ch_play

ch_play coordinates a reproducible ClickHouse experiment: it activates a declarative ch_stand, optionally installs ch_workload profiles, runs the workload, and collects ch_diag snapshots at the same time. The result is a durable run directory with JSON/HTML diagnostic reports, the reviewed plan, component versions, state, and append-only events.

The MCP server exposes only typed experiment operations. It does not expose arbitrary SQL, shell, SSH, or Docker commands.

Requirements

• Linux and Docker supported by ch_stand;
• ssh-keyscan for collection_mode: remote;
• Python 3.10 or newer;
• compatible ch_stand, ch_workload, and ch_diag console commands from the same Python environment.

Install from PyPI after publication:

python3 -m pip install ch-play

For a source checkout:

python3 -m venv .venv
.venv/bin/pip install -e '.[dev]'

Experiment manifest

The initial imdb_cluster workload requires the 4 shards x 2 replicas profile from ch_stand. Paths are resolved relative to the experiment manifest.

api_version: ch_play/v1
kind: ClickHouseExperiment

metadata:
  id: imdb-diagnostics

spec:
  artifact_root: .ch_play/experiments/imdb-diagnostics

  stand:
    config: stand/configs/sharded-replicated-8.yaml
    project: stand
    timeout_seconds: 600

  workload:
    project: workload
    profiles: [imdb_cluster]
    database: ch_workload
    scale: 0.1
    seed: 20260802
    duration_seconds: 60
    install: true
    recreate: false
    resource_monitor: true

  diagnostics:
    collection_mode: remote
    target_scope: cluster
    interval_seconds: 5
    report_name: during-workload

The ClickHouse user, cluster name, HTTP/native/SSH ports, password file, and SSH key are derived from the resolved ch_stand configuration. spec.workload.database is the database created and loaded by the workload profile; it defaults to ch_workload. Passwords are forbidden in the manifest and are never passed in process arguments or returned by MCP.

Safe execution flow

Validate and review the exact deterministic plan first:

ch-play validate experiment.yml
ch-play plan experiment.yml

Pass the returned plan_hash unchanged. A detached run is the preferred mode for MCP clients:

ch-play start experiment.yml --plan-hash sha256:... --run-id run-001
ch-play status experiment.yml --run-id run-001
ch-play events experiment.yml --run-id run-001

For a foreground terminal run, use ch-play run with the same arguments. A cancellation request is cooperative and is bound to the recorded experiment processes:

ch-play cancel experiment.yml --run-id run-001 --reason "enough samples"

The stand remains available after a completed or failed run so that it can be inspected. Removal is always explicit:

ch-play teardown experiment.yml --run-id run-001
ch-play teardown experiment.yml --run-id run-001 --clear-data --force

--clear-data permanently removes stand data and therefore additionally requires --force.

Reports

By default the run directory contains:

• during-workload.json and during-workload.html from ch_diag;
• state.json with the plan, versions, workload result, diagnostics summary, and artifacts;
• events.jsonl with ordered lifecycle events;
• diagnostic and detached-worker logs.

Reports can be inspected or compared without connecting to ClickHouse:

ch-play inspect-report path/to/report.json
ch-play compare-reports baseline.json candidate.json

The comparison explicitly marks reports non-comparable when their content version, report ID, ClickHouse version, or collection mode differs. Profile hashes, topology, scale, and seed should also be checked in state.json.

MCP server

Start the stdio server with:

ch-play-mcp

It provides these high-level tools:

• component_capabilities
• validate_experiment, plan_experiment
• start_experiment, run_experiment
• experiment_status, experiment_events, cancel_experiment
• teardown_experiment
• inspect_diagnostic_report, compare_diagnostic_reports

The server also publishes the experiment, run-state, run-event, and component-contract schemas as chplay:// resources.

Component contract

ch_play invokes the three tools through ch_play/component/v1 machine envelopes. Each component advertises the hidden interface through:

ch-stand --machine --request-id probe --component-capabilities
ch-workload --machine --request-id probe --component-capabilities
ch-diag --machine --request-id probe --component-capabilities

Every response contains exactly contract_version, component, component_version, command, request_id, status, result, artifacts, warnings, and error.