Modify (on server) outgoing service message depending on incoming one

[eugene57]

Hello!

I'd like to set message.ext.token on client (for '/meta/handshake'
message) and somehow receive from server some response to this
particular token.
So, is it possible to add extra data to outgoing (from server) service
message (e.g. successful handshake-responce) depending on the
corresponding incoming message (e.g. handshake-request)? Or should i
try another approach?

[James Coglan]

For most messages this is possible, because of the following properties of Bayeux:

* If an incoming message has an id, the same id is echoed back

* If an incoming message has a clientId, the same clientId is echoed back

* Message IDs are unique per-client

Because of this, most messages are uniquely identifiable by their combined id and clientId. But /meta/handshake has no incoming clientId, and the id is likely to be "1" -- it's the first message the client sends.

However, because we know the server will echo the id, we can do this:

* Pick a new random key for each incoming message

* Append this key to the message's id

* Store the token from the message against this key

* Let the server process the message

* Extract the original id and the key from the outgoing message

* Restore the message's real ID

* Look up the token using the key

* Attach some function of the token to the outgoing message

In code this would look something like this:

var tokens = {};

var extension = {

  incoming: function(message, callback) {

    if (message.channel === '/meta/handshake') {

      var key = pickARandomString();

      tokens[key] = message.ext.token;

      message.id = message.id + ':' + key;

    }

    callback(message);

  },

  

  outgoing: function(message, callback) {

    if (message.channel === '/meta/handshake') {

      var parts = message.id.split(':');

      message.id = parts[0];

      var token = tokens[parts[1]];

      delete tokens[parts[1]];

      message.ext = message.ext || {};

      message.ext.tokenResponse = someFunctionOf(token);

    }

    callback(message);

  }

};