MongodbCursorsTimeouts

MongodbCursorsTimeouts #

Too many cursors are timing out

Alert Rule
alert: MongodbCursorsTimeouts
annotations:
  description: |-
    Too many cursors are timing out
      VALUE = {{ $value }}
      LABELS = {{ $labels }}    
  runbook: https://srerun.github.io/prometheus-alerts/runbooks/dcu-mongodb-exporter/mongodbcursorstimeouts/
  summary: MongoDB cursors timeouts (instance {{ $labels.instance }})
expr: increase(mongodb_metrics_cursor_timed_out_total[1m]) > 100
for: 2m
labels:
  severity: warning

Here is a runbook for the Prometheus alert rule MongodbCursorsTimeouts:

Meaning #

This alert is triggered when the number of MongoDB cursor timeouts exceeds 100 in a 1-minute interval, indicating that MongoDB is experiencing performance issues.

Impact #

  • Slow query performance: Cursor timeouts can lead to slow query performance, which can impact application responsiveness and user experience.
  • Data inconsistencies: In extreme cases, cursor timeouts can cause data inconsistencies or even data loss.
  • System instability: Prolonged cursor timeouts can lead to system instability, as resources are wasted on waiting for unresponsive queries.

Diagnosis #

To diagnose the issue, follow these steps:

  1. Check MongoDB logs: Review MongoDB logs for any error messages related to cursor timeouts, slow queries, or other performance issues.
  2. Analyze query performance: Use tools like MongoDB’s built-in explain method or third-party tools like MongoDB Compass or MongoDB Atlas to analyze query performance and identify slow queries.
  3. Verify cursor configuration: Check the cursor configuration to ensure it is set up correctly and not causing unnecessary timeouts.
  4. Check system resources: Verify that the system has sufficient resources (CPU, memory, disk space) to handle the workload.

Mitigation #

To mitigate the issue, follow these steps:

  1. Optimize slow queries: Identify and optimize slow queries to reduce their execution time and prevent cursor timeouts.
  2. Adjust cursor configuration: Adjust the cursor configuration to allow for more time to execute or to increase the batch size to reduce the number of cursors.
  3. Scale MongoDB resources: Scale up MongoDB resources (e.g., increase instance size, add more nodes) to handle the workload.
  4. Implement query retries: Implement query retries to prevent data inconsistencies in case of timeouts.

Note: Refer to the MongoDB documentation and your organization’s best practices for specific guidance on resolving cursor timeouts.

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