public static Distribution getDistribution(DistributedSystem sys) {
    InternalDistributedSystem isys = (InternalDistributedSystem) sys;
    ClusterDistributionManager dm = (ClusterDistributionManager) isys.getDM();
    return dm.getDistribution();
  }
returns the JGroupMembershipManager for the given distributed system
geode-dunit/src/main/java/org/apache/geode/distributed/internal/membership/api/MembershipManagerHelper.java