public Set<InternalDistributedMember> getDepartedMembers() {
    synchronized (this) {
      if (departedMembers == null) {
        return Collections.emptySet();
      } else {
        return departedMembers;
      }
    }
  }
Returns the recipients that have departed prior to processing a reply from them.
geode-core/src/main/java/org/apache/geode/distributed/internal/ReliableReplyProcessor21.java