title: What does signal 15 mean
body: <p>I have this error after deploying my fastapi app on heroku. it say app terminated due to signal 15. Can someone help me out.</p>
  <pre><code>2021-12-03T12:17:35.623624+00:00 app[web.1]: [2021-12-03 12:17:35 +0000] [11] [INFO] Worker exiting (pid: 11)
  2021-12-03T12:17:35.623625+00:00 app[web.1]: [2021-12-03 12:17:35 +0000] [12] [INFO] Worker exiting (pid: 12)
  2021-12-03T12:17:36.000000+00:00 app[api]: Build succeeded
  2021-12-03T12:17:36.626062+00:00 app[web.1]: [2021-12-03 12:17:36 +0000] [4] [WARNING] Worker with pid 10 was terminated due to signal 15
  2021-12-03T12:17:36.629487+00:00 app[web.1]: [2021-12-03 12:17:36 +0000] [4] [WARNING] Worker with pid 9 was terminated due to signal 15
  2021-12-03T12:17:36.643503+00:00 app[web.1]: [2021-12-03 12:17:36 +0000] [4] [WARNING] Worker with pid 11 was terminated due to signal 15
  2021-12-03T12:17:36.715299+00:00 app[web.1]: [2021-12-03 12:17:36 +0000] [4] [INFO] Shutting down: Master
  2021-12-03T12:17:36.715450+00:00 app[web.1]: [2021-12-03 12:17:36 +0000] [4] [INFO] Reason: Worker failed to boot.
  2021-12-03T12:17:36.865607+00:00 heroku[web.1]: Process exited with status 3
  2021-12-03T12:17:37.090747+00:00 heroku[web.1]: State changed from up to crashed
  </code></pre>