Non-technical Description

The overnight job to synchronize connected doctors from the GMC into our system normally runs at midnight until the early morning. Today it did not run until about 10am, which means the various doctor lists in the revalidation app were not updated.

This occurred because our messaging system failed, so the relevant part of our system did not get the message to start the synchronization until far later than it should have

Trigger

A large volume of messages in RabbitMQ caused an our of memory error, preventing publishers from posting messages to their queues.

Detection

Some DLQ monitoring alerts were flagged in the morning, and a developer spotted the lack of doctors on the revalidation application

Resolution

The sync process had already started up again on its own, so there was no need to actively change anything, although one obsolete queue was purged of a high volume of messages and one service had to be restarted as it had lost connection with RabbitMQ

Timeline

All times in BST unless indicated

• 22 Oct 2023: 23:56 Restarting Consumer logs appear from recommendation service

• 22 Oct 2023: 23:56 Suspended all listeners and will no longer accept client connections logs appear from AWS RabbitMQ broker

• 23 Oct 2023: 00:01memory resource limit alarm set on node and Publishers will be blocked until this alarm clears logs appear from AWS RabbitMQ broker

• 23 Oct 2023: 00:05 Start message has been sent to start gmc sync log appears in recommendation service, but this message is not delivered until much later

• 23 Oct 2023: 09:44-ish Messages shovelled from ESR DLQ, possibly freeing up a little space?

• 23 Oct 2023: 09:52 GMC Client Service receives a message to start the doctor sync, and the sync begins.4

• 23 Oct 2023: 11:00-ish reval.queue.gmcsync.connection purged of 1.6 million messages

Root Cause(s)

5 Whys

1. Why didn’t the GMC Overnight Sync Job Start on time? - Because RabbitMQ ran out of memory

2. Why did RabbitMQ run out of memory? - Because of millions of messages in the following queues: esr.queue.audit.neo, tis.queue.cdc.created, reval.queue.gmcsync.connection

3. Why were there millions of messages in these queues? - Why so many? Is this normal? Why was it taking so long to process them?

4. Why

5. Why

Causes

• RabbitMQ ran out of memory

• esr.queue.audit.neo and tis.queue.cdc.created had millions of messages (recorded at about 10:30 am) (Active queues)

• reval.queue.gmcsync.connection had 1.6 million messages (Obsolete unused queue)

Action Items

Lessons Learned

• Don’t leave obsolete queues lying around - especially when they’re still being published to