Managing Unresolved Pending Tasks in Cassandra’s Nodetool CompactionStats

Problem:

During a diagnostic call, it was observed that nodetool compactionstats was displaying 3,493 pending tasks related to the skyrt_prod1.event_state table. Despite new tasks being initiated and processed, these existing pending tasks remained uncleared, with no tasks currently in progress. This issue raised concerns about potential underlying problems with Cassandra’s compaction process.

Solution:

The accumulation of pending tasks is often linked to issues with stale data in Cassandra’s tables. To address this, the following approach was recommended:

1. Repair and Cleanup: Conduct thorough repairs and cleanups across all nodes to resolve potential data stability issues. This step is crucial to ensure that the system can effectively process and clear the backlog of pending tasks.
2. Monitoring: Temporarily postpone further actions until all nodes have undergone the repair process. During this period, it’s essential to monitor the servers using nodetool tpstats, collecting output data regularly to assess whether the pending tasks are being processed.
3. Review and Final Actions: Once the repair process is complete, review the status of the pending tasks. In an optimal scenario, all previous tasks should be resolved by this stage. If stale pending tasks persist, additional steps will be taken to address them.

Conclusion:

By implementing the recommended repair and monitoring strategy, the pending tasks issue in nodetool compactionstats is expected to be resolved. This proactive approach not only aims to clear the existing backlog but also ensures that the system remains stable and efficient, preventing similar issues from arising in the future. Following the full execution of the repair process, it is anticipated that the pending tasks will no longer pose a problem.