MongodbReplicationLag

MongodbReplicationLag #

Mongodb replication lag is more than 10s

Alert Rule
alert: MongodbReplicationLag
annotations:
  description: |-
    Mongodb replication lag is more than 10s
      VALUE = {{ $value }}
      LABELS = {{ $labels }}    
  runbook: https://srerun.github.io/prometheus-alerts/runbooks/dcu-mongodb-exporter/mongodbreplicationlag/
  summary: MongoDB replication lag (instance {{ $labels.instance }})
expr: avg(mongodb_replset_member_optime_date{state="PRIMARY"}) - avg(mongodb_replset_member_optime_date{state="SECONDARY"})
  > 10
for: 0m
labels:
  severity: critical

Meaning #

The MongodbReplicationLag alert is triggered when the average replication lag between the primary and secondary nodes in a MongoDB replica set exceeds 10 seconds. This lag can indicate issues with data replication, network connectivity, or node performance, which can lead to data inconsistencies and availability problems.

Impact #

A prolonged MongoDB replication lag can have significant impacts on the availability and integrity of the data:

  • Data inconsistencies: If the lag persists, it can lead to divergent data sets between nodes, which can cause errors, data loss, or inconsistencies.
  • Data availability: In the event of a node failure, a significant replication lag can delay the recovery process, leading to extended downtime and data unavailability.
  • Performance degradation: Replication lag can cause increased latency, reduced throughput, and decreased system performance.

Diagnosis #

To diagnose the root cause of the MongodbReplicationLag alert, follow these steps:

  1. Check the MongoDB replica set status using rs.status() or MongoDB Atlas/Cloud Manager.
  2. Verify network connectivity and latency between nodes using tools like ping or mtr.
  3. Investigate node performance, including CPU, memory, and disk usage, using tools like top or htop.
  4. Review MongoDB logs for errors or warnings related to replication.
  5. Check the output of mongodb_replset_member_optime_date metrics to identify the specific nodes experiencing lag.

Mitigation #

To mitigate the MongodbReplicationLag alert, follow these steps:

  1. Investigate and address any network connectivity issues between nodes.
  2. Optimize node performance by adjusting resource allocation, tuning MongoDB configuration, or upgrading hardware.
  3. Check and fix any MongoDB configuration issues, such as incorrect replica set configuration or outdated MongoDB versions.
  4. Consider increasing the syncSource timeout to allow for more time for replication to catch up.
  5. If the lag persists, consider reconfiguring the replica set or adding more nodes to improve replication performance.

Remember to consult the MongoDB documentation and expert resources for further guidance on troubleshooting and optimizing MongoDB replication performance.

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