Vitest utilities for writing tests against real Postgres, Prisma, Redis and ClickHouse - we don't mock (see the root CLAUDE.md), we boot containers. Also exposes a duration-weighted shard sequencer for splitting slow suites across CI shards.

Most tests share one set of containers per vitest worker (booted once, reset between tests) - this is much faster than a container per test. Reach for an isolated variant only when a test needs it.

"shared (clone)" = one Postgres per worker with a template database; each test gets a fast CREATE DATABASE ... TEMPLATE clone, so schema isn't re-pushed per test.

If a test spawns work that outlives the test body - a RunEngine, a redis-worker Worker, a BatchQueue - and that work isn't fully drained before the test ends, you must use an isolated redis fixture (isolatedRedisTest / containerTestWithIsolatedRedis).

On the shared fixture, the leaked background loop keeps polling the one worker-scoped redis after the test's clients close, bleeding into the next test. The symptom is an intermittent "Connection is closed" error or a test that hangs until its timeout. FLUSHALL between tests does not fix this - it clears data, not live connections/loops, so per-test key prefixes won't help either. A plain db/redis test with no lingering background work is fine on the shared fixtures.

CI splits the slow suites with vitest --shard=i/N. DurationShardingSequencer replaces vitest's default file-count split with a duration-weighted one: it reads test-timings.json at the repo root ({ "<repo-relative path>": <ms> }) and greedily bin-packs files so each shard does roughly equal work, not an equal number of files. The packing is deterministic, so every shard computes the same bins and runs each file exactly once.

Configs opt in via:

import { DurationShardingSequencer } from "@internal/testcontainers/sequencer";
// in defineConfig:
test: {
  sequence: {
    sequencer: DurationShardingSequencer,
  },
}

New test files are discovered by vitest's glob and sharded automatically. A file with no entry in test-timings.json is given the median duration as a fallback, so it's still placed on exactly one shard - correctness never depends on the timings being present or current.

What the timings affect is balance. A new heavy test estimated at the median can be under-weighted and land on an already-full shard, making that shard slower. There's headroom between the current makespan and the CI budget to absorb this, so it tolerates drift - but if a shard creeps toward the budget, refresh the timings.

Measure each shard with the JSON reporter and write per-file endTime - startTime (ms), keyed by repo-relative path, back into test-timings.json. Set GITHUB_ACTIONS=true so suites that skipIf(CI) are excluded, matching what actually runs on CI:

GITHUB_ACTIONS=true pnpm exec vitest run --reporter=json --outputFile=/tmp/run.json

Stale entries for deleted/renamed files are harmless (they're simply ignored). This is a periodic chore, not a per-PR one.