I've built this before, but it's not entirely simple.
The first step is to add an API that returns the git SHA of the current revision of code deployed. For example, you deploy AAAA. Now you deploy BBBB and that will be returned. For example, let's assume you added the api "/checks/version" that returns the SHA.
Here's a sample Rails controller to implement this API. It assumes capistrano REVISION file is present, and reads current release SHA into memory at app load time:
class ChecksController
VERSION = File.read(File.join(Rails.root, 'REVISION')) rescue 'UNKNOWN'
def version
render(:text => VERSION)
end
end
You can then poll the local unicorn for the SHA via your API and wait for it to change to the new release.
Here's an example using Capistrano, that compares the running app version SHA to the newly deployed app version SHA:
namespace :deploy do
desc "Compare running app version to deployed app version"
task :check_release_version, :roles => :app, :except => { :no_release => true } do
timeout_at = Time.now + 60
while( Time.now < timeout_at) do
expected_version = capture("cat /data/server/current/REVISION")
running_version = capture("curl -f http://localhost:8080/checks/version; exit 0")
if expected_version.strip == running_version.strip
puts "deploy:check_release_version: OK"
break
else
puts "=[WARNING]==========================================================="
puts "= Stale Code Version"
puts "=[Expected]=========================================================="
puts expected_version
puts "=[Running]==========================================================="
puts running_version
puts "====================================================================="
Kernel.sleep(10)
end
end
end
end
You will want to tune the timeouts/retries on the polling to match your average app startup time. This example assumes a capistrano structure, with app in /data/server/current
and a local unicorn on port 8080
.