OPQ: One Pooled Queue

Elixir Queue!

A simple, in-memory queue with worker pooling and rate limiting in Elixir. OPQ leverages Erlang’s queue module and Elixir’s GenStage.

Originally built to support Crawler.

Features

• A fast, in-memory FIFO queue.
• Worker pool.
• Rate limit.
• Timeouts.
• Pause / resume / stop the queue.

See Hex documentation.

Installation

def deps do
  [{:opq, "~> 4.0"}]
end

Usage

A simple example:

{:ok, opq} = OPQ.init()

OPQ.enqueue(opq, fn -> IO.inspect("hello") end)
OPQ.enqueue(opq, fn -> IO.inspect("world") end)

Specify module, function and arguments:

{:ok, opq} = OPQ.init()

OPQ.enqueue(opq, IO, :inspect, ["hello"])
OPQ.enqueue(opq, IO, :inspect, ["world"])

Specify a custom name for the queue:

OPQ.init(name: :items)

OPQ.enqueue(:items, fn -> IO.inspect("hello") end)
OPQ.enqueue(:items, fn -> IO.inspect("world") end)

Start as part of a supervision tree:

Note, when starting as part of a supervision tree, the :name option must be provided.

children = [
  {OPQ, name: :items}
]

Specify a custom worker to process items in the queue:

defmodule CustomWorker do
  def start_link(item) do
    Task.start_link(fn ->
      Agent.update(:bucket, &[item | &1])
    end)
  end
end

Agent.start_link(fn -> [] end, name: :bucket)

{:ok, opq} = OPQ.init(worker: CustomWorker)

OPQ.enqueue(opq, "hello")
OPQ.enqueue(opq, "world")

Agent.get(:bucket, & &1) # => ["world", "hello"]

Rate limit:

{:ok, opq} = OPQ.init(workers: 1, interval: 1000)

Task.async(fn ->
  OPQ.enqueue(opq, fn -> IO.inspect("hello") end)
  OPQ.enqueue(opq, fn -> IO.inspect("world") end)
end)

If no interval is supplied, the ratelimiter will be bypassed.

Check the queue and number of available workers:

{:ok, opq} = OPQ.init()

OPQ.enqueue(opq, fn -> Process.sleep(1000) end)

{status, queue, available_workers} = OPQ.info(opq) # => {:normal, #OPQ.Queue<[]>, 9}

Process.sleep(1200)

{status, queue, available_workers} = OPQ.info(opq) # => {:normal, #OPQ.Queue<[]>, 10}

If you just need to get the queue itself:

OPQ.queue(opq) # => #OPQ.Queue<[]>

Queue

OPQ implements Enumerable, so you can perform enumerable functions on the queue:

{:ok, opq} = OPQ.init()

queue = OPQ.queue(opq)

Enum.count(queue) # => 0
Enum.empty?(queue) # => true

Stop the queue:

{:ok, opq} = OPQ.init()

OPQ.enqueue(opq, fn -> IO.inspect("hello") end)
OPQ.stop(opq)
OPQ.enqueue(opq, fn -> IO.inspect("world") end) # => (EXIT) no process...

Pause and resume the queue:

{:ok, opq} = OPQ.init()

OPQ.enqueue(opq, fn -> IO.inspect("hello") end) # => "hello"
OPQ.pause(opq)
OPQ.info(opq) # => {:paused, {[], []}, 10}
OPQ.enqueue(opq, fn -> IO.inspect("world") end)
OPQ.resume(opq) # => "world"
OPQ.info(opq) # => {:normal, {[], []}, 10}

Configurations

| :name | atom/module | pid | The name of the queue. | :worker | module | OPQ.Worker | The worker that processes each item from the queue. | :workers | integer | 10 | Maximum number of workers. | :interval | integer | 0 | Rate limit control - number of milliseconds before asking for more items to process, defaults to 0 which is effectively no rate limit. | :timeout | integer | 5000 | Number of milliseconds allowed to perform the work, it should always be set to higher than :interval.

Changelog

Please see CHANGELOG.md.

License

Licensed under MIT.