ClickHouse DB::Exception: No active replicas

The "DB::Exception: No active replicas" error in ClickHouse occurs when the system cannot find any active replicas for a distributed table or when attempting to perform operations on a replicated table.

Impact

This error can significantly impact database operations, potentially causing:

• Read and write failures
• Data inconsistency across replicas
• Service disruptions for applications relying on affected tables

Common Causes

1. Network issues between ClickHouse nodes
2. Zookeeper connection problems
3. Misconfigured replication settings
4. All replicas are down or unreachable
5. Insufficient replica count for the replication factor

Troubleshooting and Resolution Steps

1. Check network connectivity:

   • Ensure all ClickHouse nodes can communicate with each other
   • Verify firewall settings are not blocking inter-node communication

2. Verify Zookeeper connection:

   • Check Zookeeper cluster health
   • Confirm ClickHouse can connect to Zookeeper

3. Review replication configuration:

   • Check <remote_servers> section in config.xml
   • Verify shard and replica configurations

4. Inspect replica status:

   • Use SYSTEM REPLICAS query to check replica states
   • Restart any stopped replicas

5. Ensure sufficient replicas:

   • Add more replicas if the current count is below the replication factor

6. Check ClickHouse logs:

   • Look for specific error messages or warnings related to replication

7. Verify data consistency:

   • Use SYSTEM SYNC REPLICA to synchronize data if needed

Best Practices

• Regularly monitor replica health and status
• Implement proper alerting for replication issues
• Use a higher number of replicas than the minimum required for better fault tolerance
• Perform regular backups to ensure data safety

Frequently Asked Questions

Q: Can I read from a table when there are no active replicas?
A: No, ClickHouse requires at least one active replica to perform read operations on replicated tables.

Q: How can I prevent "No active replicas" errors?
A: Implement robust monitoring, ensure proper network connectivity, maintain a healthy Zookeeper cluster, and have more replicas than the minimum required.

Q: Will data be lost if all replicas become inactive?
A: Data is not necessarily lost, but it becomes inaccessible until at least one replica is brought back online and synchronized.

Q: How long does it take for a replica to become active after restarting?
A: The time varies depending on factors like data volume and network speed. It can range from seconds to hours for large datasets.

Q: Can I add a new replica to resolve this error?
A: Yes, adding a new replica can help, but ensure it's properly configured and synchronized with existing data before it becomes active.