---
name: "Adr 0008 Contract Enforcment With Tests"
description: "Contract testing to enforce event and replay invariants"
depth: 2
links: ["../index.md", "./adr-0006-event-store-append-only.md", "./adr-0007-projection-replay.md"]
---

# ADR-0009: Contract Testing Enforcement of Event & Replay Invariants

**status:** accepted
**date:** 08.05.2026
**deciders:** Kami

---

## context

System correctness depends not only on design-time invariants (ADR-0007, ADR-0008) but also on continuous enforcement across implementations.

Multiple EventStore and Replay implementations exist (in-memory, sqlite, future persistence layers). Without shared contract tests, invariants may diverge silently.

This ADR defines the role of contract tests as enforcement layer for system invariants.

---

## content

### 1. contract-first enforcement model

* all EventStore implementations MUST satisfy shared contract tests
* all replay implementations MUST satisfy shared replay contracts

### 2. EventStore contract requirements

Contract tests MUST enforce:

* append-only behavior
* idempotency of event_id
* strict ordering per session
* deterministic read consistency
* correct sequence continuity

### 3. Replay contract requirements

Contract tests MUST enforce:

* deterministic reconstruction of state
* identical results across repeated replay executions
* correctness over full event streams
* correctness over partial event streams (cursor-based replay)

### 4. implementation independence

* contract tests MUST NOT depend on specific implementation details
* tests operate only through interfaces (EventStore, EventReplayer, Projection)

### 5. cross-backend equivalence

* in-memory and persistent stores MUST produce identical replay results for identical inputs

---

## consequences

**positive:**

* prevents divergence between implementations
* guarantees correctness across persistence backends
* enforces architectural invariants at CI level
* makes replay correctness regression-resistant

**negative:**

* increases test suite complexity
* requires careful abstraction of test fixtures
* slower feedback cycles for persistence changes

---

## status

accepted