RequestLocals

If you have ever needed to use a global variable in Rails, you know it sucks.

One common solution is the awesome request_store:

def self.foo
  RequestStore.fetch(:foo) { 0 }
end

def self.foo=(value)
  RequestStore.store[:foo] = value
end

request_store scopes values to the execution context handling the request. When work moves to another thread, that thread accesses a different store.

Add this line to your Gemfile:

gem 'request_store_rails'

And change the code to this:

def self.foo
  RequestLocals.fetch(:foo) { 0 }
end

def self.foo=(value)
  RequestLocals.store[:foo] = value
end

Everywhere you used RequestStore.store, change it to RequestLocals.store. Now your variables will actually be stored in a true request-local way.

The middleware stores each request's context in ActiveSupport::IsolatedExecutionState, the same mechanism used by Rails' CurrentAttributes. Existing RequestLocals usage automatically follows the thread or fiber isolation level configured by Rails, without application changes.

When explicitly spawning a new thread, propagate the current store id with RequestLocals.set_current_store_id so the new thread can access the same request-local values.

A good way to apply this pattern is by encapsulating it into a helper class:

# Public: Custom thread class that allows us to preserve the request context.
class ThreadWithContext

  # Public: Returns a new Thread that preserves the context of the current request.
  def ThreadWithContext.new(*args)
    store_id = RequestLocals.current_store_id
    Thread.new {
      RequestLocals.set_current_store_id(store_id)
      yield *args
    }
  end
end

RequestLocals[:foo] = 1

ThreadWithContext.new {
  puts RequestLocals[:foo] # => 1
}

The gem does not provide such construct to avoid name collisions, you are free to reuse the snippet above and adjust it to match your use case.

If you are feeling adventurous, you could try using this fire and forget script and make all of your threads request aware, or should I say prepend and forget 😄? Probably not something to be used in a production environment, but whatever floats your boat ⛵

Have in mind that the RequestLocals.fetch(:foo) { 'default' } operation is atomic, while RequestLocal[:foo] ||= 'default' is not. In most scenarios, there is not a lot of difference, but if you are in a concurrent environment make sure to use the one that is more suitable for your use case 😉

While the plan is not to achieve 100% compatibility, this gem usually works well as a drop-in replacement. If you are using gems that rely on RequestStore but for some reason you need them to use the appropriate request/thread scope, you can try something like this on application.rb or similar:3

if RequestStore != RequestLocals
  RequestStore::Railtie.initializers.clear
  Kernel.suppress_warnings { RequestStore = RequestLocals }
end

If your code depends on these global variables, it's likely that you'll need to avoid collisions in Sidekiq workers (which would happen if the current store id is nil).

You can use the following middleware, using the job id to identify the store:

class Sidekiq::Middleware::Server::RequestStoreRails
  def call(_worker, job, _queue)
    RequestLocals.set_current_store_id(job['jid'])
    yield
  ensure
    RequestLocals.clear!
    RequestLocals.set_current_store_id(nil)
  end
end

Make sure to configure it as server middleware:

Sidekiq.configure_server do |config|
  config.server_middleware do |chain|
    chain.add Sidekiq::Middleware::Server::RequestStoreRails
  end
end

The inspiration for this gem, tests, and a big part of the readme were borrowed from the really cool request_store gem. Thanks Steve 😃

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.

Copyright (c) 2015 Máximo Mussini

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