docs: public long openSession(long index, long timestamp, RaftSession session) {
      log.debug("Opening session {}", session.sessionId());
  
      // Update the state machine index/timestamp.
      tick(index, timestamp);
  
      // Set the session timestamp to the current service timestamp.
      session.setLastUpdated(currentTimestamp);
  
      // Expire sessions that have timed out.
      expireSessions(currentTimestamp);
  
      // Add the session to the sessions list.
      session.open();
      service.register(sessions.addSession(session));
  
      // Commit the index, causing events to be sent to clients if necessary.
      commit();
  
      // Complete the future.
      return session.sessionId().id();
    }