The "DB::Exception: Shard is unavailable" error in ClickHouse occurs when a distributed query is unable to reach one or more shards in a cluster. This typically happens in a distributed table setup where some shards are not responding or are inaccessible.
Impact
This error can significantly impact query performance and data consistency in a distributed ClickHouse setup. It may lead to:
- Incomplete query results
- Increased query latency
- Potential data inconsistencies across the cluster
Common Causes
- Network connectivity issues between ClickHouse nodes
- Shard server is down or not running
- Misconfiguration in the cluster definition
- Firewall or security group settings blocking communication
- Insufficient resources on shard servers causing timeouts
Troubleshooting and Resolution Steps
Check network connectivity:
- Verify network connectivity between ClickHouse nodes
- Ensure there are no firewall rules blocking communication
Verify shard server status:
- Check if all shard servers are running
- Review server logs for any errors or crashes
Review cluster configuration:
- Examine the cluster definition in config.xml
- Ensure all shard addresses and ports are correct
Check server resources:
- Monitor CPU, memory, and disk usage on shard servers
- Increase resources if necessary to handle the workload
Adjust timeouts:
- Increase distributed_directory_monitor_sleep_time_ms and distributed_directory_monitor_batch_inserts settings if needed
Restart problematic shards:
- If a specific shard is consistently unavailable, try restarting the ClickHouse server on that node
Use system.clusters table:
- Query system.clusters to check the status of all shards in the cluster
Best Practices
- Implement proper monitoring and alerting for your ClickHouse cluster
- Regularly check and maintain network infrastructure
- Use replication to improve fault tolerance
- Implement proper load balancing across shards
- Keep ClickHouse software and configurations up to date
Frequently Asked Questions
Q: How can I identify which shard is unavailable?
A: You can query the system.clusters table to see the status of all shards in your cluster. Look for shards with a 'shard_num' that matches the one mentioned in the error message.
Q: Can I skip unavailable shards and continue the query?
A: Yes, you can use the 'skip_unavailable_shards' setting to allow queries to run even if some shards are unavailable. However, this may lead to incomplete results.
Q: How does ClickHouse handle shard failures in distributed queries?
A: By default, ClickHouse will attempt to execute the query on all available shards. If a shard is unavailable, it will throw this exception unless 'skip_unavailable_shards' is enabled.
Q: What's the difference between a shard being unavailable and a replica being unavailable?
A: A shard represents a portion of data in a distributed table, while a replica is a copy of that data. If all replicas of a shard are unavailable, the shard becomes unavailable.
Q: How can I improve the resilience of my ClickHouse cluster to prevent shard unavailability?
A: Implement proper replication, use multiple replicas per shard, set up monitoring and alerting, and ensure adequate resources and network connectivity for all nodes in the cluster.
