code: public void shutdown() {
          logger.info("Closing Retrieval Manager");
          workerPool.shutdown();
  
          try {
              workerPool.awaitTermination(30, TimeUnit.MINUTES);
          } catch (InterruptedException e) {
              // Exit wait on interruption
          }
  
          complete = true;
      }
docs: Stops the retrieval, no further files will be retrieved after those
  which are in progress have completed.