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Why it was created

ClickHouse may exceed max_execution_time and max_concurrent_queries limits due to various reasons:

  • max_execution_time may be exceeded due to the current implementation deficiencies.
  • max_concurrent_queries works only on a per-node basis. There is no way to limit the number of concurrent queries on a cluster if queries are spread across cluster nodes.

Such “leaky” limits may lead to high resource usage on all the cluster nodes. After facing this problem we had to maintain two distinct HTTP proxies in front of our ClickHouse cluster - one for spreading INSERTs among cluster nodes and another one for sending SELECTs to a dedicated node where limits may be enforced somehow. This was fragile and inconvenient to manage, so chproxy has been created :)