A webhook is an HTTP callback: one system sends your endpoint an event payload when something happens.
Instead of repeatedly asking an API “anything new?”, your app receives pushed events as they occur.
Why teams use webhooks
Webhooks are useful when you need near-real-time reactions without heavy polling.
Common examples:
- payment succeeded/failed,
- user signed up or changed plan,
- email delivered/bounced/complained,
- document processed and ready,
- CI/CD pipeline status changes.
Webhook vs polling API
| Approach | How it works | Typical tradeoff |
|---|---|---|
| Polling | Your app requests updates repeatedly | Simpler control flow, higher API/load cost |
| Webhook | Provider pushes updates to your endpoint | Lower latency, requires robust receiver design |
For event-driven systems, webhooks usually reduce latency and infrastructure overhead.
How webhooks work (end-to-end)
- You register a callback URL with a provider.
- A business event occurs in the provider system.
- Provider sends an HTTP request (often
) to your callback URL. - Your endpoint validates, records, and processes the payload.
- Your endpoint returns success/failure status so the provider can decide retry behavior.
The most reliable consumers acknowledge quickly and process asynchronously.
Webhook payloads and delivery semantics
Most providers do at-least-once delivery, which means duplicates can occur.
Design implications:
- make handlers idempotent,
- use event IDs for deduplication,
- tolerate out-of-order delivery,
- store raw payloads for replay and audit.
Security checklist for webhooks
- Verify provider signatures (HMAC/JWT or equivalent).
- Enforce HTTPS and reject insecure origins.
- Validate timestamp to prevent replay attacks.
- Restrict inbound source ranges if provider supports it.
- Never trust payload fields without schema validation.
“Secret URL only” is not enough for production security.
Reliability checklist
- Return 2xx quickly after minimal validation.
- Push heavy work to queue/job worker.
- Implement dead-letter and replay path.
- Track error rate, retry depth, and processing lag.
- Keep a manual reprocess tool for incident response.
Webhooks for email workflows
Email systems use webhooks for delivery observability and automation:
- delivered,
- bounced,
- complaint,
- opened/clicked (where supported),
- inbound message received.
That enables stateful workflows like suppression lists, escalations, and customer-visible status updates.
Related guides:
Testing webhook integrations safely
A robust test setup should validate:
- signature verification logic,
- duplicate event handling,
- timeout/retry behavior,
- schema evolution compatibility.
For email events, MailSlurp can generate realistic message and delivery events so teams can test webhook consumers before production release.
Anti-patterns to avoid
- synchronous downstream API calls inside webhook request path,
- no deduplication key strategy,
- treating missing events as impossible,
- no replay tooling during incidents.
Final take
Webhooks are a powerful event integration pattern, but only when handlers are built for security, retries, and idempotency. Treat the webhook consumer like production infrastructure, not a thin demo endpoint.