MongoDB replication - oplogSize

Background An oplog is a write operation log that MongoDB uses to store data modifications that need to be replicated out to other nodes in the configured set. This oplog is a capped collection which means it will never grow in size beyond a certain point - once it reaches it’s max size, old data drops off the end as new data is added so it keeps cycling round. The size of the oplog basically determines how long a secondary node can be down for and still be able to catch up when it comes back online. [Read More]