服务器发送事件，服务器端代码

Question

Implementing server-sent events is on the face of it quite a simple task. Borrowing the examples in the Mozilla documentation the client side code would be along the lines of

var evtSource = new EventSource("ssedemo.php");
evtSource.onmessage = function(e){//do stuff with e.data here}

What I have difficulty understanding is what happens server side. The things that puzzle me

• So you want to keep sending out events from ssdemo.php means that you need to run it in a loop and let it sleep when it isn't sending
• But by default Apache is setup to kill scripts that take "too long" to execute so this cannot be an infinite loop unless you setup that script run that way
• If I have 10 users who come in requesting the same SSE service (ssdemo.php) would it then mean that I would have 10 instances of that looped script?

I suspect that my understanding of how the server side code works, should be coded is either flawed or naive or both. I would much appreciate any pointers to the right way to do this.

Answer 1

Your understanding is correct. PHP needs to keep running, and in PHP you will need a loop, and you'll quickly run out of free Apache threads.

If you need to handle lots of connections you need to use event-based server like Node.js or Tornado that can handle lots of open connections.

If you'd rather use PHP, then a partial solution is to close connection after few seconds. The browser will reconnect, so you'll get a hybrid of polling and SSE.

In PHP you can check sys_getloadavg() to decide whether you can keep connection open or you're running short on free processes.