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Unidirectional SSE (EventSource)

Server-Sent Events or EventSource is a well-known HTTP-based transport available in all modern browsers and loved by many developers.

Default unidirectional SSE (EventSource) connection endpoint in Centrifugo is:

/connection/uni_sse
info

Only parts of EventSource spec that make sense for Centrifugo are implemented. For example Last-Event-Id header not supported (since one connection can be subscribed to many channels) and multiline strings (since we are passing JSON-encoded objects to the client) etc.

Connect command​

Unfortunately SSE specification does not allow POST requests from a web browser, so the only way to pass initial connect command is over URL params. Centrifugo is looking for cf_connect URL param for connect command. Connect command value expected to be a JSON-encoded string, properly encoded into URL. For example:

const url = new URL('http://localhost:8000/connection/uni_sse');
url.searchParams.append("cf_connect", JSON.stringify({
    'token': '<JWT>'
}));

const eventSource = new EventSource(url);

Refer to the full Connect command description – it's the same as for unidirectional WebSocket.

The length of URL query should be kept less than 2048 characters to work throughout browsers. This should be more than enough for most use cases.

tip

Centrifugo unidirectional SSE endpoint also supports POST requests. While it's not very useful for browsers which only allow GET requests for EventSource this can be useful when connecting from a mobile device. In this case you must send the connect object as a JSON body of a POST request (instead of using cf_connect URL parameter), similar to what we have in unidirectional HTTP streaming transport case.

Supported data formats​

JSON

Options​

uni_sse​

Boolean, default: false.

Enables unidirectional SSE (EventSource) endpoint.

config.json
{
    ...
    "uni_sse": true
}

uni_sse_max_request_body_size​

Default: 65536 (64KB)

Maximum allowed size of a initial HTTP POST request in bytes when using HTTP POST requests to connect (browsers are using GET so it's not applied).

Browser example​

Coming soon.