Rails Disco - A distributed party with commands, events and projections
Rails Disco is based on Ruby on Rails and makes event sourcing easy. Greg Young showed the advantages of event sourcing multiple times. GOTO Conference Talk (Chicago 2014)
Rails Disco consists out of three main parts: commands, events and projections.
Commands will be created and executed by actions of your controller, instead of directly manipulating your model. These commands are only the order to do something and after possible validations, the framework executes them by creating an event and finally manipulating the model.
The events will be all stored in a separate database and also published to all projections, where they can be processed to update the projections model/database
Finally projections are your representation of your data, they get the events and process them, to get the needed information for building up their models.
At the moment Rails Disco uses Rails 4. Maybe it works with Rails 3.2, but we didn't test that.
Any Server which is capable of streaming, e.g. puma or thin (standard Rails server WEBrick will not work). If you are facing problems installing puma on Windows, here is a tutorial.
Install Rails Disco at the command prompt
gem install rails-disco
At the command prompt, create a new Rails Disco application.
disco new myapp
myappis the name of you application.
(Note: You can also add Rails Disco to an existing Rails application. Simply omit the application name and run the command inside your application.)
Change directory to
myappand migrate the databases:
cd myapp rake disco:migrate:setup
This will operate on the Rails (= projection) and the domain database. You can configure the domain database and more Rails Disco related configurations in
If you just want to look a some standard server output, start the disco server (Remember to use a server which is capable of streaming, which means not WEBrick). Else go ahead and skip this point.
This will start the domain, the projection and the web server, but you won't see much of the disco yet.
For a humble start, let's create the scaffold for a simple blog system:
disco generate scaffold Post title:string text:text
The syntax is leaned to Rails' generate style and it basically creates a resource Post with a title and a text attribute.
Now that we have something to rely on, lets migrate and see it in action:
rake disco:migrate disco server
Go to http://localhost:3000/posts and you'll see an empty list of our posts with a link to create a new one. Go ahead and create one. If you watch the console output, you can see that an event is created, published and processed by a projection.
If you look at your databases, you see in both of them a table
posts, which contains your freshly created post. The domain database also contains a table
domain_events. There you find an event for your post creation. Lets see this in action.
Clear your projection database and restart the server.
rake disco:db:drop rake disco:migrate disco server
You will see some console output, the projection requests the missing posts from the domain. Finally the state of your projection database will be restored.