Client connection reuse and server-side query cancellation#152
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joe-clickhouse wants to merge 4 commits intomainfrom
Open
Client connection reuse and server-side query cancellation#152joe-clickhouse wants to merge 4 commits intomainfrom
joe-clickhouse wants to merge 4 commits intomainfrom
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Pull request overview
This PR hardens the mcp_clickhouse MCP server for sustained/high-concurrency usage by reusing ClickHouse clients across tool calls, implementing true server-side query cancellation, and aligning network timeouts to prevent worker exhaustion.
Changes:
- Added a cached
clickhouse_connectclient layer keyed by resolved client config, with eviction on liveness/connection failures and limited retries for safe metadata tools. - Implemented query ID tracking and server-side cancellation via
KILL QUERYon MCP timeouts. - Introduced
CLICKHOUSE_MCP_MAX_WORKERSand alignedsend_receive_timeouttoquery_timeout + 5unless explicitly overridden.
Reviewed changes
Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.
Show a summary per file
| File | Description |
|---|---|
mcp_clickhouse/mcp_server.py |
Implements client caching, active query tracking, cancellation via KILL QUERY, timeout alignment, and worker pool sizing/logging. |
mcp_clickhouse/mcp_env.py |
Adds CLICKHOUSE_MCP_MAX_WORKERS configuration property. |
mcp_clickhouse/__init__.py |
Exposes additional helpers at package level via imports/__all__. |
README.md |
Documents the updated timeout behavior, cancellation semantics, and new max-workers config. |
tests/test_client_cache.py |
Adds coverage for caching behavior, eviction, and timeout alignment. |
tests/test_query_cancellation.py |
Adds coverage for query_id propagation, active query tracking, and timeout-triggered cancellation. |
tests/test_context_config_override.py |
Ensures test isolation by clearing the client cache around config override tests. |
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Summary
The goal of this PR is to harden the MCP server for heavy, sustained usage by reusing client connections, adding real server-side query cancellation, and preventing worker thread pool exhaustion from zombie queries.
clickhouse_connectclients by config key instead of creating a new client on every tool call. Eliminates hundreds of ms of overhead per call that a significant contributor to intermittent timeouts under sustained use.query_idto every query and issueKILL QUERYon timeout instead of the no-opfuture.cancel(). Timed-out queries no longer continue running as zombies consuming worker threads and ClickHouse server resources.send_receive_timeouttoquery_timeout+ 5, unless explicitly overridden so worker threads unblock shortly after the MCP timeout fires, preventing thread pool exhaustion.list_databasesandlist_tablesretry once with a fresh client.run_queryevicts but does not retry because writes could duplicate.ContextVaris available before dispatching to the worker thread, fixing a latent bug where PR Client config override support via MCP Context Session states #115 overrides were silently missed inside the executor.