Node server application invoking client callback ignored

Question

I'm trying to run an external command from node on a server and have the output of that command sent back to the client line by line. That is, I want to send the output as it is produced, not when the command has finished.

The server side code works fine -- the command is run and the output is read line by line. I'm passing a client callback to the server to call when new data arrives. This works for the first line of output, but not for any subsequent lines. I have verified that the server indeed receives subsequent lines of output, they just never arrive at the client.

The server code looks like this (simplified).

var spawn = require('child_process').spawn;

function run(cmd, cb) {
  var child = spawn(cmd, []),
  child.stdout.on('data', function(data) { cb(String(data)); });
}

socket.on("foo", function(cb) { return run("ls", cb); });

The client side uses this code (again simplified).

socket.emit("foo", function(data) {
  console.log(data);
});

I do not get any error messages on the server or client side. Any insight into this would be appreciated.

Update

I solved this issue by using the socket explicitly to send data back to the client. For reference, the server code now looks like this.

function run(cmd, cb) {
  var child = spawn(cmd, []),
  child.stdout.on('data', function(data) {
    socket.emit("output", String(data));
  });
}

And the client.

socket.emit("foo", function(data) {
  //
});
socket.on("output", function(data) {
  console.log(data);
});

Answer 1

Since a callback has to be interpreted by the client and actually sent to the server, you cannot call a callback twce due to the way Socket.IO is written. Here's the official statement:

An acknowledgement function can only be called once.

Read here for more information.