OpenShift and Node Affinity

In order to decide on which node given Pod should be running, OpenShift looks at so called Predicates and Priority Functions. A predicate must match the one configured in DeploymentConfiguration and Priority Function is responsible for choosing the best node for running Pods.

Let’s assume that we have a sample policy (similar to one provided in OpenShift manual), that uses site as a Predicate along with rack and machine as Priority Functions. Now let’s assume we have two nodes:

And two DeploymentConfiguration with Node Selectors (this tells OpenShift on which nodes given DeploymentConfiguration wishes to run) defined as follows:

With the above example only DeploymentConfiguration 1 will be scheduled (on Node 1), since site matches the predicate. In this case rack and machine parameters are not used (because we have only one node).

Note that the default OpenShift’s configuration uses region (as a Predicate) and zone (as a Priority Function). However it can be reconfigured very easily. 

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Some OpenShift deployments might span multiple racks in a data center or even multiple sites. It is important to tell Infinispan where physical machines are located, which will allow to choose better nodes for backing up your data (in distribution mode). 

As the matter of fact, Infinispan uses site, rack and machine. The main goal is to avoid backing up data on the same host.

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