NotificationControllerSample

@RestController
@RequestMapping("/notifications")
public class NotificationControllerSample implements MessageListener {
  private static final long TIMEOUT = 360 * 60 * 1000;//6 hours
  private final Multimap<String, DeferredResult<ResponseEntity<ApolloConfigNotification>>>
      deferredResults = Multimaps.synchronizedSetMultimap(HashMultimap.create());
  private static final ResponseEntity<ApolloConfigNotification>
      NOT_MODIFIED_RESPONSE = new ResponseEntity<>(HttpStatus.NOT_MODIFIED);

  @RequestMapping(method = RequestMethod.GET)
  public DeferredResult<ResponseEntity<ApolloConfigNotification>> pollNotification(
      @RequestParam(value = "appId") String appId,
      @RequestParam(value = "cluster") String cluster,
      @RequestParam(value = "namespace", defaultValue = ConfigConsts.NAMESPACE_DEFAULT) String namespace,
      @RequestParam(value = "dataCenter", required = false) {
    //calcuate watched keys, will load from db in some cases
    List<String> watchedKeys = findWatchedKeys(appId, namespace, dataCenter);
    
    DeferredResult<ResponseEntity<ApolloConfigNotification>> deferredResult =
        new DeferredResult<>(TIMEOUT, NOT_MODIFIED_RESPONSE);

    //register all keys
    for (String key : watchedKeys) {
      this.deferredResults.put(key, deferredResult);
    }

    deferredResult.onCompletion(() -> {
      //unregister all keys
      for (String key : watchedKeys) {
        deferredResults.remove(key, deferredResult);
      }
    });

    return deferredResult;
  }

  private List<String> findWatchedKeys(String applicationId, String namespace,
                                                String dataCenter) {
    List<String> watchedKeys = Lists.newArrayList();
    //do more logic, may need to load from db in some cases
    return watchedKeys;
  }

  @Override
  public void handleMessage(String key, String channel) {
    ResponseEntity<ApolloConfigNotification> notification =
        new ResponseEntity<>(new ApolloConfigNotification(key, HttpStatus.OK);

    //create a new list to avoid ConcurrentModificationException
    List<DeferredResult<ResponseEntity<ApolloConfigNotification>>> results =
        Lists.newArrayList(deferredResults.get(key));

    for (DeferredResult<ResponseEntity<ApolloConfigNotification>> result : results) {
      result.setResult(notification);
    }
  }
}