a rate-limiting work queue with foreground and background operations
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Gem Version


A thread-safe rate-limited work queue, with foreground and background operations


Add this line to your application's Gemfile:

gem 'throttle-queue'

And then execute:

$ bundle

Or install it yourself as:

$ gem install throttle-queue


Sometimes server resources are rate-limited. If you as a client exceed the server's limit, you become temporarily black-listed. A popular set of free APIs, which rhymes with Foogle Maps, has strict limits in place after which you start seeing 403s instead of 200s.

Let's say, for the above reason or another, you want to accumulate a cache of objects no faster than N times a second. You're happy to startup your app quickly and let your cache grow in the background. When a user requests a resource, you can bump it to the front of your queue and block until it is ready.


Create a queue and add background work

require 'throttle-queue'

q = ThrottleQueue.new 3
files.each {|file|
	q.background(file) {
		fetch file

Get user input and take action right away

q.foreground(user_file) {
	fetch user_file

Wait for everything to finish



Each resource is assumed to have a unique identifier, e.g. a filename or a reproducible hash value. The queue does not check if the resource exists first, but it will check if the id has already been queued. If an id is already in the queue and the same id is added as a background job, the block already in the queue is held onto; the new block is dropped. If an id is in the background queue, adding it to the foreground will cause the background block to be dropped. If an id is in the foreground queue, adding it again from another thread will cause the new block to be dropped, and the existing block to be waited upon.

Once an id has made it through the queue and been processed, the same id can be added again and will be blindly processed again. It is assumed the user of the object knows whether the resource already exists, and will decide whether or not a given id should be added.

Multiple Processes

Just bring in the multi-process-aware queue wrapper

require 'throttle-queue/multi-process'

q = ThrottleQueue::MultiProcess.new 3
files.each {|file|
	q.background(file) {
		fetch file

Use this queue the same way in each process. Make sure each queue id has the same meaning in each process (e.g. file paths should be absolute or relative to the same working directory). If your processes race to stack up a bunch of background work, the first process to add each work will be the only one to execute its block.

Do not fork after a multi-process queue is created. If your process forks itself, create each queue after the fork.


  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