Webhooks vs server-sent events (SSE)
Webhooks and server-sent events both push updates the moment something happens, but to different receivers. A webhook is an HTTP POST sent to another server's endpoint, one request per event. Server-sent events stream messages to a client that holds a connection open, usually a browser. Use webhooks to notify backends, SSE to feed live UIs.
How each one delivers an event
A webhook is initiated by the sender each time. Your system records an event, looks up the subscriber's registered endpoint URL, and sends a signed POST with the event as the body. The receiver is a server: it is always reachable at a stable URL, it acknowledges with a status code, and if it is down the sender retries later. Nothing stays open between events.
SSE inverts who connects. The client opens a GET request, the server responds with Content-Type: text/event-stream, and the connection stays open while the server writes messages into it as they occur. That model assumes a client that is present and connected: close the tab and delivery simply stops. There is no endpoint to call back later, which is why SSE has no equivalent of a retry queue.
That difference in receiver is the whole decision. A browser cannot expose a URL for a webhook to POST to, and another company's backend is not going to hold a streaming connection open to you around the clock. The transports are not interchangeable; they serve opposite sides of the stack.
The differences that matter in practice
Delivery guarantees look completely different. Because each webhook is a discrete request with a response, the sender knows whether it landed and can retry with backoff when it did not, alert the subscriber after repeated failures, and keep a delivery log per event. With SSE, the browser's EventSource reconnects automatically after a drop and can resume from the last message ID, but once nobody is listening, events are gone unless the server buffers them itself.
Security models differ too. Webhook receivers verify an HMAC signature on each request, since the endpoint is reachable by anyone on the internet. An SSE stream is just an authenticated HTTP response, so it inherits whatever cookies or tokens already protect the rest of your API.
Operationally, webhooks cost you a sending pipeline (queues, retries, key management), while SSE costs you held-open connections and proxy configuration: buffering has to be off, and HTTP/1.1's six-connections-per-domain cap argues for serving streams over HTTP/2.
Choosing between them, and using both
Ask who consumes the event. Another company's system, a partner integration, or any backend that must act on the event reliably: webhooks. Your own frontend showing live order status, notification badges, or streaming model output: SSE. If the client also needs to send data back constantly, neither fits and you want a socket; see WebSockets vs SSE for that comparison, or how ChatGPT streams responses for a production example.
Plenty of products end up with both, and they compose naturally: a webhook from a payment provider hits your backend, your backend records it, and an SSE stream pushes the update to the dashboard the user is watching. Each transport covers the hop the other cannot.
If webhooks are the side you are building, the pipeline behind them, retries, signatures, per-endpoint delivery logs, is what Svix provides as a service, so sending your first event is one API call rather than a queueing project.