after_sending.js

// This version calls the _write callback after the data is sent.

Duplex = require('stream').Duplex;

var BCSocket = function(...) {
  Duplex.call(this, {objectMode: true});
  this.buffer = [];
};
BCSession.prototype = Object.create(Duplex.prototype);

BCSession._write = function(chunk, encoding, callback) {
  this.buffer.push(chunk);
  this.once("buffer flushed", callback);
};

exports.middleware = function(req, res) {
  // This is about 1000 lines more complex in practice, but you get the idea

  // Read messages & write reply
  this.push(req.body.messages);
  whenBufferNotEmpty(function() { // keep the request until we have data to send
    process.nextTick(function() {
      res.end(socket.buffer);
    });
  });

  this.emit("buffer flushed"); // call _write callback
};


//// malfunctioning user code

// In this version of events, only one message will ever be sent in any response.

// Eg, if you do this:
session.write("reply1");
session.write("reply2");
// -> only "reply1" will be sent on the next hanging GET, and "reply2" will be sent on the *next* get.

immediate.js

// This version calls the _write callback immediately

Duplex = require('stream').Duplex;

var BCSocket = function(...) {
  Duplex.call(this, {objectMode: true});
  this.buffer = [];
};
BCSession.prototype = Object.create(Duplex.prototype);

BCSession._write = function(chunk, encoding, callback) {
  this.buffer.push(chunk);
  callback();
};

exports.middleware = function(req, res) {
  // This is about 1000 lines more complex in practice, but you get the idea

  // Read messages & write reply
  this.push(req.body.messages);
  whenBufferNotEmpty(function() { // keep the request until we have data to send
    res.end(socket.buffer);
  });
};


//// malfunctioning user code

// If you do this, 'a' is never written.
session.end('a', function() {
  process.exit();
});


// If you do this, only one reply will be sent now.
session.on("readable", function() {
  data = session.read();
  session.write("reply1");
  session.write("reply2");
});