testland/test-isolation-patterns
Pure reference catalog of test-isolation and fixture-lifecycle patterns - the four-phase test pattern (Meszaros), fixture scope (per-test / per-describe / shared / global), the Fresh-Fixture vs Shared-Fixture trade-off (Fowler), parallel-safety patterns, and cleanup discipline (afterEach / afterAll / tagged-cleanup), plus a pattern-selection guide and a worked leaking-state diagnosis. The database-isolation strategies (transaction-rollback / database-per-worker / template-database) and network / external-service stubbing live in references/. This is the architecture-tier reference, not a file-level fixture-coupling style rule. Use when designing fixture scope and isolation strategy, auditing fixture coupling or retry/wait policy, or moving a suite to parallel execution.
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database-store-isolation.mdreferences/
Database and external-store isolation
Deep reference for test-isolation-patterns SKILL.md. Consult after picking fixture scope, when the system under test reads or writes a database, cache, or queue. This is the dominant source of test flake at scale; five canonical strategies, each with trade-offs.
Transaction-rollback (the default)
Each test runs in a transaction; teardown rollbacks. Works for: relational DBs with full transaction support. Doesn't work for: DDL changes, multiple DB connections, queues, caches.
Database-per-test-worker
Each parallel worker gets its own database (named app_test_worker_1, app_test_worker_2, etc.). Created once at startup; reused across tests within the worker; dropped at suite end. Works for: parallel execution with mutation-heavy tests. Cost: pre-suite setup time + N× DB storage.
Template database / pristine clone
Pre-create a template database with seed data; clone it per test (or per worker). PostgreSQL's CREATE DATABASE … TEMPLATE template_db is the canonical mechanism. Works for: tests needing complex seed state. Cost: template maintenance.
Containerised DB-per-test
Each test gets a fresh Docker container (Testcontainers is the canonical library). Maximum isolation; highest cost. Works for: integration tests where the DB version / extensions / config matter. Don't use for: unit tests.
In-memory substitution
Use SQLite in-memory instead of the production DB engine. Fast; works for simple SQL. Doesn't work for: production-specific features (PostgreSQL JSON, Postgres extensions, MySQL spatial types). Cited as an anti-pattern by Fowler on integration tests when the production engine has features the in-memory substitute lacks.
Anti-patterns
| Anti-pattern | Why it fails |
|---|---|
| Tests that mutate a shared DB without isolation | Cross-test coupling; the dominant source of flake at scale |
| In-memory substitution masking production-engine differences | Tests pass locally; fail in production |
| Transaction-rollback for tests that do DDL (CREATE TABLE in test) | DDL is auto-commit in most engines; rollback doesn't undo it |
| Database-per-worker without a maximum-worker limit | Storage explodes; CI cost surges |
| Containerised DB-per-test for unit tests | 5-second container startup × 1000 unit tests = unworkable |