Mandatory placement occurs when the constraint’s score is +INFINITY or -INFINITY. In such cases, if the constraint can’t be satisfied, then the rsc resource is not permitted to run. For score=INFINITY, this includes cases where the with-rsc resource is not active.

If you need resource A to always run on the same machine as resource B, you would add the following constraint:

<rsc_colocation id="colocate" rsc="A" with-rsc="B" score="INFINITY"/>

Remember, because INFINITY was used, if B can’t run on any of the cluster nodes (for whatever reason) then A will not be allowed to run. Whether A is running or not has no effect on B.

Alternatively, you may want the opposite — that A cannot run on the same machine as B. In this case, use score="-INFINITY".

<rsc_colocation id="anti-colocate" rsc="A" with-rsc="B" score="-INFINITY"/>

Again, by specifying -INFINITY, the constraint is binding. So if the only place left to run is where B already is, then A may not run anywhere.

As with INFINITY, B can run even if A is stopped. However, in this case A also can run if B is stopped, because it still meets the constraint of A and B not running on the same node.