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Simpler than actors, easier than threads. Get threaded!


Why wait? If you're doing IO in MRI, or really anything in JRuby you can speed up your programs dramatically by using threads. Threads however are a low level primitive in Ruby that can be difficult to use. The Threaded library implements a few common thread patterns into an easy to use interface. This lets you focus on writing your code and Threaded will worry about running that code as fast as possible.


In your Gemfile:

gem 'threaded'

Then run $ bundle install

Simple Promises

Throw tasks you want to get worked on in the background into a Threaded.later block:

promise = Threaded.later do
  require "YAML"
  YAML.load `curl 2>/dev/null`

Then when you need the value use the value method:

promise.value # => ["ruby-2.0.0", "ruby-1.9.3", # ...

It's secretly doing all of that work in the background letting your main Ruby thread focus on the work you care about most.

Keep your Promises

Promises will block when executed inside of one another, this means you can put promises in your promises and they'll always be executed in the correct order.

curl = Threaded.later do
  `curl 2>/dev/null`

yaml = Threaded.later do
  require "YAML"
  YAML.load curl.value

This code guarantees that the block in curl gets executed before the YAML.load. Of course, the outcome is the same:

yaml.value # => ["ruby-2.0.0", "ruby-1.9.3", # ...

While a contrived example, you can use this type of promise chaining to parallelize complex tasks.

By the way, if you call Threaded.later and never call value on the returned object it may run but is not guaranteed to. So if you value your "promises" then you'll always keep them.

Promise STDOUT behavior

A Threaded later block is supposed to look and feel like regular code. The biggest difference is that as soon as you define a Threaded.later {} block it begins to run in the background. Nothing throws off the illusion of "normal" code than sporadic random lines in your STDOUT. So by default Threaded captures all promise stdout and only outputs it when value is called.

task = Threaded.later do
  puts "HEY YOU GUYS!!!!"
  10 * 10

At this point the task has already run, but "HEY YOU GUYS!!!" is nowhere to be seen in STDOUT. It will show up as soon as you request the value

puts task.value
# => 100

If you don't like this voodoo and want to see really jumbled up STDIO, it's okay. You can set Threaded.sync_promise_io = false

task = Threaded.later do
  puts "HEY YOU GUYS!!!!"
  10 * 10
# => "HEY YOU GUYS!!!"
# => 100

You can also set this value in a config block.

Noisey Shell Sessions

Output from other shell sessions will still show up as soon as they execute.

Threaded.later do
  `git clone [email protected]:schneems/threaded.git`
=> #<Threaded::Promise:0x007fd51c0a9b98 @mutex=#<Mutex:0x007fd51c0a9b20>, @has_run=false, @running=true, @result=nil, @error=nil, @job=#<Proc:[email protected]/Users/schneems/Documents/projects/threaded/lib/threaded.rb:71>>
remote: Counting objects: 221, done.
remote: Compressing objects: 100% (130/130), done.
remote: Total 221 (delta 115), reused 188 (delta 83)
Receiving objects: 100% (221/221), 29.62 KiB | 0 bytes/s, done.
Resolving deltas: 100% (115/115), done.

You can get around this by redirecting the IO:

out = `git clone [email protected]:schneems/threaded.git 2>/dev/null`
puts out

In bash 1 is stdout, 2 is stderror. You can redirect a stream to another, for example to merge stderr into stdout you could run a command with 2>&1 at the end. To completely disregard output you can send it to /dev/null on unix systems.

Note: git does some weird things when you try to redirect stderr to stdio 2>&1 (

By using a library to direct the sub shell stdin, stdout, stderr like Open3:

require 'open3'
Open3.popen3("git clone [email protected]:schneems/threaded.git") do |stdin, stdout, stderr, wait_thr|

Or by running the command in a --quiet option if it has one:

`git clone [email protected]:schneems/threaded.git --quiet`

Background Queue

The engine that powers the Threaded promise is also a publicly available background queue! You may be familiar with Resque or sidekiq that allow you to enqueue jobs to be run later, threaded has something like that. The main difference is that threaded does not persist values to a permanent store (like Resque or PostgreSQL). Here's how you use it.

Define your task to be processed:

class Archive
  def, branch = 'master')
    repo = Repository.find(repo_id)

It can be any object that responds to call but we recommend a class or module which makes switching to a durable queue system (like Resque) easier.

Then to enqueue a task to be run in the background use Threaded.enqueue:

repo = Repo.last
Threaded.enqueue(Archive,, 'staging')

The first argument is a class that defines the task to be processed and the rest of the arguments are passed to the task when it is run.


The default number of worker threads is 16, you can configure that when you start your queue:

Threaded.config do |config|
  config.size = 5

Want a different logger? Specify a different Logger:

Threaded.config do |config|
  config.logger =

As soon as you call enqueue a new thread will be added to your thread pool if it is needed. If you wish to explicitly start all threads you can call Threaded.start. You can also inline your config if you want when you start the queue:

Threaded.start(size: 5, logger:

For testing or guaranteed code execution use the inline option:

Threaded.inline = true

This option bypasses the queue and executes code as it comes.

Thread Considerations

This worker operates in the same process as your app, that means if your app is CPU bound, it will not be very useful. This worker uses threads which means that to be useful your app needs to either use IO (database calls, file writes/reads, shelling out, etc.) or run on JRuby or Rubinius.

All other threading concerns remain true so be careful for using things like Dir.chdir inside of threaded as it changes the directory for all threads. Also don't modify shared data (unless you've got a mutex around it and know what you're doing).

It is possible for you to enqueue more things in your queue than can be processed before your program exits (you hit CTRL+C, or get an exception). When your program exits all jobs promises and enqueued jobs go away as they are not persisted to disk. If you care about your data getting run always call value on promises. When value is called on promises if it has not already started running it will be run immediately. This functionality allows for the chaining of promises.

To truly preserve data you've enqueued into Threaded's background queue you need to switch to a durable queue backend like Resque. Alternatively use a promise and call value on the Threaded.later promise object.