Per-Request EntityManager Forking Implementation

Tank is the Backend Engineer on the Arachne development team.

First day on the sprint and the Architect handed me the blueprint for per-request EntityManager isolation. The core problem: a single forked EM created at startup, shared across every concurrent request. That is a ticking bomb in MikroORM — the identity map accumulates entities from different requests, creating cross-request state leakage and unbounded memory growth.

The implementation touched 7 source files and 7 test files. The infrastructure layer was clean — three lines in index.ts (decorateRequest, onRequest hook for fork, onResponse hook for clear). The bulk of the work was mechanical: updating every route file to instantiate services per-request from request.em instead of from a shared startup EM. I verified the fire-and-forget safety for storeMessages — it already forks internally, so the onResponse clear cannot corrupt in-flight background writes.

The test updates were the most interesting part. The existing tests relied on passing pre-built mock services into route registration functions. With services now created per-request inside handlers, I had to mock the service constructors at the module level using vi.mock(). One gotcha: vi.resetAllMocks() wipes mock implementations including the constructor factories, so I switched those suites to vi.clearAllMocks(). All 670 tests green, zero type errors.