What Are Webhooks and How They Work

Webhooks are a simple and efficient way for applications to communicate about events as they happen. Instead of a client repeatedly asking a server whether something changed (polling), the server sends an HTTP request — the webhook — to a URL you provide when a specified event occurs. Think of it as a push notification for machines: an ordered package of data is delivered to your specified endpoint automatically.

At a basic level, a webhook involves three components: the event source (the system where the event happens), the webhook payload (the data describing the event), and the webhook receiver (your endpoint that accepts and processes the incoming HTTP request). For example, when a customer places an order in an e-commerce platform, the platform can POST a webhook containing order details to a fulfillment system that immediately begins packing and shipping.

Why choose webhooks? They are:

• Real-time: Events are delivered as they happen so receivers can react quickly.
• Efficient: No repeated polling, which reduces unnecessary API calls and load.
• Simple: Based on standard HTTP; easy to implement and debug.

Common examples where webhooks shine:

• Source code hosting: Platforms like Git hosting services send webhooks when a push, pull request, or issue event happens so CI systems can run builds or notifications can be posted.
• Payments: Payment gateways POST webhooks to notify merchants of successful charges, disputes, or subscription changes.
• E-commerce: Order created, canceled, or refunded events trigger fulfillment or accounting workflows.
• Messaging and notifications: Chat platforms or CRM systems inform downstream apps of new messages or contact updates.

Typical webhook request details:

• HTTP method: Usually POST (sometimes PUT).
• Headers: Metadata such as content-type (JSON), an HMAC signature or API key to validate authenticity, and sometimes a timestamp.
• Payload: A JSON body describing the event — for example, order id, status, customer data, and timestamp.

Webhooks vs. polling: Polling asks “Has anything changed?” at intervals (e.g., every minute). Webhooks say “Here is what changed” immediately. Polling can be simpler for very small systems but becomes wasteful and slow as scale or latency requirements increase. Webhooks reduce latency and server load but require you to host a stable HTTP endpoint and do a bit more work on reliability and security.

Key considerations when using webhooks:

• Delivery guarantees: Most webhook providers implement retry logic if delivery fails, but your endpoint should be idempotent (able to safely process duplicate events) and respond quickly (typically within a few seconds) to avoid retries or timeouts.
• Security: Validate incoming requests. Common approaches include HMAC signatures (provider signs the payload; receiver verifies using a shared secret), token headers, TLS (HTTPS), and timestamp checks to avoid replay attacks.
• Schema stability and versioning: Payloads can evolve. Good providers version their webhook payloads or document changes and allow clients to opt into new versions so receivers don't break unexpectedly.
• Monitoring and logging: Because webhooks are asynchronous, failures can be subtle. Log deliveries and responses, and monitor metrics like failure rate, latency, and retries so you can detect and respond to problems.

Real-world example: Git hosting service and CI integration. When you push code, the hosting service sends a webhook to the continuous integration system with details about the commit. The CI server verifies the signature, pulls the relevant repository state, and starts a build. The webhook-based approach avoids repeatedly checking the repo for changes and ensures near-instant build triggering.

Limitations to be aware of:

• Endpoint availability: Your service must be reachable by the provider over the internet (or via a private connection if supported). Development endpoints behind NAT or on localhost need tunneling tools to receive real webhooks.
• Rate limits and batching: High-volume event sources can generate many webhooks; providers may batch events or rate-limit deliveries. Design your processing to handle bursts gracefully.
• Payload size limits: Providers often limit the size of webhook payloads, and some will include references (IDs) instead of full objects to reduce size.

Getting started with webhooks is approachable: pick a provider that supports webhooks, create a receiver endpoint (an HTTPS URL), configure the events you want, and implement validation and idempotent handling. Use tooling like request inspectors or local tunnels during development, and add monitoring and alerting for production reliability.

In short, webhooks are a powerful, lightweight pattern for event-driven integrations that enable real-time automation across distributed systems. When designed with security, reliability, and observability in mind, they dramatically simplify how services communicate about changes and trigger workflows.