cl-sse is an partial implementation of the Server Side Events protocol as per https://www.w3.org/TR/eventsource/. It is a toolkit for Common Lisp service implementations that want to send or receive events according to the protocol. There is a small demo you can try with a browser (instructions below).

The protocol originated for use in browsers, where most browsers support a javascript EventSource implementation that can be used to receive pushed event dispatches from a web server.

With appreciation to https://gist.github.com/jareware/aae9748a1873ef8a91e5 for a browser eventsource example.

The server.lisp module is for the push piece of the protocol. You could probably make do with format, but this piece ensures that protocols are observed if you aren't familiar with SSE, though it does differ in that it requires an event name/type where the spec consider this optional.

The client.lisp module is for CL services that want to receive SSE pushes. SSE clients outside of javascript don't seem to be very common, but it definitely has uses. For example a server might use SSE to receive job dispatches in a microservice architecture. In fact, if you were to use SSE for job dispatches, browsers could easily be workers.

I was tempted to have the event properties be nil, instead of empty strings, when they weren't in the event, however I opted instead to follow the spec a bit more closely, for no particularly good reason. As there are no users of this code it's something of a moot point, it was mostly for example purposes.

Shows SSE in action in a browser. Use as follows:

1. (ql:quickload :sse-demo)
2. (sse:demo/start-server) ; starts a hunchentoot server (in demo::http-server)
3. Connect a browser to localhost:8080/sse and watch the demo unfold.

By connecting to the SSE page, the browser will execute some javascript code that connects to SSE service on the /events endpoint, and responds to a loop emitting sse events coming from the server. After receiving 10 events the server hangs up, and the browser re-acquires the /events endpoing and it starts over gain.

4. When you're done, kill your browser tab, and (sse:demo/stop-server) to kill the web service.