API vs Webhook

Pull on demand vs push on event

APIs and webhooks are both ways for systems to communicate, but they differ in direction and timing. An API is a request-driven interface where you ask for data when you need it (pull model), while a webhook is an event-driven notification where a service pushes data to you when something happens (push model).

API

An API (Application Programming Interface) is a set of defined endpoints that allow one system to request data or actions from another. The client initiates each interaction by sending a request and receiving a response. REST and GraphQL are common API styles. APIs follow a synchronous, pull-based pattern: the caller decides when to fetch data and must poll if it wants to detect changes.

Webhook

A webhook is a user-defined HTTP callback. When a specific event occurs in a source system (a payment completes, a commit is pushed, a form is submitted), the source sends an HTTP POST request to a URL you have registered. Webhooks invert the communication pattern: instead of polling an API repeatedly, you receive data the moment it becomes available. They are sometimes called 'reverse APIs.'

Key Differences

- **Direction**: APIs are pull-based (client requests data). Webhooks are push-based (server sends data when an event occurs). - **Initiation**: With APIs, the client decides when to communicate. With webhooks, the server decides when to notify. - **Real-time**: Webhooks deliver data in near real-time. APIs require polling to approximate real-time behavior. - **Complexity**: APIs need request logic, error handling, and rate limiting on the client. Webhooks need an HTTP endpoint, signature verification, and retry handling on the receiver. - **Reliability**: API calls get immediate responses. Webhook deliveries can fail silently if the receiver is down, requiring retry mechanisms. - **Discovery**: APIs are documented with endpoints and schemas. Webhooks are configured per integration and often lack standardized discovery.

When to Use Each

**Use an API** when you need data on demand, want to query or search with specific parameters, need synchronous responses, or are building interactive user experiences. **Use a webhook** when you need real-time notifications of events, want to avoid the overhead of constant polling, are integrating with external services that support them (Stripe, GitHub, Slack), or building event-driven architectures.

Analogy

**An API** is like calling a restaurant to check if your table is ready: you call when you want, and they answer in the moment. **A webhook** is like giving the restaurant your phone number so they text you automatically when your table is ready: no repeated calls needed.