Rails asset precompilation happening even if there is no change in assets - ruby-on-rails

Question

I am running a rails stack on AWS Opsworks. In before_symlink.rb deploy hook, I have the following code.
rails_env = new_resource.environment["RAILS_ENV"]
shared_path = "#{new_resource.deploy_to}/shared"
# create shared directory for assets, if it doesn't exist
directory "#{shared_path}/assets" do
mode 0770
action :create
recursive true
not_if do
Dir.exists?("#{shared_path}/assets")
end
end
# symlink current deploy's asset folder to shared assets each deploy
link "#{release_path}/public/assets" do
to "#{shared_path}/assets"
end
# precompile assets into public/assets (which is symlinked to shared assets folder)
execute "rake assets:precompile" do
cwd release_path
command "bundle exec rake assets:precompile --trace"
environment 'RAILS_ENV' => rails_env
end
The problem is that assets are precompiled all the time even if there is no change, inspite of being in a symlinked shared folder. I am running on Rails 4.1.2 and I guess Rails is intelligent enough to compile only changed assets after Rails 4? Why is this happening?
UPDATE
This is now solved.
I missed out symlinking /tmp/cache to /shared/tmp/cache. This is where sprockets stores cached files.
Updated code.
# Precompile assets. Assets are compiled into shared/assets and shared between deploys.
rails_env = new_resource.environment["RAILS_ENV"]
shared_path = "#{new_resource.deploy_to}/shared"
# create shared directory for assets, if it doesn't exist
directory "#{shared_path}/assets" do
mode 0770
action :create
recursive true
not_if do
Dir.exists?("#{shared_path}/assets")
end
end
#create shared directory to store sprockets cache
directory "#{shared_path}/tmp/cache" do
mode 0770
action :create
recursive true
not_if do
Dir.exists?("#{shared_path}/tmp/cache")
end
end
# symlink current deploy's asset folder to shared assets each deploy
link "#{release_path}/public/assets" do
to "#{shared_path}/assets"
end
# symlink current deploy's sprockets cache folder to shared cache folder on each deploy
link "#{release_path}/tmp/cache" do
to "#{shared_path}/tmp/cache"
end
# precompile assets into public/assets (which is symlinked to shared assets folder)
execute "rake assets:precompile" do
cwd release_path
command "bundle exec rake assets:precompile --trace"
environment 'RAILS_ENV' => rails_env
end

Answer 1

OpsWorks rebuilds your entire rails app upon each deployment and keeps a number of backup copies. It should create it in a path like:
/srv/www/yourappname/current
You can confirm this by ssh into your server. If you go to path
ls -la /srv/www/yourappname/releases
You should see a number of folder with datetime stamps names.
If you look at contents they will each contain your entire rails app.
This makes it easy to roll back if something goes wrong during deployment.
Since each folder is basically a new install of your app, asset pre-compilation needs to happen otherwise the current version won't have any assets. This type of deployment doesn't just have the same directory do a
git pull origin master
UPDATE: It looks like this was an issue with Sprockets that was resolved in this pull request so make sure your gem sprockets version includes this commit. It looks like it was included in v4.0.0beta1 & v4.0.0beta2 so try that or if not, try downgrade to >=2.12.4 as per this comment