Clear Memcached on Heroku Deploy

Question

What is the best way to automatically clear Memcached when I deploy my rails app to Heroku?

I\'m caching the home page, and when I make changes and redeploy, the pag

Answer 1

I just had this problem as well but wanted to stick to the git deployment without an additional script as a wrapper.

So my approach is to write a file during slug generation with an uuid that marks the current precompilation. This is impelmented as a hook in assets:precompile.

# /lib/tasks/store_asset_cacheversion.rake
# add uuidtools to Gemfile

require "uuidtools"

def storeCacheVersion
  cacheversion = UUIDTools::UUID.random_create
  File.open(".cacheversion", "w") { |file| file.write(cacheversion) }
end

Rake::Task["assets:precompile"].enhance do
  puts "Storing git hash in file for cache invalidation (assets:precompile)\n"
  storeCacheVersion
end

Rake::Task["assets:precompile:nondigest"].enhance do
  puts "Storing git hash in file for cache invalidation (assets:precompile:nondigest)\n"
  storeCacheVersion
end

The other is an initializer that checks this id against the cached version. If they differ, there has been another precompilation and the cache will be invalidated.

So it dosen't matter how often the application spins up or down or on how many nodes the worker will be distributed, because the slug generation just happens once.

# /config/initializers/00_asset_cache_check.rb

currenthash = File.read ".cacheversion"
cachehash   = Rails.cache.read "cacheversion"

puts "Checking cache version: #{cachehash} against slug version: #{currenthash}\n"

if currenthash != cachehash
  puts "flushing cache\n"
  Rails.cache.clear
  Rails.cache.write "cacheversion", currenthash
else
  puts "cache ok\n"
end

I needed to use a random ID because there is as far as I know no way of getting the git hash or any other useful id. Perhaps the ENV[REQUEST_ID] but this is an random ID as well.

The good thing about the uuid is, that it is now independent from heroku as well.

Answer 2

I deploy my applications using a bash script that automates GitHub & Heroku push, database migration, application maintenance mode activation and cache clearing action.

In this script, the command to clear the cache is :

heroku run --app YOUR_APP_NAME rails runner -e production Rails.cache.clear

This works with Celadon Cedar with the Heroku Toolbelt package. I know this is not a Rake-based solution however it's quite efficient.

Note : be sure you set the environment / -e option of the runner command to production as it will be executed on the development one otherwise.

Edit : I have experienced issues with this command on Heroku since a few days (Rails 3.2.21). I did not have time to check the origin the issue but removing the -e production did the trick, so if the command does not succeed, please run this one instead :

heroku run --app YOUR_APP_NAME rails runner Rails.cache.clear

Answer 3

25 Jan 2013: this is works for a Rails 3.2.11 app running on Ruby 1.9.3 on Cedar

In your Gemfile add the following line to force ruby 1.9.3:

ruby '1.9.3'

Create a file named lib/tasks/clear_cache.rake with this content:

if Rake::Task.task_defined?("assets:precompile:nondigest")
  Rake::Task["assets:precompile:nondigest"].enhance do
    Rails.cache.clear
  end
else
  Rake::Task["assets:precompile"].enhance do
    # rails 3.1.1 will clear out Rails.application.config if the env vars
    # RAILS_GROUP and RAILS_ENV are not defined. We need to reload the
    # assets environment in this case.
    # Rake::Task["assets:environment"].invoke if Rake::Task.task_defined?("assets:environment")
    Rails.cache.clear
  end
end

Finally, I also recommend running heroku labs:enable user-env-compile on your app so that its environment is available to you as part of the precompilation.

Answer 4

Aside from anything you can do inside your application that runs on 'application start' you could use the heroku deploy hooks (http://devcenter.heroku.com/articles/deploy-hooks#http_post_hook) that would hit a URL within your application that clears the cache

Answer 5

Since the heroku gem is deprecated, an updated version of Solomons very elegant answer would be to save the following code in lib/tasks/heroku_deploy.rake:

namespace :deploy do
    task :production do
        puts "deploying to production"
        system "git push heroku"
        puts "clearing cache"
        system "heroku run rake cache:clear"
        puts "done"
    end
end

namespace :cache do
  desc "Clears Rails cache"
  task :clear => :environment do
    Rails.cache.clear
  end
end

then instead of git push heroku master you type rake deploy:production in command line. To just clear the cache you can run rake cache:clear

Answer 6

What I ended up doing was creating a new rake task that deployed to heroku and then cleared the cache. I created a deploy.rake file and this is it:

namespace :deploy do

    task :production do
        puts "deploying to production"
        system "git push heroku"
        puts "clearing cache"
        system "heroku console Rails.cache.clear"
        puts "done"
    end

end

Now, instead of typing git push heroku, I just type rake deploy:production.