--- title: "Move unit tests to a parallel-by-default framework, gated on ten clean runs" description: "Migrate serial tests to a framework that runs them in parallel, and ship each package after ten green runs." url: https://starsling.dev/ci-playbooks/techniques/parallel-test-framework canonicalUrl: https://starsling.dev/ci-playbooks/techniques/parallel-test-framework --- # Move unit tests to a parallel-by-default framework, gated on ten clean runs Migrate serial tests to a framework that runs them in parallel, and ship each package after ten green runs. ## How to choose an approach This technique currently has one documented company example. Validate its prerequisites, tradeoffs and original CI environment against your repository before adapting it. - **[DoorDash](https://starsling.dev/ci-playbooks/doordash#parallel-test-framework)**: Use it when A test step runs mostly serially, and the test framework or a newer one can run the same tests in parallel. Check before adopting: Parallel runs expose hidden order, shared-state and timing assumptions; fix each root cause, and leave tests on the old framework where the new one lacks a needed feature. ## Apply this technique ````text Evaluate and apply this CI technique to my repository: Move unit tests to a parallel-by-default framework, gated on ten clean runs. Migrate serial tests to a framework that runs them in parallel, and ship each package after ten green runs. Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/parallel-test-framework.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. ```` ## How DoorDash used this technique Read what DoorDash changed and follow the original sources. ### DoorDash: Move unit tests to a parallel-by-default framework, gated on ten clean runs Mostly serialized XCTest runs had stretched DoorDash's iOS CI test step into double-digit minutes. DoorDash migrated its shared iOS packages to Swift Testing, which runs synchronous and asynchronous tests in parallel by default. A Cursor rule made the mechanical edits, and the migration prompt told the agent to run each package's tests with Bazel after every file and fix failures before moving on. A package counted as migrated once its tests passed ten times in a row, which surfaced tests that depended on order, shared state or timing. The unit test step in CI got about 60% faster, and overall builds for shared iOS packages about 40% faster. Evidence: Company-reported result or observation The unit test step in CI became roughly 60% faster. ([Source](https://careersatdoordash.com/blog/doordash-using-cursor-to-accelerate-migration-to-swift-testing/)) This contributed to an average of about 40% faster overall builds for shared iOS packages. ([Source](https://careersatdoordash.com/blog/doordash-using-cursor-to-accelerate-migration-to-swift-testing/)) Use it when: A test step runs mostly serially, and the test framework or a newer one can run the same tests in parallel. Before applying it: Parallel runs expose hidden order, shared-state and timing assumptions; fix each root cause, and leave tests on the old framework where the new one lacks a needed feature. [Read more in the DoorDash playbook](https://starsling.dev/ci-playbooks/doordash#parallel-test-framework) [Primary source](https://careersatdoordash.com/blog/doordash-using-cursor-to-accelerate-migration-to-swift-testing/) ## CI optimization is continuous work. Apply this technique yourself, or use StarSling Runners for faster GitHub Actions and optional agent-driven optimization PRs, which teams on eligible paid plans can turn on. [Get self-improving CI](https://starsling.dev/products/runners) ## Related techniques ### [Build the test database once for all workers](https://starsling.dev/ci-playbooks/techniques/coordinate-test-setup) Have one parallel worker apply the schema while the others wait for it, so more workers stop repeating the same setup. 1 implementation · 1 company (Ramp) ### [Isolate state before running tests in parallel](https://starsling.dev/ci-playbooks/techniques/isolated-parallel-tests) Give concurrent tests independent databases, services, or fixtures so they can share runner capacity without interfering with each other. 5 implementations · 5 companies (DoorDash, Ramp, Shopify, Stripe, Uber) ### [Match test concurrency to the runner](https://starsling.dev/ci-playbooks/techniques/tune-test-workers) Measure worker counts against available CPU and memory. More workers help only while the runner has capacity. 4 implementations · 4 companies (Anthropic, Mastra, OpenAI, Stripe) ## Company Playbooks ### [Anthropic](https://starsling.dev/ci-playbooks/anthropic) Run only affected tests ### [DoorDash](https://starsling.dev/ci-playbooks/doordash) Run build and test in parallel, sharded across CI agents ### [Linear](https://starsling.dev/ci-playbooks/linear) Benchmark the same job on faster hardware ### [Mastra](https://starsling.dev/ci-playbooks/mastra) StarSling runners for the slowest test workflow ### [OpenAI](https://starsling.dev/ci-playbooks/openai) Rank CI fixes by the minutes they save ### [Ramp](https://starsling.dev/ci-playbooks/ramp) Move a CPU-bound test suite to a faster CPU ### [Shopify](https://starsling.dev/ci-playbooks/shopify) Let an agent iterate against one CI metric ### [Spotify](https://starsling.dev/ci-playbooks/spotify) Build arm64 on native arm64 runners ### [Stripe](https://starsling.dev/ci-playbooks/stripe) Run only the tests a change can reach ### [Uber](https://starsling.dev/ci-playbooks/uber) Build the merge-queue speculations most likely to be needed ## Related guides Implementation guides for this technique. ### [Shard tests across parallel jobs](https://starsling.dev/best-practices/github-actions/shard-tests) Split a long suite with a matrix and your test framework’s sharding support. ### [Cut repeated test setup](https://starsling.dev/best-practices/github-actions/cut-repeated-test-setup) Move shared fixture work out of individual test cases and keep necessary cleanup. ### [Diagnose slow tests in CI](https://starsling.dev/github-actions/problems/slow-tests) Check common causes in order and confirm each with workflow evidence. ### [Optimize Playwright tests](https://starsling.dev/github-actions/optimizations/optimize-playwright) Choose trace and video retention and install the browsers your suite uses. ### [Make Vitest run mode explicit](https://starsling.dev/github-actions/optimizations/optimize-vitest) Pin run mode to the test command and bound the job with a timeout. ### [Parallelize independent jobs](https://starsling.dev/github-actions/optimizations/parallelize-independent-jobs) Run independent validation lanes concurrently and keep genuine job dependencies.