ioquatix / request_store

Per-request global storage for Rack.

RequestStore

Ever needed to use a global variable in Rails? Ugh, that's the worst. If you need global state, you've probably reached for Thread.current. Like this:

def self.foo
  Thread.current[:foo] ||= 0
end

def self.foo=(value)
  Thread.current[:foo] = value
end

Ugh! I hate it. But you gotta do what you gotta do...

The problem

Everyone's worrying about concurrency these days. So people are using those fancy threaded web servers, like Thin or Puma. But if you use Thread.current, and you use one of those servers, watch out! Values can stick around longer than you'd expect, and this can cause bugs. For example, if we had this in our controller:

def index
  Thread.current[:counter] ||= 0
  Thread.current[:counter] += 1

  render :text => Thread.current[:counter]
end

If we ran this on MRI with Webrick, you'd get 1 as output, every time. But if you run it with Thin, you get 1, then 2, then 3...

The solution

Add this line to your application's Gemfile:

gem 'request_store'

And change the code to this:

def index
  RequestStore.store[:foo] ||= 0
  RequestStore.store[:foo] += 1

  render :text => RequestStore.store[:foo]
end

Yep, everywhere you used Thread.current just change it to RequestStore.store. Now no matter what server you use, you'll get 1 every time: the storage is local to that request.

API

The fetch method returns the stored value if it already exists. If no stored value exists, it uses the provided block to add a new stored value.

top_posts = RequestStore.fetch(:top_posts) do
  # code to obtain the top posts
end

Rails 2 compatibility

The gem includes a Railtie that will configure everything properly for Rails 3+ apps, but if your app is tied to an older (2.x) version, you will have to manually add the middleware yourself. Typically this should just be a matter of adding:

config.middleware.use RequestStore::Middleware

into your config/environment.rb.

No Rails? No Problem!

A Railtie is added that configures the Middleware for you, but if you're not using Rails, no biggie! Just use the Middleware yourself, however you need. You'll probably have to shove this somewhere:

use RequestStore::Middleware

No Rails + Rack::Test

In order to have RequestStore storage cleared between requests, add it to the app:

# spec_helper.rb

def app
  Rack::Builder.new do
    use RequestStore::Middleware
    run MyApp
  end
end

Using with Sidekiq

This gem uses a Rack middleware to clear the store object after every request, but that doesn't translate well to background processing with Sidekiq.

A companion library, request_store-sidekiq creates a Sidekiq middleware that will ensure the store is cleared after each job is processed, for security and consistency with how this is done in Rack.

Semantic Versioning

This project conforms to semver. As a result of this policy, you can (and should) specify a dependency on this gem using the Pessimistic Version Constraint with two digits of precision. For example:

spec.add_dependency 'request_store', '~> 1.0'

This means your project is compatible with request_store 1.0 up until 2.0. You can also set a higher minimum version:

spec.add_dependency 'request_store', '~> 1.1'

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

Don't forget to run the tests with rake.