--- title: "Keep unit tests independent of slow services" description: "Use controlled substitutes for external dependencies when the test is intended to verify local behavior, keeping integration coverage separately." url: https://starsling.dev/ci-playbooks/techniques/mock-external-dependencies canonicalUrl: https://starsling.dev/ci-playbooks/techniques/mock-external-dependencies --- # Keep unit tests independent of slow services Use controlled substitutes for external dependencies when the test is intended to verify local behavior, keeping integration coverage separately. ## How to choose an approach Compare the examples by repository setup, prerequisites, and risks before adopting one. Each company implemented this in its own environment; measure each change against your own baseline. - **[OpenAI](https://starsling.dev/ci-playbooks/openai#mock-dependencies)**: Use it when A slow test calls a real dependency outside the code it exists to verify. Check before adopting: Keep one job that exercises the real dependency, nightly or after merge. A deterministic mock proves the code under test, and the live job proves the service. - **[Uber](https://starsling.dev/ci-playbooks/uber#separate-unit-dependencies)**: Use it when Unit or component checks wait for database servers whose engine-specific behavior they never exercise. Check before adopting: Keep real-engine integration tests and compare their discovered test sets and combined coverage, since unit-only coverage shifts when tests move. The server-startup test also drops a Cassandra schema-version assertion; keep an equivalent engine-specific check in the integration lane. ## Apply this technique ````text Evaluate and apply this CI technique to my repository: Keep unit tests independent of slow services. Use controlled substitutes for external dependencies when the test is intended to verify local behavior, keeping integration coverage separately. Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/mock-external-dependencies.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 companies used this technique Read what each company changed and follow the original sources. ### OpenAI: Mock the slow dependency OpenAI added a missing mock to a 63-test resolver suite and kept all 63 tests. In the public evals repository, the workflow validates new evaluations against dummy chat and completion models, so the check runs without a live model call. Evidence: Company-reported result or observation 63-test resolver suite: 592.6s to 29.8s after adding the missing mock ([Source](https://developers.openai.com/blog/maintaining-software-with-the-gpt-6-family#other-ideas-to-try)) Use it when: A slow test calls a real dependency outside the code it exists to verify. Before applying it: Keep one job that exercises the real dependency, nightly or after merge. A deterministic mock proves the code under test, and the live job proves the service. [Read more in the OpenAI playbook](https://starsling.dev/ci-playbooks/openai#mock-dependencies) [Primary source](https://developers.openai.com/blog/maintaining-software-with-the-gpt-6-family#other-ideas-to-try) ### Uber: Keep unit tests independent of database servers Cadence removed Cassandra, MySQL, PostgreSQL and MongoDB services from its unit-test container. Component tests that only needed persistence switched to SQLite, and database-specific persistence and CLI tests moved into the integration tree, whose Makefile gained an explicit discovery pass. OPT87's boundary-isolation recipe is adapted with a local SQLite store. Evidence: Source PR/commit evidence The PR reports unit and integration test runs and a comparison of coverage reports. ([Source](https://github.com/cadence-workflow/cadence/pull/6800)) Use it when: Unit or component checks wait for database servers whose engine-specific behavior they never exercise. Before applying it: Keep real-engine integration tests and compare their discovered test sets and combined coverage, since unit-only coverage shifts when tests move. The server-startup test also drops a Cassandra schema-version assertion; keep an equivalent engine-specific check in the integration lane. [Read more in the Uber playbook](https://starsling.dev/ci-playbooks/uber#separate-unit-dependencies) [Primary source](https://github.com/cadence-workflow/cadence/pull/6800) ## 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 ### [Balance test work across shards](https://starsling.dev/ci-playbooks/techniques/balance-test-shards) Use measured runtimes or smaller test files to reduce the slowest shard, which determines when the suite finishes. 4 implementations · 4 companies (Linear, OpenAI, Ramp, Stripe) ### [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) ### [Complete the fixture before its timeout fires](https://starsling.dev/ci-playbooks/techniques/finish-fixtures-before-timeout) Exercise the event that advances the test, so it stops waiting on the timer that rescues it. 1 implementation · 1 company (Uber) ## 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. ### [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. ### [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. ### [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.