code: public Broadcastable openChannel(String ccid, String atmosphereTrackingId) {
  
          throwExceptionIfTrackingIdnotSet(atmosphereTrackingId);
  
          log.debug("GET Channels open long poll channelId: {} trackingId: {}", ccid, atmosphereTrackingId);
          // NOTE: as of Atmosphere 0.8.5: even though the parameter is set
          // not to create the broadcaster if not
          // found, if the
          // broadcaster is found, but set to "destroyed" then it is recreated
          // TODO when is a broadcaster "destroyed" ???
          Broadcaster broadcaster = BroadcasterFactory.getDefault().lookup(BounceProxyBroadcaster.class, ccid, false);
          if (broadcaster == null) {
              log.error("no broadcaster registered for channel {}", ccid);
              // broadcaster not found for given ccid
              throw new JoynrHttpException(Status.BAD_REQUEST, JOYNRMESSAGINGERROR_CHANNELNOTFOUND);
          }
  
          // this causes the long poll, or immediate response if elements are
          // in the cache
          return new Broadcastable(broadcaster);
      }
docs: /*
  Opens a channel for long polling.
  
  @param ccid
  the ID of the channel
  @param atmosphereTrackingId
  the tracking ID
  @return
  @throws JoynrHttpException
  if no channel with the given ID was found (e.g. because it
  wasn't created before) or if the tracking ID wasn't set