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

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

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

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

• Oct 23, 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

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

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

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

• Oct 24, 2023: Functionality restored (overnight sync job ran successfully)

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 (is this the real reason?)

3. Why were there millions of messages in these queues? - NeoAudit containers have a habit of dropping connection to RabbitMQ, CDC rabbit router was unable to consume/publish messages from the queue

4. Why do the containers drop connection to RabbitMQ? - This requires an investigation, it’s a not uncommon issue where RabbitMQ and ECS drop connections without an obvious cause

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

• There’s no such thing as too much alerting