Isolate state before running tests in parallel

Give concurrent tests independent databases, services, or fixtures so they can share runner capacity without interfering with each other.

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Prompt

Evaluate and apply this CI technique to my repository: Isolate state before running tests in parallel.

Give concurrent tests independent databases, services, or fixtures so they can share runner capacity without interfering with each other.

Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/isolated-parallel-tests.md

Inspect this repository’s CI workflows, build and test commands, dependencies, and recent run timings. Identify where this technique applies and explain the expected benefit using this repository’s evidence.
Measure a baseline from recent comparable runs: elapsed time, queue time, runner minutes and failures where available. If data is unavailable, say what cannot be measured.
Compare relevant company approaches, checking prerequisites, platform differences and tradeoffs. Rank applicable options by evidence, fit and measurable opportunity; do not present company results as predictions for this repo.
Recommend the smallest safe change, show what checks and fallback paths remain, and ASK ME WHICH CHANGE TO APPROVE BEFORE EDITING ANY FILES.
Only after approval, work on a new feature branch, make one reversible change, and open a reviewable pull request. Never merge or push to a default or protected branch.
Keep required checks, test coverage, deployment and release behavior intact. Validate the change and compare equivalent runs where possible; report limitations and what should be monitored after merge.

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How companies used this technique

Read what each company changed and follow the original sources.

Run isolated end-to-end journeys in parallel

The packaged end-to-end suite ran 66 journeys on one worker. Each journey already had its own home directory, workspace, runtime and server port, so the PR set Playwright to three workers with fullyParallel. Global setup sweeps orphaned processes once and installs the Electron runtime once, before any worker starts.

Use it when: A browser or desktop suite runs on one worker while its tests share no files, ports or processes.
Before applying it: Parallel journeys are safe when each owns its state; a shared file, port or install step becomes a race. Measure the worker count on your own runner.

Give parallel test workers more databases

After the CPU change, raising the pytest-xdist worker count left the suite's time flat at first. Profiling showed the test database buckling under the load: it hit lock contention once more than 20 workers shared it. Ramp ran more test databases, measured the ratio of workers to databases, settled on about 12 workers per database, and tuned Postgres connection settings.

Together with the faster CPUs, the database changes took Ramp to a ~70% speedup of the test suite two days in, a saving of about 30 minutes. Source

Use it when: Tests run in parallel workers against one shared database or service, and adding workers leaves the job's time flat while the database is saturated.
Before applying it: Every extra database uses CPU and memory the test workers would otherwise get; Ramp saw slight regressions past its measured ratio, so keep the count that measured fastest.

Isolate tests before adding workers

The CLI end-to-end suite shared one development store. Shopify/cli#7279 gave each test its own store and each worker its own ports; #7309 then turned on fullyParallel and raised Playwright workers from one to five. The default branch runs the suite fully parallel with four workers.

The PR reports about 10 minutes with 5 workers, down from about 27 minutes with 1 worker. Source

Use it when: Tests run one at a time because they share ports, temporary files, accounts or database state.
Before applying it: More workers add load on shared external APIs; keep teardown observable and keep genuinely serial tests on a serial lane.

Give concurrent tests their own state

Stripe sync-engine gives each end-to-end test file its own Postgres container on a random port and enables file parallelism; webhook tests run in a separate sequential configuration because they share a remote endpoint. TaxJar, a Stripe company, made its remote transaction IDs unique per version and build so its WooCommerce 7.x and 9.x test lanes can run in parallel.

Use it when: A shared database, port or remote account forces the whole suite to run serially.
Before applying it: Keep intentionally shared-state tests in a serial lane, keep teardown reliable, and use isolated sandbox accounts for remote services.

Give concurrent suites their own services

SubmitQueue replaced shared manual service setup with a Compose stack per test context. Docker allocates an ephemeral host port and the test discovers it at runtime, so the suites run as separate parallel CI jobs. OPT88's resource-isolation step is adapted; the readiness and image-tag fixes on this page complete it.

The PR creates separate CI jobs for the suites and an isolated Compose project per test context. Source

Use it when: Independent integration suites collide on one port, database or container name, forcing serial execution.
Before applying it: Use collision-proof namespaces and guaranteed cleanup after every run. Separate data and image tags as well as ports, and use a protocol-level readiness probe in place of a fixed startup sleep.

How companies used this technique

Run isolated end-to-end journeys in parallel

Give parallel test workers more databases

Isolate tests before adding workers

Give concurrent tests their own state

Give concurrent suites their own services

Related techniques

Build the test database once for all workers

Match test concurrency to the runner

Move unit tests to a parallel-by-default framework, gated on ten clean runs

Company Playbooks

Related guides

Shard tests across parallel jobs

Cut repeated test setup

Diagnose slow tests in CI

Optimize Playwright tests

Make Vitest run mode explicit

Parallelize independent jobs