Okay , this is a different kind of question

Suppose a server has connections to 5000 clients,
and it has sent them data . How does the server keep track of which clients to refresh when new data arrives .. or more specifically how do the clients know that the server has new data without the server clogging the network , i.e without sending any message....

I know there is a solution by the clients hooking onto events , but any information , url , would be greatly , deeply appreciated!