# Policy Engine · Change Events Prep (POLICY-ENGINE-30-003)

- **Date:** 2025-11-20
- **Depends on:** Overlay projection (30-001) and simulation bridge (30-002)
- **Working directory:** `src/Policy/StellaOps.Policy.Engine`

## Event envelope
- Topic: `policy.overlay.changed`
- Payload fields:
  - `tenant` (string)
  - `ruleId` (string)
  - `version` (int) — matches projection version
  - `changeType` (`created|updated|deprecated|deleted`)
  - `projection` (OverlayProjection) — single item; for `deleted`, include last-known `projection` with `decision: "defer"`.
  - `delta` (object, optional) — produced when triggered from simulation `whatif`: `baselineDecision`, `candidateDecision`, `diffReason`.
  - `artifacts` — `evidenceHash`, `treeDigest?`, `dsseEnvelopeHash?`
  - `emittedAt` (ISO-8601 UTC)
  - `correlationId` (string) — propagate from incoming request or run id.

## Delivery guarantees
- At-least-once on the bus; projection storage is source of truth. Events carry idempotency key `tenant:ruleId:version`.
- Ordering per `ruleId` is preserved by publishing after storage commit.

## Consumers
- Scheduler uses events to enqueue re-evaluation jobs keyed by `tenant` + `ruleId`.
- UI/CLI preview listens for `changeType` ≠ `created` to refresh overlays without polling.

## Acceptance for prep completion
- Topic, payload, and idempotency key are fixed; downstream scheduler/UI/CLI can wire without further contract churn.