Why it was created
max_execution_timemay be exceeded due to the current implementation deficiencies.
max_concurrent_queriesworks 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 :)