Webhook Provider

Emit events to customer endpoints reliably, securely, and observably. Consuming webhooks is covered by idempotent-financial-workflows. Signing formulas and event schema template are in [REFERENCE.md](REFERENCE.md).

Payload Signing

• Sign: HMAC-SHA256(secret, "v1:" + timestamp + ":" + raw_body)
• Header: X-Webhook-Signature: t=<unixts>,v1=<hexdigest>
• Timestamp in the signed string prevents replay attacks.
• Dual-secret verification during rotation (old + new accepted for 24 h).
• Ed25519 option: publish a public key — customer verifies without a shared secret.

Delivery & Retry

• Success = any 2xx within timeout (30 s recommended).
• Retry on 4xx (except 410), 5xx, timeout, network error.
• Backoff + jitter: 30 s → 5 min → 30 min → 2 h → 8 h → 24 h.
• After max retries: dead-letter queue + dashboard + optional alert.
• On 410: auto-disable the endpoint. Never retry 2xx.

Endpoint Health & Circuit Breaker

Track success/failure ratio over a sliding window (last 100 deliveries). Above threshold (e.g. >50% for 1 h): disable, notify customer, require explicit re-enable. On re-enable, replay dead-lettered events (bounded, customer-configurable window).

Ordering & Idempotency

• At-least-once delivery; include idempotency_key in every payload — customers must deduplicate.
• Per-resource delivery queue keyed by resource ID; cross-resource order not guaranteed (document this).
• Monotonic sequence per resource so consumers detect gaps.

Event Schema

Fields: id, type (e.g. invoice.paid), createdat (ISO 8601 UTC), apiversion, idempotency_key, data. Emit a new event type for breaking changes — never mutate an existing schema. Publish an AsyncAPI/OpenAPI catalog (template in REFERENCE.md).

Fan-Out

Event → internal bus (Kafka, SQS, Postgres LISTEN/NOTIFY) → delivery worker per subscription. Never deliver synchronously from the originating handler. Outbox: write the event in the same DB transaction as the state change; a relay publishes to the bus.

Customer Controls

• Register endpoint URL; select subscribed event types.
• Delivery log with request + response bodies (retain 72 h); manual replay of any event.
• Test-mode ping on registration; pause/resume without losing queued events.

Testing

• Unit: HMAC sign/verify; retry backoff math.
• Integration: tunnel (ngrok, Hookdeck); emit a test event; assert signature verifies.
• Failure: 500 → retry fires; 410 → endpoint disables.
• Replay: disable, emit events, re-enable, assert ordered delivery of all missed events.

Guardrails

• Never log raw bodies beyond retention (PII risk); never deliver synchronously (latency cascades).
• Never reuse idempotency keys across event types; document retry and ordering guarantees explicitly.