Non-technical Description

An edge case sequence of events meant that the synchronisation of data for 'Person' ran on both load-balanced servers in the overnight jobs, creating duplicates. This is meant to be prevented by a locking mechanism to ensure the jobs only run on one of the servers. The team are investigating what contributed to the edge case scenario occurring, in order to mitigate against it reoccurring, and strengthening the logic governing the locking mechanism.

Trigger

• Teams notification

• 

  Slack notification

  

Detection

• A user reported in Teams Support Channel. The issue was also raised in the TIS tis-dev-team Slack channel.

• The overlapping jobs could be viewed in the server logs
  

  

  
  and also in the monitoring-prod Slack channel (started 1:29 AM and 1:33 AM):

  

Resolution

• TIS Team manually re-ran the Person Sync job from the Sync administration panel: https://apps.tis.nhs.uk/sync/

Timeline

• 04 Feb 2021 00:21 - Out of Memory Error on HEE-TIS-VM-PROD-APPS-BLUE

• 04 Feb 2021 01:29 - PersonElasticSearchSyncJob : Sync [Person sync job] started on HEE-TIS-VM-PROD-APPS-GREEN

• 04 Feb 2021 01:33 - PersonElasticSearchSyncJob : Sync [Person sync job] started on HEE-TIS-VM-PROD-APPS-BLUE

• 04 Feb 2021 08:21 - Notification on Teams

• 04 Feb 2021 09:25 - Job run again

Root Cause(s)

• Job ran in parallel, one on each of the servers.

• The ‘locking’ to prevent this only takes account of scheduled runs

• Container restarted in the ~10 minute window where this problem would occur

• This was triggered by an OutOfMemoryError:

  2021-02-04 00:21:19.677  INFO 1 --- [onPool-worker-2] s.j.PersonPlacementEmployingBodyTrustJob : Querying with lastPersonId: [49403] and lastEmployingBodyId: [287]
  2021-02-04 00:21:27.760  INFO 1 --- [onPool-worker-2] u.n.t.s.job.TrustAdminSyncJobTemplate    : Time taken to read chunk : [12.95 s]
  java.lang.OutOfMemoryError: Java heap space
  Dumping heap to /var/log/apps/hprof/sync-2021-01-19-11:29:40.hprof ...
  Unable to create /var/log/apps/hprof/sync-2021-01-19-11:29:40.hprof: File exists
  Terminating due to java.lang.OutOfMemoryError: Java heap space

Action Items

Lessons Learned

• The ‘locking’ to prevent the job running in parallel only takes account of scheduled runs. Any container restarts or manually running the job can cause duplication if it overlaps with the job running on the other server instance.

• We need a more robust solution for preventing duplication of jobs running.