Rails - Avoid simultaneous execution of controller action

Question

class ExampleController < ApplicationController

  def getAPI
      #retrieve and process data from API
  end

end

How can I avoid that the controller action gets executed simultaneous? Would a class variable work?

class ExampleController < ApplicationController
  @@in_progress = false

  def getAPI
      render and return if @@in_progress
      @@in_progress = true
      #retrieve and process data from API
      @@in_progress = false
  end

end

Are class variables persistent even if the applications runs with multiple processes of e.g. passenger. Is this a good idea at all?

Will this work if multiple users request the same controller action or does it only avoid simultaneous execution for one user?

Easy, it is enough to run just one single-threaded server worker to serve your app. Now all your requests are automatically serialized. — Sergio Tulentsev, Feb 05 '18 at 13:41
But the proper way would be to fix whatever is wrong when you have concurrent requests. — Sergio Tulentsev, Feb 05 '18 at 13:42
@SergioTulentsev the problem is, that I have to wait for the API until I can decide if I can continue or not. — 0lli.rocks, Feb 05 '18 at 13:53

score 0 · Answer 1 · answered Feb 05 '18 at 13:53

You have to take care of this problem with the right tools depending on many factors.

Is your code running on a single, multi-threaded process? Then mutexes could be helpful
Is your controller action not to be run concurrently because of database operations only? Then a transaction with a lock might be your solution
Is your code running on multiple processes, and maybe on different servers at the same time? Then an external lock should be considered (ex: https://redis.io/topics/distlock)

Edit: Please do not implement a mutex-like mechanism of your own (@@in_progress). There are tools that handle this stuff atomically and they are well-tested.

Edit2: If your controller action is to wait for a synchronous response from an API, this is a job to be run asynchronously. Consider using tools like Sidekiq.

Rails - Avoid simultaneous execution of controller action

1 Answers1