Deciding When to Trade Data Freshness for Performance in MongoDB-as-a-Service

MongoDB is a popular document store that is also available as a cloud-hosted service. MongoDB internally deploys primary-copy asynchronous replication, and it allows clients to vary the Read Preference, so reads can deliberately be directed to secondaries rather than the primary site. Doing this can sometimes improve performance, but the returned data might be stale, whereas the primary always returns the freshest data value. While state-of-practice is for programmers to decide where to direct the reads at application development time, they do not have full understanding then of workload or hardware capacity. It should be better to choose the appropriate Read Preference setting at runtime, as we describe in this paper.We show how a system can detect when the primary copy is saturated in MongoDB-as-a-Service, and use this to choose where reads should be done to improve overall performance. Our approach is aimed at a cloud-consumer; it assumes access to only the limited diagnostic data provided to clients of the hosted service.