I recently set up a new Rails site on a Joyent Accelerator for a client.  The accelerator size selected had more than enough capacity, so we decided to set up the staging instance of the site on the same box.  There is a great set of instructions on the Joyent wiki for how to properly set up a Rails site that will be deployed via Capistrano to an Accelerator, so start there if you've never set up a site on Joyent before.  There are actually very few tweaks to those instructions you need to follow to set up multiple site instances on one Accelerator - in my case a "staging" and a "production" environment for a single application.

mongrel_cluster.yml
The default place to store this file is under /config.  We are actually going to set up a separate mongrel file for each environment, so create a file called mongrel_cluster_[environment].yml  (e.g. mongrel_cluster_staging.yml) under /config/accelerator/environments in your Rails application.  Set the number of servers, the port, and the environment variables as you would normally.  We'll be moving this file back to its expected location during the deployment process.

Now copy over the three files mentioned on the wiki into the /accelerator folder and make the needed changes from the instructions.  We'll just be adding a few additional modifications.

accelerator_tasks.rb
Since we have moved the location of the mongrel configuration file we need to let the "create_vhost" task know where to read in the YAML file from as it uses variables from that file to properly configure apache.  So update this line:

1  cluster_info = 
   YAML.load(File.read("config/accelerator/environments/mong
   rel_cluster_#{application}.yml"))

apache_vhost.erb
No changes needed.  W00t!

smf_template.erb
This file is the template used to set up all the dependant services that your Rails app will need.  The problem with the default version is that the mongrel service needs to have a distinct name for each application that is set up on the box otherwise bad conflicts happen.  (This is also mentioned on the Joyent wiki page.)  Update this line to make sure you have a distinct name for each environment:

1  <dependent name='mongrel_multi-user_<%= application %>' 
   restart_on='none' grouping='optional_all'>

Now we move onto the final step.  We have moved the location of the mongrel configuration file so that we can support multiples of that file in our source control system.  However, the mongrel service on the deployed server is still looking for that file in the old location with the old name.  So, we are going to add a simple task to deploy.rb to fix this at deploy time.  At the bottom of your file add a new custom task:

1  namespace :custom do
2    task :movemongrelconfig, :roles => :app do
3      # Move the env specific file to the general location
4      run "mv 
       #{release_path}/config/accelerator/environments/mongr
       el_cluster_#{application}.yml 
       #{release_path}/config/mongrel_cluster.yml"
5    end
6  end 

And call the new task at the end of the deployment process:

1  after :deploy, 'custom:movemongrelconfig'

I'm sure that there are other ways to get multiple site instances running, but this one was relatively simple and has been working well for us for some time now.