Ensures DB migration only happens on first instance (#248)

lib/tasks/cf.rake

@@ -0,0 +1,8 @@
+namespace :cf do
+  desc "Only run on the first application instance"
+  task on_first_instance: :environment do
+    # We expect this information to be always available or break otherwise
+    instance_index = JSON.parse(ENV["VCAP_APPLICATION"])["instance_index"]
+    exit(0) unless instance_index.zero?
+  end
+end

manifest.yml

@@ -1,10 +1,10 @@
 ---
 defaults: &defaults
   buildpacks:
-    - https://github.com/cloudfoundry/ruby-buildpack.git
+    - ruby_buildpack
   processes:
     - type: web
-      command: rake db:migrate && bin/rails server
+      command: bundle exec rake cf:on_first_instance db:migrate && bin/rails server
       instances: 2
       memory: 512M