposes this functionality. Depending on the tool, creating a set A B may be equivalent to A then B, or B then A.