that means i need to keep the connection open.
The question of the connection being open or not doesn't matter. You're going to need to have the connection open at the very latest you want to send a response from the server to the client.
and to send trigger back to the client with results ,
what about if i have timer that is starting at the client , should i start to calculate it in the server also ?
is there any danger of i keep open connection to the server all the time ? and do updaters?
is web sockets are good solution ?
Hmm. I thought I could decipher your post but I'm lost tbh.
If you're using HTTP, you should know HTTP is a stateless protocol. Revolving largely on POST and GET. http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol
Why don't you have the clients send a GET message and have the server send them the calculated result in the response?
What it it all boils down to is not letting the Clients alter the server in any way. They can request information but not alter it.