Web Browsers

Refer to the documentation for EventSource for information on how to use the EventSource API in your web application.

const sse = new EventSource("/sse");

sse.onmessage = (e) => {
  console.log(e.data);
};

sse.addEventListener("custom-event", (e) => {
  console.log(e.data);
});

Cross-Origin Requests

The server supports cross-origin requests via the Access-Control-Allow-Origin header. By default, it is set to * to allow all origins.

Cross-Origin requests are configurable with the --cors-* family of options.

NOTE: Access-Control-Allow-Credentials cannot be set to true if either of Access-Control-Allow-Origin or Access-Control-Allow-Headers is set to * (any).

Last-Event-ID

The native browser EventSource API does not support manually setting Last-Event-ID when establishing a new connection. It will set it automatically when reconnecting after a connection loss. However, it is often useful to set it manually to resume from a specific message ID or to "catch-up" any messages missed since the last connection. For this use-case, the server supports setting Last-Event-ID via the query parameter ?last_event_id=.

const sse = new EventSource("/sse?last_event_id=some-id");

When both the Last-Event-ID header and ?last_event_id= query parameter are present, the header takes precedence.

The server itself does not store any message history. Message history and "catch-up" functionality must be implemented in the Lua script via the catchup(sub, last_event_id) function.