MongoDB - how to make Replica set Step down truly seamless

Question

The problem is - when your Replica Set is forced to step down while your application is running, all mainstream Mongo clients will throw at least one exception per connection. This happens because their database connections are hardwired to the physical server which used to be the primary, and no longer accepts queries. So, while MongoDB architects might think that the StepDown process does not create any downtime, in reality if you handle connections according to their documentation, each step down will cause a full blown crash for at least one user, and might even create a data integrity issue. I hope, this can be avoided with a simple wrapper that captures some specific Mongo exceptions and handles them by automatically re-connecting to the Replica Set, and re-running the failed query. If you already have a solution for this, please share! I am particularly interested in a solution that works with any major Mongo driver for Node.JS.

Answer 1

You are correct -- this is the exact behavior I experienced with both mainstream ODMs as well as the official native MongoDB driver for Node.js.

Replica set step-downs would cause my outstanding queries to fail with "Could not locate any valid servers in initial seed list", "sockets closed", and "ECONNRESET" before additional queries would get buffered up even though bufferMaxEntries is correctly configured.

Therefore, I developed Monkster to provide seamless replica set step-down and overall high-availability for MongoDB clusters for Node.js developers using the popular Monk ODM.

Monkster is a Node.js package that provides high availability for Monk, the wise MongoDB API. It implements smart error handling and retry logic to handle temporary network connectivity issues and replica set step-downs seamlessly.

https://www.npmjs.com/package/monkster