> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt

# Fix Replication Lag

At time `T`, the last write operation applied on the specified [secondary](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-secondary) of replica set `ABC` was behind the most recent operation applied on the [primary.](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-primary)

## Alert Conditions

You can configure alert conditions in the project-level [alert settings](https://www.mongodb.com/docs/cloud-manager/tutorial/manage-alert-configurations.md#std-label-configure-alerts) page to trigger alerts.

To learn more about the alert condition, see [Replication Lag is.](https://www.mongodb.com/docs/cloud-manager/reference/alerts.md#std-label-replication-oplog)

## Common Triggers

- An idle replica set. The reported replication lag is actually just the time since the last write. Replication lag is calculated between the last operation time on the primary and the time of the last operation received by the secondary. If a replica set is only written to once every 10 minutes, the replication lag will be 10 minutes just after the write is made to the primary and just prior to the next write being replicated to the secondary.

- The secondary is under-provisioned, which means it needs more allocated resources, and cannot keep up with the primary (common if using secondaries for read scaling).

- There is insufficient bandwidth, or some other networking problem, between the primary and secondary.

## Fix the Immediate Problem

- Adjust the settings for this alert to only trigger if the replication lag persists for longer than 2 minutes. This will reduce the chances of a false positive.

- Resolve networking issues between the primary and secondary.

To learn more, see [Troubleshoot Replica Sets](https://www.mongodb.com/docs/manual/tutorial/troubleshoot-replica-sets.md#std-label-replica-set-troubleshooting-check-replication-status) in the MongoDB manual.

## Implement a Long-Term Solution

- Increase bandwidth between the primary and secondary.

- Move (or upgrade in place) the secondary to a machine that is identically (or better) provisioned to the current primary.

## Monitor Your Progress

View the following charts to monitor your progress:

- Network

  Monitor network metrics to track network performance.

- Replication Headroom

  Monitor replication headroom to determine whether the secondary might fall off the oplog.

- Replication Lag

  Monitor replication lag to determine whether the secondary might fall off the oplog.

To learn more, see [View Deployment Metrics.](https://www.mongodb.com/docs/cloud-manager/tutorial/view-diagnostics.md#std-label-monitor-cluster-metrics)