HashQueue

Simple namespaced Queue for highly concurrent environments. Maintains separate queues for different keys accessible from numerous threads without a trouble. Think of it as an extension to stdlib's Queue class. It could be even used as its replacement. Features nice locking capabilities, read on.

Add this line to your application's Gemfile:

gem 'hash_queue', '~> 0.1'

And then execute:

$ bundle

Or install it yourself as:

$ gem install hash_queue

For those living on the edge:

gem 'hash_queue', git: 'git@github.com:mikekreeki/hash_queue.git'

..with Bundler 1.1 or newer:

gem 'hash_queue', github: 'mikekreeki/hash_queue'

To initialize new HashQueue just simply do:

require 'hash_queue'

hash_queue = HashQueue.new

To queue stuff it's just as easy as:

hash_queue[:my_queue].queue Stuff.new

# or

hash_queue.queue :my_queue, Stuff.new

You can also queue multiple items in a single method call:

hash_queue[:my_queue].queue_many :foo, :bar, :xyz

# Ruby itself provides some handy magic syntax for expanding arrays into arguments. The result is identical.
stuff = [:foo, :bar, :xyz]
hash_queue[:my_queue].queue_many *stuff

Keys (or namespaces if you prefer) can be anything you want. Usually those will be symbols or strings but don't need to be. Objects, classes, numbers or even true or nil will work. Same applies for queued items (but beware if you don't pop with :size option, you logically won't be able to distinguish if nil you get back is nil you queued there or just null return value of popping from an empty queue)

#queue is aliased as #enqueue, #push and #<< for convenience. #queue_many is aliased as #enqueue_many and #push_many accordingly.

HashQueue instances offer some handy methods.

Returns combined sizes of all queues managed.

hash_queue = HashQueue.new
hash_queue.queue :animals, "tenderlove's cat"
hash_queue.queue :favourite_rubies, 'rubinius'
hash_queue.size  # => 2

Also aliased as #count and #length.

Returns true or false whether HashQueue instance is empty or not.

hash_queue = HashQueue.new
hash_queue.empty? # => true
hash_queue.queue :animals, "tenderlove's cat"
hash_queue.empty? # => false

Removes all managed queues.

hash_queue = HashQueue.new
hash_queue.queue :animals, "tenderlove's cat"
hash_queue.empty? # => false
hash_queue.clear
hash_queue.empty? # => true

When using lots of different keys (e.g. hostnames) your HashQueue instance will eventually grow in size. #clean removes empty queues to save precious RAM and speed things up. It's just a maintenance method, it won't break anything, you can use individual queues later the same way you're doing now, they will be automatically redeclared when you touch them.

Returns maintained keys.

hash_queue = HashQueue.new
hash_queue.queue :foo, 'Now or never.' 
hash_queue.keys # => [:foo]

Not very interesting but comes handy, for interesting stuff check Popping and Locking section down below.

To get a specific queue you want to put your hands on, just do:

hash_queue[:my_queue]

It also features some not that fancy methods like #size, #empty? and #clear.

It's also possible to bypass namespacing capabilities and use HashQueue's individual queue directly as a replacement for stdlib's Queue. Just initialize new queue:

queue = HashQueue::Queue.new
queue.push :foo
queue.size # => 1
queue.pop # => :foo
queue.empty? # => true

There are two way to pop stuff from HashQueue instance. One way is to pop from all the queues at once. When you call pop on HashQueue instance it will pop one item from each individual queue it manages (unless it's locked). Always returns an Array.

hash_queue = HashQueue.new
hash_queue.queue :animals, :cat
hash_queue.queue :animals, :dog
hash_queue.queue :rubyists, :tenderlove
hash_queue.queue :rubyists, :yehuda
hash_queue.queue :rubyists, :matz

hash_queue.pop # => [:cat, :tenderlove]
hash_queue.pop # => [:dog, :yehuda]
hash_queue.pop # => [:matz]

#pop method takes optional :size option with which you can specify how many items you want to pop out of each queue.

hash_queue = HashQueue.new

10.times { |i| hash_queue.queue :queue_1, i }
10.times { |i| hash_queue.queue :queue_2, i + 100 }

hash_queue.pop(size: 2) # => [0, 1, 100, 101]

Or you can provide :blocking option. When called with blocking: true it won't return until there's something to return. When there's nothing to be returned (either all individual queues are empty, locked or there aren't any) the call holds until something is queued from a background thread or some queue is unlocked.

hash_queue = HashQueue.new

Thread.new {
  sleep 1
  @hash_queue[:foo].queue :bar
}   
     
@hash_queue.pop(blocking: true) # waits until a background thread queues something and then returns, [:bar] in this case

You can also pop items from individual queues:

hash_queue = HashQueue.new
3.times { hash_queue[:foo].queue Object.new }

hash_queue[:foo].pop # => #<Object:0x000001008d4658>
hash_queue[:foo].pop(size: 2) # => [#<Object:0x000001008ae228>, #<Object:0x000001008ae200>]

You can use same options as mentioned above plus you can pass in a block into play. Block passed to HashQueue::Queue#pop yields items that otherwise would be directly returned, e.g. popped from the queue, and method actually pops values only if return value of a block evaluates to true.

food = HashQueue::Queue.new
food.push :carrot

food.pop do |snack|
  current_user.favors? snack
end

Use #peek to look what is in the head of the queue but without removing it from there.

queue = HashQueue::Queue.new
queue.push :foo

queue.peek # => :foo
queue.size # => 1

You can peek on more items in the head of the queue specifying :size option. Be aware, using #peek your code might be subject to race condition. If you want to pop only when some condition is met, use #pop with a block mentioned above.

HashQueue was designed for a specific use case and for that it provides a flexible locking facility.

You can lock a specific queue so you won't be able to pop until you unlock it.

hash_queue = HashQueue.new
hash_queue[:foo].queue Object.new
hash_queue[:foo].lock
hash_queue[:foo].pop # => nil
hash_queue[:foo].unlock
hash_queue[:foo].pop # => #<Object:0x000001008a5bf0>

And you can place multiple locks on a queue and then you'll need to unlock them all to be able to pop stuff out again.

hash_queue = HashQueue.new
hash_queue[:foo].queue Object.new
hash_queue[:foo].lock(3)
hash_queue[:foo].pop # => nil
hash_queue[:foo].unlock
hash_queue[:foo].pop # => nil
hash_queue[:foo].unlock
hash_queue[:foo].unlock
hash_queue[:foo].pop # => #<Object:0x000001008a5bf0>

Both #lock and #unlock take as an argument number of locks you want to put or remove from the queue. Defaults to 1. There's even a convenient #unlock_all method and #count_locks that returns current number of locks placed on the queue. You can always check whether a specific queue is locked with #locked?.

hash_queue[:foo].lock
hash_queue[:foo].locked? # => true
hash_queue[:foo].unlock
hash_queue[:foo].locked? # => false

Here it gets a little bit tricky. When popping stuff out of a queue you can specify lock: true option. That means while popping it will place the same number of locks on the queue as the number of items it pops out.

hash_queue[:foo].locked? # => false
hash_queue[:foo].pop(size: 2, lock: true)
hash_queue[:foo].locked? # => true
hash_queue[:foo].count_locks # => 2

While it seems complicated the use case is fairly simple. Locking facility ensures that only a certain number of items could be out of a specific queue at one time. Imagine a crawler, you have opened 200 parallel connections but want to make sure you won't open more than 10 connections to one server at the time. You can use HashQueue as a local queue for URLs to be crawled, hostnames using as a key to keep separate queues for each hostname. You pop out 10 URLs placing 10 locks on a hostname, when a page is processed then unlock one lock.

You may see a flaw in this schema, because I lied a little. The issue with this example is that you can't pop new URLs to process until all previous 10 have been processed, right? You would have to wait until all locks have been unlocked. Well, HashQueue got this covered - You can pop from a queue while the queue is locked!

Imagine a queue with a lot of items in it. When you tell a queue to pop X items but the queue has Y locks on it, it just pops X-Y items. When you look in specs you'll find something like this:

it 'should allow you to pop items even if locked when you want more items then there are locks but taking existing locks into account' do        
  @hash_queue[:foo].lock(2)
  @hash_queue[:foo].pop(size: 10).size.must_equal 8
end

Requires Ruby 1.9. Tested under MRI 1.9.3 and JRuby.

• Someone please check and correct my grammar in readme. English is not my native language. I will send you my love and hug over Twitter.

To run the specs just run:

rake

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Added some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

This library is released under the MIT license.

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