How to instantly notify logged in user of a short message

Short task description: I want one signed in user to be able to send an instant short plain text message to another signed in user. The solution needs to be easily scalable and not too resource demandinng in terms of bandwidth and server load (and $$).

The first idea was do client polling but this idea was quickly abandoned since it didn't meet scalability requirement. So, after that I went into research and came accross a number of concepts including sockets, node.js, xmpp. The amount of information is a bit overwhelming, so I was hoping for some advice to point me in the right directions. Hopefully something with readily available hosting solutions.

@epasca开发者_如何学Crello: thanks for quick response. I did, but not in detail. Before going in-depth into any technology, I want to be know that this is actually what I need.

Most of the examples concetnrate on instant chat but my requirements are somewhat different. I don't need every signed in user to see a message, but only one particular user, for whom it was meant, while there can be, say, 100 000 users logged in...

@Saeed Neamati: thanks! Yes, I pretty much understand the two client-server communication options and have come to the conclutions that the pulling is a no-go. What I am trying to find now is the most scalable (that's the main prerequisite) and (hopefully) easy to implement push option. For instance, the socket option is relatively easy but it seems like it's not going to scale well due to server overload (or am I wrong). The node.js (at least by concept description) should be better at that, but I wanted to get some confirmation to this assumption. With xmpp - I'm not even sure how relevant it is to my task and how to approach it.

@andyuk: Andy thanks, yes socket.io is also something that I came accross while doing research. As far as I understand it requires a server module that needs to run on a host. Do you know if possible to run on any server or do I need to look for a specialized hosting company? THe socket.io site for some reason doesn't work on my PC (neither IE or FF).

Did you look at the source code of nodejs chat.?

Look, you only have two options for client-server communication. Either client starts a request (an HTTP request on the web), which is called pull model (like client pulls the request out of the server), and server responds to that, or server starts a response directly without receiving any request (an HTTP response on the web), which is called push model (like the server pushes the data out to the client).

What you described as polling is actually the pull model, and indeed it takes lots of resources from the server.

But on the other hand, when you want to use push model, your server should know the client. In other words, we know that HTTP (based on TCP/IP) is a stateless protocol, which means that after each request, your connection is closed, and server loses you and forgets about you.

If you want the server to know the client, you should keep the connection open. This is usually done via some HTTP headers like Keep-Alive and Connection.

But to do that you should read Comet Programming. However, this reduces your scalability, because more connections are kept open for a one-to-one map between connection and client (To understand this better, you can think of connections as doors of the server. The more you occupy the door as a client, the less other clients can use it).

Checkout socket.io. If web sockets isn't supported by the browser it will fallback to the next best transport technology.

There is even a chat example included in the source code.

As for your concerns about scalability, node.js is perfect for this due to it's event driven, non-blocking nature. Handling many open connections is one of Node's real strengths.

Plurk uses Node.js for their real time chat features and they support 100k+ users.