# posix-mqueue

Minimal wrapper around the [POSIX message queue](pmq). The POSIX message queue
offers:

* Persistence. Push messages while nothing is listening.
* Simplicity. Nothing to set up. Built into Linux.
* IPC. Blazingly fast communication between processes on the same machine.
* Blocking and non-blocking. Listeners block until a message arrives on the
  queue. No polling. Sending messages doesn't block.

Add `gem 'posix-mqueue'` to your favorite Gemfile.

Still WIP. Not stable. Not everything works as promised.

## Usage

```ruby
require 'posix/mqueue'

m = POSIX::Mqueue.new("/whatever")
m.send "hello"
puts m.receive
# => "hello"

fork { POSIX::Mqueue.new("/whatever").send("world") }

# Blocks until the forked process pushes to the queue
m.receive
# => "world"

# Deletes the queue and any messages remaining.
# None in this case. Otherwise the queue will persist till reboot.
m.unlink

# Queue is now full by default Linux settings, see below on how to increase it.
10.times { m.send rand(100).to_s }

# #send will block until something is popped off the queue, but timedsend takes
# timeout arguments (first one is seconds, second is nanoseconds). If you don't
# want to block on send, pass 0 for both:

assert_raises POSIX::Mqueue::QueueFull do
  m.timedsend(0, 0, "I will fail")
end

# Empty the queue again
10.times { m.receive }

# Like timedsend, timedreceive takes timeout arguments and will raise
# POSIX::Mqueue::Queueempty when it would otherwise block
assert_raises POSIX::Mqueue::QueueEmpty do
  m.timedreceive(0, 0)
end

```

## mqueue

Most important information from the manpages, with a little added information
about the behavior of `posix-mqueue`.

## /proc interfaces

Linux has some default limits you can easily change.

1. `/proc/sys/fs/mqueue/msg_max`. Contains the maximum number of messages in a
   single queue. Defaults to 10. You should increase that number. `#send` will
   eventually throw an exception if the queue is full (instead of blocking). It
   does not do that right now.
2. `/proc/sys/fs/mqueue/msgsize_max`. Maximum size of a single message. Defaults
   to 8192 bytes. `posix-mqueue` allows up to 4096 bytes. You'll be able to
   change this, soon!
3. `/proc/sys/fs/mqueue/queues_max`. Maximum number of queues on the system.
   Defaults to 256. Which is probably enough.

## Virtual filesystem

The message queue is created as a virtual file system. That means you can mount
it:

```bash
# sudo mkdir /dev/queue
# sudo mount -t mqueue none /dev/queue
```

Add a queue and a few tasks, count the characters (19):

```ruby
$ irb
> require 'posix/mqueue'
=> true
> m = POSIX::Mqueue.new("/queue")
=> #<POSIX::Mqueue:0xb8c9fe88>
> m.send "narwhal"
=> true
> m.send "walrus"
=> true
> m.send "ponies"
=> true
```

Inspect the mounted filesystem:

```bash
$ ls /dev/queue/
important  mails  queue
$ cat /dev/queue/queue
QSIZE:19         NOTIFY:0     SIGNO:0     NOTIFY_PID:0
```

Here `QSIZE` is the bytes of data in the queue. The other flags are about
notifications which `posix-mqueue` does not support currently, read about them
in [mq_overview(7)][pmq].

[pmq]: http://man7.org/linux/man-pages/man7/mq_overview.7.html