How to fix "unknown role" errors in Capistrano recipes

When you have a complex recipe setup with multistage deployment you may run into this error:

Copy`role_list_from': unknown role `something' (ArgumentError)

Consider this task definition:

Copynamespace :foo do
  task :bar, :roles => :something do
    # do crazy stuff
  end
end

Whenever we call foo.bar in our recipe, Capistrano will fail if you deploy to a stage where none of the servers has the role the error complains about, "something" in this case.

However, you can hack around it Show archive.org snapshot :

1. Put a roles[:something] into your deploy.rb to avoid the above error message.\
   Now, when Capistrano enters a hook and tries to call a method that is only available for that group of servers, you'll get another error:

   Copy`foo.bar' is only run for servers matching {:roles=>:something}, but no servers matched
   

2. To properly fix it, you also need to avoid running any commands inside that task if no server matches the task's role:

   Copyroles[:something] # To make Capistrano aware of it
   # ...
   namespace :foo do
     task :bar, :roles => :something do
       next if find_servers_for_task(current_task).empty? # Skip for certain setups
       # do crazy stuff
     end
   end
   

As always, remember to properly hook.