public Set<String> getParallelGatewaySenderIds() {
    Set<String> regionGatewaySenderIds = getAllGatewaySenderIds();
    if (regionGatewaySenderIds.isEmpty()) {
      return Collections.emptySet();
    }
    Set<GatewaySender> cacheGatewaySenders = getCache().getAllGatewaySenders();
    Set<String> parallelGatewaySenderIds = new HashSet<>();
    for (GatewaySender sender : cacheGatewaySenders) {
      if (regionGatewaySenderIds.contains(sender.getId()) && sender.isParallel()) {
        parallelGatewaySenderIds.add(sender.getId());
      }
    }
    return parallelGatewaySenderIds;
  }
Monitors when other members that participate in this persistent region are removed and creates
a log entry marking the event.
geode-core/src/main/java/org/apache/geode/internal/cache/PartitionedRegion.java