The "DB::Exception: Too many simultaneous connections" error in ClickHouse occurs when the number of concurrent connections to the database exceeds the maximum limit set in the configuration.
Impact
This error can significantly impact the availability and performance of your ClickHouse database:
- New connection attempts will be rejected, potentially causing application downtime.
- Existing queries may experience increased latency due to resource contention.
- Overall system performance may degrade as resources are stretched thin.
Common Causes
- Insufficient max_concurrent_queries setting in ClickHouse configuration.
- Connection leaks in client applications.
- Inefficient connection pooling.
- Sudden spike in user activity or traffic.
- Long-running queries holding connections open.
Troubleshooting and Resolution Steps
Check current connection count:
SELECT * FROM system.metrics WHERE metric LIKE '%Connection%';Review and increase max_concurrent_queries in config.xml:
<max_concurrent_queries>100</max_concurrent_queries>Implement or optimize connection pooling in your application.
Identify and fix connection leaks in client code.
Set appropriate timeouts for queries and idle connections.
Consider using ClickHouse's HTTP interface for better connection management.
Monitor and optimize long-running queries.
Best Practices
- Regularly monitor connection usage and trends.
- Implement proper connection pooling in applications.
- Use connection multiplexing when possible.
- Set appropriate max_concurrent_queries based on hardware resources.
- Implement retry logic with exponential backoff in client applications.
Frequently Asked Questions
Q: How can I check the current number of connections in ClickHouse?
A: You can use the query SELECT * FROM system.metrics WHERE metric LIKE '%Connection%'; to view current connection metrics.
Q: What is a good value for max_concurrent_queries?
A: The optimal value depends on your hardware resources and workload. Start with a conservative value (e.g., 100) and adjust based on monitoring and performance testing.
Q: Can connection pooling help prevent this error?
A: Yes, proper connection pooling can significantly reduce the likelihood of hitting connection limits by reusing existing connections efficiently.
Q: Does this error affect read-only queries?
A: Yes, this error affects all types of queries, including read-only ones, as it's related to the total number of connections, not the type of operations being performed.
Q: How can I identify which clients or queries are using the most connections?
A: You can use the system.processes table to view current running queries and their associated information, including the user and query text. For example: SELECT * FROM system.processes ORDER BY elapsed DESC;
