Using ENV files in production on Heroku

Question

I've got the following configuration line in my production.rb environment file, as instructed in this article:

  config.cache_store = :mem_cache_store, ENV["MEMCACHEDCLOUD_SERVERS"].split(','), { :username => ENV["MEMCACHEDCLOUD_USERNAME"], :password => ENV["MEMCACHEDCLOUD_PASSWORD"] }

But when I try to deploy, I'll get an error:

Running: rake assets:precompile rake aborted!
undefined method split' for nil:NilClass /tmp/build_abdc.../config/environments/production.rb:107:inblock in '

This is because config vars are not available during compilation. There's a Heroku labs add-on that you can supposedly use to remedy this, but it comes with a warning from Heroku that "Using this labs feature is considered counter to Heroku best practices."

So then what is the best practice when it comes to using ENV vars in production config? Should they just all be wrapped in rescue handlers so that Heroku ignores them during compilation?

Maybe you can try moving the setting to an initializer: http://stackoverflow.com/questions/5810289/setting-the-cache-store-in-an-initializer — steakchaser, Feb 09 '14 at 18:26
@steakchaser - thanks but that didn't change anything- the line still gets hit in compilation and raises same error — Yarin, Feb 09 '14 at 19:12
muha! i love the asset pipeline! did you try to set `initialize_on_precompile`? i don't know if that's still a thing in rails4 but it saved my ass several times on heroku http://stackoverflow.com/questions/11889131/precompile-failing-on-heroku-with-initialize-on-precompile-set-to-false — phoet, Feb 09 '14 at 21:25
are you sure that MEMCACHEDCLOUD_SERVERS exists in your environment? — rhernando, Feb 09 '14 at 21:30
@phoet - interesting article- so it says "Your app’s config vars are not present in the environment during slug compilation, so you should take steps to handle the nil case for config vars". And then it says about `initialize_on_precompile`: "In Rails 4.x this option has been removed and is no longer needed". Huh? — Yarin, Feb 09 '14 at 21:31
@rhernando - Yes, that's not the problem- the problem is that the environment doesn't exist during slug compilation. See [here](https://devcenter.heroku.com/articles/rails-asset-pipeline#failures-in-the-assets-precompile-task) — Yarin, Feb 09 '14 at 21:33

score 1 · Accepted Answer · edited Feb 13 '19 at 01:53

We ended up just checking for the ENV var before assigment. It looks like that's the pattern you need whenever you use ENV vars in config/initializers on Heroku:

  # NOTE: ENV vars aren't available during slug compilation, so we must check if they exist:
  if ENV["MEMCACHEDCLOUD_SERVERS"]
    config.cache_store = :mem_cache_store, ENV["MEMCACHEDCLOUD_SERVERS"].split(','), { :username => ENV["MEMCACHEDCLOUD_USERNAME"], :password => ENV["MEMCACHEDCLOUD_PASSWORD"] }
  end

See also: https://devcenter.heroku.com/articles/rails-asset-pipeline#failures-in-the-assets-precompile-task

Using ENV files in production on Heroku

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