Sitecore xDB: Performance Tuning your MongoDB Driver Configuration Settings

The Goal

Working with my team on a high-traffic client's Sitecore Commerce site, we were tasked with improving MongoDB connection management on the Content Delivery servers to help alleviate pressure on the servers and MongoDB, particularly during busy times of the day, and at times when traffic surges occur due to marketing campaigns or other real-world events.

The Key Settings

We confirmed that Sitecore ships with the default MongoDB driver settings that are actually set in the driver code.  You can view the default settings in the driver code, by following this GitHub link:
https://github.com/mongodb/mongo-csharp-driver/blob/v2.0.x/src/MongoDB.Driver/MongoDefaults.cs

Working with mLab Support, we determined that our focus would be on the following:

Min Pool Size

We decided to increase the Min Pool Size from the default 0 to 20. The mLab team approved this suggestion on the basis that we observed the Content Delivery server's connection pools maxing out due to the amount of operations that were happening during the Sitecore startup process.

Max Pool Size

We increased our Max Pool Size from the default of 100 to 150 in order to better accommodate surges in connection demand. The purpose of this update was to lessen the chance of running out of connections altogether.

Connection Idle Time

We increased the Connection Idle Time from the default of 10 minutes to 25 minutes to reduce the need to create new connections during normal and high-traffic surges.

Connection Life Time

We dropped the default setting of 30 minutes down to 0 (no lifetime). This change was based on the default setting that could also be a contributing factor to our observed connection churning.

Per this thread, a MongoDB engineer (driver author) suggested that this setting is likely not needed:
https://stackoverflow.com/questions/32816076/what-is-the-purpose-of-the-maxconnectionlifetime-setting

The How

As Kam explains in his post, Sitecore exposes an empty updateMongoDriverSettings pipeline that you can hook into to modify configurations that are not available in the connection string.

I created a processor to add to this pipeline that alters the MongoClientSettings:

Finally, I added the following patch to add the processor to pipeline, allowing us to pass the updated MongoDB driver settings to the custom processor:

Final Note

A special thanks to Dan Read (Arke), Alex Mayle (Sogeti) and the mLab Support Team for their contributions.