6.6. Ordering Sets of Resources

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

Example 6.9. 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>

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

6.6.1. Ordered Set

To simplify this situation, resource sets (see Section 6.5, “Resource Sets”) can be used within ordering constraints:

Example 6.10. 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.