Elasticsearch ShardLockObtainFailedException: Shard lock obtain failed

Brief Explanation

The ShardLockObtainFailedException: Shard lock obtain failed error occurs in Elasticsearch when a node attempts to acquire a lock on a shard but fails to do so. This typically happens during index recovery or when multiple nodes are trying to access the same shard simultaneously.

Impact

This error can prevent shards from being allocated or recovered, leading to incomplete or unavailable indices. It may result in reduced search performance, data inconsistencies, or partial data unavailability until resolved.

Common Causes

Concurrent operations on the same shard
Network issues between nodes
Disk I/O problems
Insufficient disk space
Corrupted shard data
Misconfigured cluster settings

Troubleshooting and Resolution Steps

Check cluster health:
```
GET _cluster/health
```
Identify the affected index and shard:
```
GET _cat/shards?v
```
Verify node status and connectivity:
```
GET _cat/nodes?v
```
Check disk space on all nodes:
```
GET _cat/allocation?v
```
Review Elasticsearch logs for detailed error messages.
If the issue persists, try restarting the affected node.

If the problem continues, consider reallocating the shard:

POST /_cluster/reroute
{
  "commands": [
    {
      "allocate_empty_primary": {
        "index": "affected_index",
        "shard": 0,
        "node": "target_node_name",
        "accept_data_loss": true
      }
    }
  ]
}

If all else fails, you may need to rebuild the index from a snapshot or primary source.

Best Practices

Ensure adequate disk space on all nodes.
Regularly monitor cluster health and shard allocation.
Implement proper backup and snapshot strategies.
Use rolling restarts when updating or maintaining nodes.
Configure appropriate shard allocation settings to prevent overloading single nodes.

Frequently Asked Questions

Q: Can this error occur during normal cluster operations?
A: While it's not common during normal operations, it can occur during high-concurrency situations or when there are underlying issues with node communication or disk I/O.

Q: How does this error affect my data integrity?
A: The error itself doesn't cause data loss, but it can prevent access to the affected shard until resolved. Ensure you have proper backups and snapshots in place.

Q: Is it safe to use the allocate_empty_primary command to resolve this issue?
A: Use this command with caution, as it can lead to data loss. Only use it when you're certain the data can be recovered from replicas or you're willing to lose the data on that shard.

Q: How can I prevent this error from occurring in the future?
A: Implement regular maintenance, ensure adequate resources (especially disk space), and monitor your cluster health closely. Consider adjusting your shard allocation strategy if you frequently encounter this issue.

Q: Does increasing the index.unassigned.node_left.delayed_timeout setting help prevent this error?
A: While this setting can help in some scenarios by giving more time for node recovery, it doesn't directly prevent the ShardLockObtainFailedException. It's more useful for temporary node disconnections rather than lock-related issues.