Checking replication lag of nodes in a replica set

The most ideal state for a replica set is when all nodes within the cluster are in sync. In this recipe, we will look at how to check the replication lag of nodes in a replica set.

Getting ready

We will need at least a multinode replica set cluster. You can refer to the Adding a node in a replica set recipe in Chapter 4, High Availability with Replication, for more details.

How to do it...

1. Ensure all nodes are up and connect to the primary node of the replica set using mongo shell.
2. Fetch the current replication information:

rs.printReplicationInfo()

3. You should see output similar to this:

configured oplog size: 1578.62548828125MB
log length start to end: 142363secs (39.55hrs)
oplog first event time: Sun Oct 08 2017 18:54:12 GMT+0530 (IST)
oplog last event time: Tue Oct 10 2017 10:26:55 GMT+0530 (IST)
now: Tue Oct 10 2017 10:26:59 GMT+0530 (IST)

4. Check the current replication information:

rs.printSlaveReplicationInfo()

5. You should see output similar to...