HP Serviceguard manual Extended Dependencies

In a simple dependency, if pkg1 depends on pkg2, and pkg1’s priority is higher than pkg2’s, pkg1’s node order dominates. Assuming pkg1’s node order is node1, node2, node3, then:

•On startup:

◦pkg1 will select node1 to start on, provided pkg2 can run there.

◦pkg2 will start on node1, provided it can run there (no matter where node1 appears on pkg2’s node_name list).

–If pkg2 is already running on another node, it will be dragged to node1, provided it can run there.

◦If pkg2 cannot start on node1, then both packages will attempt to start on node2 (and so on).

Note that the nodes will be tried in the order of pkg1’s node_name list, and pkg2 will be dragged to the first suitable node on that list whether or not it is currently running on another node.

•On failover:

◦If pkg1 fails on node1, pkg1 will select node2 to fail over to (or node3 if it can run there and node2 is not available or does not meet all of its dependencies; etc.)

◦pkg2 will be dragged to whatever node pkg1 has selected, and restart there; then pkg1 will restart there.

•On failback:

◦If both packages have moved to node2 and node1 becomes available, pkg1 will fail back to node1 if both packages can run there;

– otherwise, neither package will fail back.

Guidelines for Simple Dependencies

As you can see from the “Dragging Rules for Simple Dependencies” (page 139), if pkg1 depends on pkg2, it can sometimes be a good idea to assign a higher priority to pkg1, because that provides the best chance for a successful failover (and failback) if pkg1 fails.

But you also need to weigh the relative importance of the packages. If pkg2 runs a database that is central to your business, you probably want it to run undisturbed, no matter what happens to application packages that depend on it. In this case, the database package should have the highest priority.

Note that, if no priorities are set, the dragging rules favor a package that is depended on over a package that depends on it.

Consider assigning a higher priority to a dependent package if it is about equal in real-world importance to the package it depends on; otherwise assign the higher priority to the more important package, or let the priorities of both packages default.

You also need to think about what happens when a package fails; see “What Happens when a Package Fails” (page 143).

Extended Dependencies

To the capabilities provided by Simple Dependencies (page 137), extended dependencies add the following:

•You can specify whether the package depended on must be running or must be down.

You define this condition by means of the dependency_condition, using one of the literals UP or DOWN (the literals can be upper or lower case). We'll refer to the requirement that

Package Configuration Planning 141