RequestLocals

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

One of the usual tricks is to go for Thread.current, or if you have done your homework, to use the awesome request_store.

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

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

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

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

• Using Thread.current, values can stick around even after the request is over, since some servers have a pool of Threads that they reuse, which can cause bugs.

• Using request_store, the storage is not actually request local. Variables are stored in Thread.current, except that the storage is cleared after each request. However, this does not work when you need to use multiple threads per request, different threads access different stores.

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

Oh yeah, everywhere you used Thread.current or RequestStore.store just change it to RequestLocals.store. Now your variables will actually be stored in a true request-local way.

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 RequestStoreRails::Middleware

The middleware in the gem sets a thread-local variable :request_store_id in Thread.current for the main thread that is executing the request.

If you need to spawn threads within a server that is already using thread-based concurrency, all you need to do is to make sure that the :request_store_id variable is set for your threads, and you will be able to access the RequestLocals as usual.

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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