Create a MessagePublisher service

[dannylamb]

Depends on #466

We need to create a service to publish messages from Drupal. It should have a basic interface

interface MessagePublisher {
  public function publish($broker_url, $queue, $msg, array $headers);
}

Our implementation class should use STOMP, and will essentially look like this:

$queue  = '/queue/foo';
$msg    = 'bar';

/* connection */
try {
    $stomp = new Stomp('tcp://localhost:61613');
} catch(StompException $e) {
    die('Connection failed: ' . $e->getMessage());
}

/* send a message to the queue 'foo' */
$stomp->send($queue, $msg);

/* close connection */
unset($stomp);

We'll need to write a Kernel test for this service, pulling it out of the dependency injection container, publishing a message with our publish function, and then reading from the queue to see if the message has been received. You can check for a message using code like this:

$queue  = '/queue/foo';
$msg    = 'bar';

/* connection */
try {
    $stomp = new Stomp('tcp://localhost:61613');
} catch(StompException $e) {
    die('Connection failed: ' . $e->getMessage());
}

/* subscribe to messages from the queue 'foo' */
$stomp->subscribe($queue);

/* read a frame */
$frame = $stomp->readFrame();

$this->assertTrue($frame->body == $msg, "Queue must contain sent message");

unset($stomp);

The only caveat here is that the broker url will come from the islandora module's configuration.

[dannylamb]

I'll tackle this one for some pre-Xmas fun.

[dannylamb]

So.... turns out the php stomp pecl extension is not supported in php7. So I need to use this: https://github.com/stomp-php/stomp-php. Which is good. It knows to block until a message has been received and handles things a bit nicer. But it's on packagist. So we need to start using composer, which leads to a lot of big changes. Like #474 and #475

[dannylamb]

So I can close this and will just re-assign the broadcast action issue to mysef: #468