@Override
    public void memberJoined(DistributionManager distributionManager,
        InternalDistributedMember id) {
      if (logger.isDebugEnabled()) {
        logger.debug("Observer for bucket {} member joined {}", bucketToMonitor, id);
      }
      synchronized (this) {
        arrivals.addAndGet(1);
        notifyAll();
      }
    }
Monitors distributed membership for a given bucket
geode-core/src/main/java/org/apache/geode/internal/cache/PRHARedundancyProvider.java