Elasticsearch IndexShardRecoveringException: Index shard recovering

Brief Explanation

The "IndexShardRecoveringException: Index shard recovering" error in Elasticsearch occurs when a shard is in the process of recovery and is not yet available for read or write operations. This error indicates that the cluster is still working on making the shard fully operational.

Impact

This error can significantly impact the availability and performance of your Elasticsearch cluster:

Queries and indexing operations targeting the recovering shard will fail
Overall cluster performance may be affected during the recovery process
Applications relying on the affected index may experience timeouts or errors

Common Causes

Node restart or failure
Cluster rebalancing
Index creation or restoration from a snapshot
Network issues causing temporary node disconnections
Insufficient disk space for shard allocation

Troubleshooting and Resolution Steps

Check cluster health:
```
GET _cluster/health
```
Identify recovering shards:
```
GET _cat/recovery?v
```
Monitor recovery progress:
```
GET _cat/shards?v
```
Ensure adequate disk space on data nodes
Verify network connectivity between nodes

If recovery is slow, consider increasing recovery throttling:

PUT _cluster/settings
{
  "persistent": {
    "indices.recovery.max_bytes_per_sec": "50mb"
  }
}

If the issue persists, check Elasticsearch logs for any related errors or warnings
Consider increasing the number of replicas to improve availability during recovery

Best Practices

Implement proper monitoring and alerting for cluster health and shard status
Use rolling restarts when updating nodes to minimize recovery time
Ensure adequate resources (CPU, memory, disk) for your cluster's workload
Regularly backup your indices using snapshots
Use index lifecycle management (ILM) to automate index management

Frequently Asked Questions

Q: How long does shard recovery typically take?
A: Recovery time depends on various factors such as shard size, network speed, and available resources. Small shards may recover in seconds, while large shards can take hours.

Q: Can I query an index while its shards are recovering?
A: You can query an index during recovery, but operations targeting recovering shards will fail with the IndexShardRecoveringException. Other available shards will still be queryable.

Q: How can I prevent this error from occurring?
A: While you can't completely prevent it, you can minimize occurrences by ensuring proper cluster sizing, using rolling restarts, and implementing good monitoring practices.

Q: Will increasing the number of replicas help with this error?
A: More replicas can improve availability during recovery, as queries can be routed to available replicas while primary shards recover. However, it also increases the overall recovery workload.

Q: Is it safe to restart a node if I encounter this error?
A: Restarting a node is generally safe but may prolong the recovery process. It's better to wait for recovery to complete unless you suspect an issue with the node itself.