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Move fast break things but with monitoring

We run a complex system with multiple services and every 2 or 3 week we  update the Java applications.  I want to do it every week as most applications are stateless and can be patched anytime but the application serving the main website is using sticky session. We are working to make it failover sessions, once we do that, we can do mid week deployment and that will allow us to go faster than 3 weeks.  This week I pushed a huge infrastructure change related to user Id generation. I had asked ops team to check the status of new relic after the midnight deployment and it looked like this so everyone was happy.


I woke up and checked new relic mobile app and things looked ok to me. After finishing my morning routines I ran my daily exception report and one thing that caught the eye was 90K exceptions in last 12 hours in one of the files I had changed.  To gauge the impact I went in new relic and it showed me an error rate of 0.07 in one of the app

I then checked new relic and I see this blip that caused 90k errors, when the blip was there the status must have been red but then it was quickly green and Ops team didnt caught it
 


The issue was due to a wrong method invocation in one of the class used by this one specific application and it took just 15 min to fix after reproducing with a testcase.  So why didnt QA/UAT/automated tests caught it, well the issue was like a heisen bug and would occur only when the object is missing in cache. 99% of the calls would go to cache and only 0.07% were going to dao layer that had the bug.  I quickly made a patch and ops deployed it and I can see the issue is now gone.

Had there been no monitoring it would have been difficult to catch these kinds of bugs. New relic and internal monitoring tools makes life easy as it exposes anomalies.

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