# Model-Based Testing: Queue Invariants Under Crash How bunqueue uses fast-check command models, a real TCP broker, SQLite, shrinking, and SIGKILL recovery to verify queue safety. Canonical: https://bunqueue.dev/architecture/model-based-testing/ --- bunqueue's example tests are supplemented by four `fast-check` models. The main asynchronous command model starts the real standalone broker on dynamic ports, uses the public MessagePack TCP protocol, writes a fresh SQLite database, and may terminate the broker with `SIGKILL` before reconnecting. Focused models exercise S3 backup/restore, worker-monitoring aggregates and enterprise telemetry conservation/cardinality. ```bash bun run test:model ``` The default broker campaign runs 150 generated histories of up to 80 commands. The backup campaign adds 50 histories of up to 30 commands, the worker monitoring campaign adds 500 histories of up to 80 actions, and the enterprise telemetry campaign adds 500 backup-state histories plus 1,000 queue-selection cases. `bun test` includes it, so `bun run test:sandbox` executes the same model inside the isolated unit container. ## Focused invariants The backup model combines a real WAL-mode SQLite database with an in-memory S3 contract. Generated histories insert rows, pin the WAL with an old reader, create and restore snapshots, prune retention, introduce stale sidecars, and corrupt payloads. Its invariants require every published backup to have metadata, every restore to equal its captured point in time, old WAL frames never to resurrect data, retention to preserve the newest set, and a failed restore never to alter live state. The monitoring model generates worker register, heartbeat, active, complete, fail and unregister transitions. After each action it reconciles registered workers, active jobs and concurrency slots with a simple map oracle, then checks the exported Prometheus gauges. This specifically protects utilization alerts from aggregate-counter drift. The enterprise telemetry model enforces two additional conservation laws: backup attempts equal successes plus failures plus at most one active attempt, the reported scheduler and last-outcome duration/timestamp/compressed-size match the modeled transition, and exported plus omitted queue labels equals the registered queue count while the emitted subset never exceeds its configured cap. ## Why a state model? Queue bugs rarely live in one operation. They appear in histories such as push → pull → update → fail → retry → crash → recover. The model stores the expected lifecycle in memory, executes each generated command on the real engine, and compares the two after every step. When a property fails, `fast-check` shrinks the history to a smaller counterexample and prints a seed and replay path. ## Safety invariants The oracle checks these contracts together: - **Conservation and no loss:** every accepted durable lifecycle is represented by exactly one waiting, prioritized, waiting-children, delayed, active, completed, or failed/DLQ job, unless an explicit remove operation retired it. - **No resurrection:** a terminal generation is never delivered again. Reusing a terminal custom ID creates a strictly newer generation; retrying DLQ or completed work is an explicit transition. - **Exclusive delivery:** two real TCP clients cannot hold the same job active concurrently. At-least-once recovery permits later redelivery, not concurrent ownership. - **Bounded recovery:** `attempts <= maxAttempts` and `stallCount <= maxStalls`; exhausted jobs enter DLQ once. - **Legal transitions:** delivery, retry, terminal completion/failure, promotion, delay, removal, and crash recovery follow the declared graph. - **Ordering:** priority first, FIFO or LIFO tie-break as configured, no delayed or TTL-expired early delivery, one active job per FIFO group, and no parent before its dependencies. - **Failure policies:** fail-parent, ignore, continue, and remove-dependency modes produce their declared parent transition. - **Resources:** custom IDs and unique keys remain exclusive; concurrency and rate tokens admit no extra job or leak when rate acquisition rejects after a concurrency acquisition; expired jobs leave memory, counters, index, write buffer, and SQLite exactly once. - **Internal coherence:** API counts, priority counts, `ShardCounters`, processing maps, dependency maps, locks, `jobIndex`, SQLite `jobs`, and SQLite DLQ agree with the same modeled state. - **Crash idempotence:** an active job becomes a bounded retry or one DLQ entry; a second restart with no intervening operation does not increment or duplicate anything. Failed jobs live in DLQ, so conservation counts that lifecycle once rather than adding a separate duplicate "failed plus DLQ" bucket. ## Commands and observability Generated commands include single and batch push/pull/ack, failure and retry, payload/progress/priority/delay changes, queue and DLQ controls, pause, concurrency and rate limits, heartbeats, flows, custom-ID reuse, invalid token operations, actual process crashes, and isolated contracts for FIFO/LIFO, groups, unique keys, dependencies, delay and TTL. After each command, the test checks: 1. `GetState`, `GetJob`, aggregate counts, and priority counts. 2. Payload generation, priority, attempts, stall count, and live lease tokens. 3. Exact `jobs` and `dlq` rows plus persisted queue controls. 4. `/stats` internal collection telemetry for counters, indexes, processing, dependency, completion, and lock cardinality. ## The 71-invariant register The production checklist contains 71 invariants across 19 categories. The main `fc.commands` lifecycle model owns core safety, ordering, limits, counters, crash recovery, dependencies, DLQ, and queue controls. Focused suites own the contracts that need different clocks or failure injection: cron, worker timeouts and fencing, protocol ambiguity, MessagePack roundtrips, migrations, SQLite integrity, WAL checkpoints, and clean-restart equivalence. The focused CLI category adds generated argv/flag properties, exact command-to-MessagePack fixtures, mutation-free read/error snapshots, API parity, disconnect cancellation, and a real-executable matrix for every command. This distinction matters: a green lifecycle model is not presented as proof of cron or migration behavior. Every checklist item must point to an executable assertion in either the generated model or its named specialist suite. The complete category-to-suite register is maintained in the internal [`model-based-testing.md`](https://github.com/egeominotti/bunqueue/blob/main/docs/features/model-based-testing.md). ## Replaying a failure Use the seed printed by `fast-check` and increase the campaign when needed: ```bash BUNQUEUE_MODEL_RUNS=500 \ BUNQUEUE_MODEL_COMMANDS=150 \ BUNQUEUE_MODEL_SEED=424242 \ bun run test:model ``` First decide whether the model or engine is wrong. A confirmed engine bug must be preserved as a deterministic `test/repro-model-*.test.ts` regression that fails before the fix. This process has already exposed payload and priority updates lost on restart, reset stall budgets, ignored attempt bounds, duplicate DLQ recovery risk, TTL rows left behind in SQLite, dependency-gated jobs left observable after queue obliteration, dangling indexes after DLQ purge, and a stale DLQ generation after terminal custom-ID reuse. It also exposed live lease and client-ownership entries retained after moving active work to delayed. The subsequent full sandbox gate found a tenth defect: low-level jobs created before `stallCount` existed violated the new SQLite `NOT NULL` boundary. Persistence now normalizes an omitted value to zero without weakening the schema. An expanded progress/persistence oracle later found an eleventh defect: `MoveToWait` completed its in-memory transition but left the SQLite row `active`. Restart recovery therefore treated a manual requeue as interrupted work. The transition now persists `state`, `run_at`, and `started_at` through the same `updateRunAt` boundary used by delayed moves. For the complete engineering procedure, see the repository's [`docs/features/model-based-testing.md`](https://github.com/egeominotti/bunqueue/blob/main/docs/features/model-based-testing.md) and [`docs/testing.md`](https://github.com/egeominotti/bunqueue/blob/main/docs/testing.md).