# Change Events Prep — PREP-POLICY-ENGINE-30-003-CHANGE-EVENTS-DEPEN

Status: Draft (2025-11-20)
Owners: Policy Guild · Scheduler Guild · Cartographer Guild
Scope: Define the change-event payload and scheduling expectations following simulation bridge (30-002).

## Dependencies
- Simulation bridge output schema (30-002).
- Scheduler delivery channel (NATS/Redis) subject names and dedupe policy.

## Draft event envelope
- `event_type`: `policy.overlay.change`
- Fields: `tenant_id`, `overlay_hash`, `policy_profile_hash`, `simulation_id?`, `changes[]` (array of `{component_purl, advisory_id, prev_status, new_status, severity_delta?, trace_ref}`), `occurred_at` (UTC), `event_id` (deterministic ID = hash of overlay_hash + timestamp).
- Transport: propose NATS subject `policy.overlay.change` with durable stream; idempotency key = `event_id`.
- Observability: counter `policy_overlay_change_total{tenant,result}`; log template includes overlay_hash, event_id, change_count.

## Acceptance to close PREP
- Subject/stream names confirmed with Scheduler.
- JSON schema stub saved at `docs/modules/policy/schemas/policy-overlay-change@draft.json`.
- Sample event at `docs/modules/policy/samples/policy-overlay-change@draft.json`.

## Handoff
This document serves as the prep artefact for PREP-POLICY-ENGINE-30-003-CHANGE-EVENTS-DEPEN. Update once 30-002 finalizes schema/subject; then unblock the implementation task.