# Approval-engine transactional outbox contract

**Owner:** gate-house
**Implementer:** approval-engine
**Evidence engine:** audit-core
**Status:** normative for GH-WP-0002-T02
**Date:** 2026-08-29
**Statute:** `net-kingdom/canon/standards/security-layer-model_v0.7.md` §9.4, §9.6
**Failure mode:** `GH-DEC-2026-002`
**Source draft:** `approval-engine/docs/outbox-contract.md` (adopted)

This is the wire. Boundary and locality are in the statute and in
`approval-engine/INTENT.md`. An implementer MUST NOT satisfy this contract by
emitting synchronously to `audit-core` inside the state-change transaction.
That path is atomic and is forbidden: an audit outage would become an inability
to revoke.

## Rule

Every issuance, use, supersession, and revocation **inserts an outbox row in
the same transaction** that mutates the approval object. The durable queue
lives in `approval-engine`'s own store. Drain is asynchronous, at-least-once.
`audit-core` dedupes on `event_id`; a replay does not fork the chain.

If the outbox insert cannot be committed, the mutation does not commit.
Emit-after-commit is a defect.

Heartbeat rows (see [`approval-emission-detection.md`](approval-emission-detection.md))
use the same table and the same at-least-once drain. They are not coupled to an
object mutation.

## Event classes

| Class | When | `audit-core` `action` |
| --- | --- | --- |
| `issuance` | object becomes `approved` (threshold met) | `approval.issuance` |
| `use` | object becomes `consumed` (public consume under [`approval-consumption.md`](approval-consumption.md)) | `approval.use` |
| `supersession` | object becomes `superseded` | `approval.supersession` |
| `revocation` | object becomes `revoked` | `approval.revocation` |
| `heartbeat` | signed *nothing to report* plus counts | `approval.heartbeat` |

Expiry is a clock crossing, persisted on observation, and is **not** an
emitted class. The validity window is already on the object.

## Outbox row

| Field | Type | Notes |
| --- | --- | --- |
| `event_id` | UUID | Stable. Drain retries reuse it. |
| `class` | enum above | |
| `approval_id` | UUID or null | Null only for heartbeat. |
| `payload` | object | The `audit-core.event.v1alpha1` record, ready to POST. |
| `created_at` | RFC 3339 UTC | |
| `drained_at` | RFC 3339 UTC or null | Set after a successful `audit-core` ack. |

## Payload (`audit-core` v1alpha1)

```json
{
  "schema_version": "audit-core.event.v1alpha1",
  "event_id": "<same as outbox.event_id>",
  "observed_at": "<created_at>",
  "tenant": "platform",
  "scope": "netkingdom-approvals",
  "source": "approval-engine",
  "actor": "<actor or null for heartbeat>",
  "action": "approval.revocation",
  "resource": "approval:<approval_id>",
  "outcome": "success",
  "reason": null,
  "details": {
    "class": "revocation",
    "approval_id": "<uuid>",
    "binding_digest": "sha256:…",
    "request_digest": null,
    "superseded_by": null
  }
}
```

No secret values. `details` may add non-secret identifiers; it MUST NOT add a
validity verdict for consumers to branch on. `audit-core` MUST NOT expose an
approval-validity query; this payload does not invite one.

For `use` events, `details.request_digest` is the digest the PEP presented at
consume (see the consumption contract). It is null on other classes.

## Drain

1. Select undrained rows, oldest first.
2. POST each payload to `audit-core`.
3. On success, set `drained_at`.
4. On failure, leave the row; retry later. **Do not** roll back the object
   mutation — it already committed with the row.

An `audit-core` outage therefore cannot block a revocation. This engine's
own store being unavailable can, and should: the change could not have been
recorded anyway. That is the whole of `GH-DEC-2026-002`.

## Forbidden shapes

- `audit-core` HTTP (or any client) inside `BEGIN` … `COMMIT` of a mutation.
- Best-effort publish after commit with no row.
- A second, non-local queue as the durability mechanism.
- Deduping in `approval-engine` instead of relying on `event_id` at `audit-core`.