Capistrano 3: Run task only on a single server from a pool of servers assigned a role

capistrano, capistrano3

Solution

I assume that you are referring to your production configuration, with so many web servers. In this case, your `config/deploy/production.rb` probably contains many lines like this:

server 'web_1', roles: %w(web)
server 'web_2', roles: %w(web)
server 'web_3', roles: %w(web)
...

Simply make one of these servers primary, so it looks like:

server 'web_1', roles: %w(web), primary: true
server 'web_2', roles: %w(web)
server 'web_3', roles: %w(web)
...

Then change your task so it looks like this:

task :checkout_project_properties do
    on primary(:web) do
        execute("checkout-project-properties #{uc_stage} #{repo} #{branch}")
    end
end

Problem

I have 20 servers that are in the "web" role. I have a task I need to perform on only one of them as the change affects shared storage. My current solution is a hack to get around this (below). Looking for a better way, I don't have a ton of ruby or cap experience. ``` task :checkout_project_properties do num_runs = 0 on roles(:web), in: :sequence do if num_runs > 0 abort('Only running on one server. Exiting') end execute("checkout-project-properties #{uc_stage} #{repo} #{branch}") num_runs += 1 end end ```

Original source