title: GCP Pub/Sub ack for multiple subscribers
body: <h2><strong>Background</strong>:</h2>
  
  <p>I have a publisher with 2 subscribers on GCP pub/sub, both the subscribers run a NodeJS code &amp; acknowledge once the process is done.</p>
  
  <h2><strong>Issues:</strong></h2>
  
  <ol>
  <li>How can I know which subscriber has ack the message?</li>
  <li>Say I have only 1 subscriber and I have sent 10 messages at once (in parallel) and 2 have failed, how can I know which 2 messages were failed? Is there any mapping between ackId and message sent on GCP pub/sub?</li>
  </ol>