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.

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Prompt

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.

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How companies used this technique

Read what each company changed and follow the original sources.

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.

63-test resolver suite: 592.6s to 29.8s after adding the missing mock Source

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.

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.

The PR reports unit and integration test runs and a comparison of coverage reports. Source

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.

How companies used this technique

Mock the slow dependency

Keep unit tests independent of database servers

Related techniques

Balance test work across shards

Build the test database once for all workers

Complete the fixture before its timeout fires

Company Playbooks

Related guides

Cut repeated test setup

Shard tests across parallel jobs

Diagnose slow tests in CI

Optimize Playwright tests

Make Vitest run mode explicit