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docs/decisions/adr-0008-contract-enforcment-with-tests.md

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+---
+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