private Cluster cluster(UpgradeCheckRequest prerequisiteCheck) throws AmbariException {
    return clustersProvider.get().getCluster(prerequisiteCheck.getClusterName());
  }
This checks if the source and target version has an entry for each OS type in the cluster.
ambari-server/src/main/java/org/apache/ambari/server/checks/MissingOsInRepoVersionCheck.java