# OPQ: One Pooled Queue
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## Elixir Queue!
A simple, in-memory queue with worker pooling and rate limiting in Elixir. OPQ leverages Erlang's [queue](http://erlang.org/doc/man/queue.html) module and Elixir's [GenStage](https://github.com/elixir-lang/gen_stage).
Originally built to support [Crawler](https://github.com/fredwu/crawler).
## Features
- A fast, in-memory FIFO queue.
- Worker pool.
- Rate limit.
- Timeouts.
- Pause / resume / stop the queue.
## Installation
```Elixir
def deps do
[{:opq, "~> 3.2"}]
end
```
## Usage
### A simple example:
```elixir
{: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:
```elixir
{:ok, opq} = OPQ.init()
OPQ.enqueue(opq, IO, :inspect, ["hello"])
OPQ.enqueue(opq, IO, :inspect, ["world"])
```
### Specify a custom name for the queue:
```elixir
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.
```elixir
children = [
{OPQ, name: :items}
]
```
### Specify a custom worker to process items in the queue:
```elixir
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:
```elixir
{: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:
```elixir
{:ok, opq} = OPQ.init()
OPQ.enqueue(opq, fn -> Process.sleep(1000) end)
{queue, available_workers} = OPQ.info(opq) # => {:normal, {[], []}, 9}
Process.sleep(1200)
{queue, available_workers} = OPQ.info(opq) # => {:normal, {[], []}, 10}
```
### Stop the queue:
```elixir
{: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:
```elixir
{: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
| Option | Type | Default Value | Description |
|--------------|-------------|----------------|-------------|
| `: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](CHANGELOG.md).
## License
Licensed under [MIT](http://fredwu.mit-license.org/).
