BackboneSyncRails

Use this in a Rails 3.1 app. Right now the only supported pubsub messaging system is Faye http://faye.jcoglan.com/.

This assumes you already have a Backbone.js + Rails app.

0. Install the gem, say in your Gemfile:

   gem 'backbone_sync-rails', '~> 0.0.1'

1. Run a Faye server. It's pretty straightforward, check out example_faye/run.sh in this repo.

2. Tell your app where the faye server is. This may differ per Rails.env. For now, let's say we add config/initializers/backbone_sync_rails_faye.rb with:

   BackboneSync::Rails::Faye.root_address = 'http://localhost:9292'

3. Pull in the javascripts:

   //= require extensions/backbone.collection.idempotent
   //= require backbone_sync-rails/rails_faye_subscriber

4. Open a connection to Faye from your clients, somewhere on your page (in the layout?):

   <script type="text/javascript" src="<%= BackboneSync::Rails::Faye.root_address %>/faye.js"></script>
   

5. Observe model changes in Rails, and broadcast them. The gem provides the guts of an observer for you, so add a file like app/models/user_observer.rb:

   class UserObserver < ActiveRecord::Observer
     include BackboneSync::Rails::Faye::Observer
   end

   and enable it in config/application.rb like any good observer:

   module MyApp
     class Application < Rails::Application
       # snip...
   
       # Activate observers that should always be running.
       config.active_record.observers = :user_observer
   
       # snip...
     end
   end

6. Instantiate a new BackboneSync.RailsFayeSynchronizer for each instance of a Backbone collection you instantiate. You could do this in the collection's constructor, or do it by hand:

   // For simplicitly, here it is in a router, or app bootstrap
   this.users = new MyApp.Collections.UsersCollection();
   var fayeClient = new Faye.Client('<%= BackboneSync::Rails::Faye.root_address %>/faye');
   new BackboneSync.RailsFayeSubscriber(this.users, {
     channel: 'users', // Set to Rails model.class.table_name, or override Model.faye_channel
     client: fayeClient
   });
   this.wizards.reset(options.users);

7. Check it out! Open two browsers, do some stuff in one, and see your changes cascade to the other. Your Backbone views will need to observe events on the collection like change, add, and remove.

If you're on a version of Rails < 3.1, you'll probably have to copy some files into your app by hand, like the vendor/assets files. You'll probably have to require the lib/backbone_sync-rails/faye.rb file yourself, too.

I wrote an untested example application that uses CoffeeScript and the backbone-rails generators:

https://github.com/jasonm/wizards

In short, I augment the Backbone.Collection.prototype._add function so that adding multiple models to the same collection with the same id attribute (or your idAttribute-specified attribute of choice) will pass silently.

In long:

In a distributed messaging system, messages should be idempotent: this means that, for any message, an actor should be able to execute that message several times with no ill effect.

Why? Consider the following situation.

1. There are two clients, Alice and Bob.
2. Alice creates a new model in Backbone.
3. The server receives her request and persists it. It also distributes a "create" message to all subscribed clients. 4. Alice's new model is added to her local collection in the normal due course of Backbone.Model.prototype.save.
4. Bob receives the create message and creates a model in his local collection.
5. All is well until this point. Now, Alice receives the create message (she is subscribed just as Bob is) and creates a duplicate model into her collection.

There is actually a race condition in that Alice's HTTP request to create (and therefore her normal save()-based addition to the collection)_ may complete before or after the pubsub notification informs her collection to add a new member.

One approach to solving this would be for each update message to be tagged with its originating client, and for each client to filter out those messages. This would prove difficult, particularly since, in this implementation, the ActiveModel::Observer subclass is decoupled from the originating client.

The change made in vendor/assets/javascripts/extensions/backbone.collection.idempotent.js is to make Backbone.Collection.prototype.add idempotent with respect to the server-side id attribute, and neatly addresses the issue.

I'm more than happy to hear about better approaches from people with more experience in distributed messaging systems.

Copyright (c) 2011 Jason Morrison. See MIT-LICENSE for details.