Verify that the client has received the message from the server

Question

In a chat service that I'm building, I need to send messages directly from the server.

I have found no solution, there is an example in the documentation:

// SERVER
io.sockets.on('connection', function (socket) {
    socket.on('ferret', function (name, fn) {
        fn('woot');
    });
});

// CLIENT
socket.on('connect', function () { // TIP: you can avoid listening on `connect` and listen on events directly too!
    socket.emit('ferret', 'tobi', function (data) {
        console.log(data); // data will be 'woot'
    });
});

but does the opposite of what I need!

I need to do the emit from the server and receive a confirmation of receipt from the client!

Is there a way to do this?

Answer 1

there is no guarantee as the connection can be killed before the servers message reaches the client. thus there is also no event on the server like "clientGotMessage". if a message MUST reach the user there is no other way than to tell the server that you received the message on the client.

You can do this 'easy' by sending a number down. client and server keep track of that number. each time the server sends, it counts up, each time the client receives, it counts up. When the client sends something, it sends the number, so the server will see if the client has everything. If the client missed a message, the next message will have a number that the client wont accept and request the lost message from the server.