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Webhook vs API: what's the difference?

Both move data between two systems over HTTP, but they point in opposite directions. With an API, your application sends a request and waits for the answer. With a webhook, the other system sends the request to you, unprompted, the moment an event occurs. An API answers "what is true right now?"; a webhook tells you "this just changed."

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Who starts the conversation​

That difference in direction decides almost everything else. An API call is initiated by the client, so the client controls when it happens, how often, and what it asks for. Authentication runs in the familiar direction: you present a key or token, the server checks it, and you get a response you can retry immediately if it fails.

A webhook inverts all of that. The provider initiates the request against a webhook endpoint you registered, so you do not choose the timing and you cannot ask for anything. You get whatever the payload contains. Authentication also runs backwards: the sender proves who it is by signing the request, and you verify that signature before you trust the body. Retries are the sender's responsibility too, which is why a webhook provider's retry policy matters as much as its payload format.

Strictly speaking a webhook is not the opposite of an API, it is part of one. When a provider "sends you a webhook," it is calling an HTTP endpoint you built, so the provider is acting as the client and your service is the API. Some documentation calls this a push API or a reverse API. Same mechanism, different name.

What each direction is good at​

APIs are the right tool when your application decides it needs data: rendering a page, backfilling a report, looking up a record a user just searched for. They are also the only option when you need to write, since a webhook only carries notifications outward.

Webhooks are the right tool when the other system decides something happened and you would otherwise have no way to know. A payment settles, a build finishes, a customer cancels. Without webhooks the only alternative is polling the API on a timer, which either burns most of its requests on empty responses or leaves you minutes behind. Push delivery is also what makes event-driven architecture practical across company boundaries, where you cannot put both services on the same message broker.

Neither replaces the other. A typical Stripe integration reads customers and invoices through the REST API and learns about payment outcomes through webhooks. The webhook is the trigger; the API call that follows fetches whatever detail the payload left out.

The tradeoffs webhooks bring​

Receiving webhooks is harder than calling an API, and the difficulty is worth knowing before you commit to it.

  • You need a public URL. Your endpoint has to be reachable from the internet, which complicates local development and rules out services that sit entirely behind a firewall.
  • Delivery is best effort. If your endpoint is down when the event fires, the event is lost unless the sender retries, and retries eventually give up. Production receivers pair them with a dead letter queue.
  • Order is not guaranteed. Retries and parallel delivery mean a subscription.updated event can land before the subscription.created that preceded it, so use timestamps or IDs rather than arrival order.
  • Duplicates happen. A slow 2xx response looks like a failure to the sender, so the same event can arrive twice. Handling has to be idempotent.
  • Traffic arrives on the sender's schedule. A batch job on their side can turn into a spike on yours, and you cannot slow it down the way you can pace your own API calls.

An API call has none of these problems, which is the honest argument for polling in low-volume integrations. The argument stops working as soon as freshness matters or the number of objects you would have to poll grows.

Choosing between them​

Ask who knows first. If your application knows when it needs the data, call the API. If the other system knows and you would only find out by asking on a loop, take the webhook. For a stream of updates in both directions inside a single session, such as a live dashboard or a chat client, neither fits well and a WebSocket is the better answer.

Most integrations end up using both, and the work shifts to running the receiving side well: verifying signatures, absorbing spikes, retrying into your own systems, and keeping a record of what arrived. Svix Ingest handles that side, and if you are on the sending side, Svix delivers webhooks to your users with retries and signatures built in. Our what is a webhook lesson covers the mechanics in more depth.