Friday, 25 October 2019

New adventures in microservices - reducing inter-service calls

Not so long ago I was working on a product that internally exposed an API to allow clients to keep in sync with a user's most recently read documents.  I expect most readers of this blog will have used Amazon's Kindle or a similar online reading application so I won't have to explain any of the fundamentals of this functionality.

On the surface this "recently read" service was quite simple - just read the user's most recent records out of a specifically designed database table and present it to the client application.

A complicating factor in this particular system was that some documents belong to a group rather than an individual user, and as a result the rights to access documents could change over time.

I consider this to be a case study of when catering for an edge case can lead to unnecessary pressure on core systems.  Approximately 75% of calls to the documents service were permission checks from the recently read service.

All of the clients of the recently read service would silently ignore any reference to a document that the client did not have, so the permission checks were completely unnecessary.

After consulting with the various teams involved I removed the permission checking calls and as a result the response time of the recently read service improved, and the load on the documents service reduced significantly.  As a result the documents service was able to run with fewer instances.

This was one of the rare cases when the best way to improve the performance of a service call was to remove it altogether.

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