web, the server updates the client on event: how?

[DiTBho]

Usually the web server serves a page at the client's request.
I need the server to update the client in case of an event.

how? 

[sleemanj]

Usually the web server serves a page at the client's request.

The simplest way is to poll for updates.

But you can also look to Push notifications, or Websockets.

[Nominal Animal]

I'd recommend Websockets.  The idea is that the page contains JavaScript that connects to the server using Websockets, and assigns a function to handle the MessageEvents for that socket.  That way, whenever the server sends something, that Javascript handler gets called.

See the link for further info, guides, and so on.