code: private void heartbeatWhenMaster(long now) {
          Collection<MemberImpl> members = clusterService.getMemberImpls();
          for (MemberImpl member : members) {
              if (!member.localMember()) {
                  try {
                      logIfConnectionToEndpointIsMissing(now, member);
                      if (suspectMemberIfNotHeartBeating(now, member)) {
                          continue;
                      }
  
                      pingMemberIfRequired(now, member);
                      sendHeartbeat(member);
                  } catch (Throwable e) {
                      logger.severe(e);
                  }
              }
          }
      }
docs: Sends heartbeat to each of the cluster members.
  Checks whether a member has failed to send a heartbeat in time
  (see {@link #maxNoHeartbeatMillis})
  and removes that member from the cluster.
  <p></p>
  This method is only called on the master member.
  
  @param now the current cluster clock time