Task Subscription API

The Task Subscription API allows you to subscribe to different tasks. The process-engine-api adapter implementation then takes care of the delivery of the tasks matching the intended subscription.

Delivery semantics

A task subscription delivers the tasks that are available in the process engine and match the subscription. When you create a new subscription, the adapter delivers already active matching tasks and then continues to deliver matching task changes while the subscription is active. A subscription is therefore not limited to future task events.

If the application creates a new subscription, still-active tasks can be delivered again even if the previous application instance already processed them. Consumers that send notifications, forward tasks, or synchronize tasks to another system must handle repeated deliveries idempotently. Persist enough processing state outside the application process, for example a last synchronization timestamp or a per-task processing marker. Compare this state with the adapter-provided created date or updated date from TaskInformation.meta, and skip deliveries that were already handled.

@Component
@Slf4J
@RequiredArgsConstructor
public class TaskSubscriber {

  private final TaskSubscriptionApi taskSubscriptionApi;
  private final Map<TaskInformation, Map<String, ?>> userTasks = new ConcurrentHashMap<>();


  @SneakyThrows
  public void subscribe() {
    taskSubscriptionApi.subscribeForTask(
      new SubscribeForTaskCmd(
        CommonRestrictions.builder().build(),
        TaskType.USER, // user tasks only
        null, // all of them
        null, // all variables
        (taskInfo, variables) -> {
          if (!userTasks.containsKey(taskInfo)) { // don't replace existing tasks
            log.info("[TASK LIST]: Received user task {} with meta {}", taskInfo.getTaskId(), taskInfo.getMeta());
            userTasks.put(taskInfo, variables);
          }
        },
        (taskId) -> {
          log.info("[TASK LIST]: Removing task {}", taskId);
          userTasks.keySet().stream().filter(info -> info.getTaskId().equals(taskId)).findFirst().ifPresent(userTasks::remove);
        }
      )
    ).get();
  }
}