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Webhook vs Callback: what's the difference?

A callback is a function you hand to other code so it can run it later, inside a single program. A webhook is an HTTP request one service sends another when an event happens. Both are ways of saying "tell me when this is done," but a callback stays in your process and a webhook crosses the network.

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What a webhook does​

Webhooks are a method of communication between two applications or services, where one service sends an HTTP request to another when a specific event occurs. The receiving service, also known as the subscriber, registers a webhook endpoint that listens for incoming requests. Webhooks are often used in event-driven architectures and are an efficient way to keep systems in sync, as they enable real-time updates and notifications.

What a callback does​

Callbacks, on the other hand, are functions that are passed as arguments to another function and are executed at a later time or upon the completion of a specific task. In the context of programming, callbacks are often used for asynchronous operations, such as handling the response from an API request or processing user input in a graphical user interface.

The two meanings of "callback"​

The word carries a second, unrelated sense that causes most of the confusion here. In OAuth flows and payment redirects, a "callback" means a callback URL: an address you register with a provider so it can send the user, or a one-off result, back to you over HTTP. That kind of callback is much closer to a webhook than to a language-level callback function. The practical difference is scope and lifetime. A callback URL usually completes one transaction for one user and then stops mattering; a webhook endpoint is a standing subscription that keeps receiving events for as long as it is registered.

How they compare​

Both webhooks and callbacks handle events, and both involve one component telling another that something changed. The differences that matter in practice:

  1. Communication method: Webhooks use HTTP requests for communication between services, whereas callbacks involve passing a function as an argument within the same application or programming environment.
  2. Scope: Webhooks are generally used for communication between separate applications or services, often across different domains or environments. Callbacks are used within a single application or environment, usually for managing asynchronous operations or handling user interactions.
  3. Language and platform: Webhooks are language-agnostic and can be used for communication between services written in different programming languages or running on different platforms. Callbacks are typically language-specific, as they involve passing a function within the same programming environment.
  4. Failure handling: A callback either runs or it does not, and the surrounding code sees the error immediately. A webhook can fail on the wire, so the sender owns a retry policy and the receiver has to expect duplicates.

Choosing between them​

If both pieces of code run in the same process, you want a callback: it is faster, simpler, and the error path is visible to the caller. If the two sides are separate services, especially across an organizational boundary, you want a webhook, and with it the delivery concerns a network hop brings. Svix handles that side for teams sending webhooks to their own users, with signatures, retries, and delivery logs built in.