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Asynchronous

This package contains generic classes and interfaces which can be used to process messages asynchronously using a SimpleBus MessageBus instance.

@TODO The intro should explain what it does.

Publishing messages

When a Message should not be handled by the message bus (i.e. command or event bus) immediately (i.e. synchronously), it can be published to be handled by some other process. This library comes with three strategies for publishing messages:

  1. A message will always also be published.
  2. A message will only be published when the message bus isn't able to handle it because there is no handler defined for it.
  3. A message will be published only if its name exists in a predefined list.

Strategy 1: Always publish messages

This strategy is very useful when you have an event bus that notifies event subscribers of events that have occurred. If you have set up the event bus, you can add the AlwaysPublishesMessages middleware to it:

use SimpleBus\Message\Bus\Middleware\MessageBusSupportingMiddleware;
use SimpleBus\Asynchronous\MessageBus\AlwaysPublishesMessages;
use SimpleBus\Asynchronous\Publisher\Publisher;
use SimpleBus\Message\Message;

// $eventBus is an instance of MessageBusSupportingMiddleware
$eventBus = ...;

// $publisher is an instance of Publisher
$publisher = ...;

$eventBus->appendMiddleware(new AlwaysPublishesMessages($publisher));

// $event is an object
$event = ...;

$eventBus->handle($event);

The middleware publishes the message to the publisher (which may add it to some a queue of some sorts). After that it just calls the next middleware and lets it process the same message in the usual way.

By applying this strategy you basically allow other processes to respond to any event that occurs within your application.

Strategy 2: Only publish messages that could not be handled

This strategy is useful if you have a command bus that handles commands. If you have set up the command bus, you can add the PublishesUnhandledMessages middleware to it:

use SimpleBus\Message\Bus\Middleware\MessageBusSupportingMiddleware;
use SimpleBus\Asynchronous\MessageBus\PublishesUnhandledMessages;
use SimpleBus\Asynchronous\Publisher\Publisher;
use Psr\Log\LoggerInterface;
use Psr\Log\LogLevel;

// $commandBus is an instance of MessageBusSupportingMiddleware
$commandBus = ...;

// $publisher is an instance of Publisher
$publisher = ...;

// $logger is an instance of LoggerInterface
$logger = ...;

// $logLevel is one of the class constants of LogLevel
$logLevel = LogLevel::DEBUG;

$commandBus->appendMiddleware(new PublishesUnhandledMessages($publisher, $logger, $logLevel));

// $command is an object
$command = ...;

$commandBus->handle($command);

Because of the nature of commands (they have a one-to-one correspondence with their handlers), it doesn't make sense to always publish a command. Instead, it should only be published when it couldn't be handled by your application. Possibly some other process knows how to handle it.

If no command handler was found and the command is published, this will be logged using the provided $logger.

Strategy 3: Only publish predefined messages

This strategy is useful when you know what messages you want to publish.

use SimpleBus\Message\Bus\Middleware\MessageBusSupportingMiddleware;
use SimpleBus\Asynchronous\MessageBus\AlwaysPublishesMessages;
use SimpleBus\Asynchronous\Publisher\Publisher;
use SimpleBus\Message\Message;
use SimpleBus\Message\Name\MessageNameResolver;

// $eventBus is an instance of MessageBusSupportingMiddleware
$eventBus = ...;

// $publisher is an instance of Publisher
$publisher = ...;

// $messageNameResolver is an instance of MessageNameResolver
$messageNameResolver = ...;

// The list of names will depend on what MessageNameResolver you are using.
$names = ['My\\Event', 'My\\Other\\Event'];

$eventBus->appendMiddleware(new PublishesPredefinedMessages($publisher, $messageNameResolver, $names));

// $event is an object
$event = ...;

$eventBus->handle($event);

Consuming messages

When a message has been published, for instance to some kind of queue, another process should be able to consume it, i.e. receive and process it.

A message consumer actually consumes serialized envelopes, instead of the messages themselves. A consumer then restores the Envelope by deserializing it and finally it can restore the Message itself by deserializing the serialized message carried by the Envelope.

To ease integration of existing messaging software with SimpleBus/Asynchronous, this library contains a standard implementation of a SerializedEnvelopeConsumer. It deserializes a serialized Envelope, then lets the message bus handle the Message contained in the Envelope.

use SimpleBus\Asynchronous\Consumer\StandardSerializedEnvelopeConsumer;
use SimpleBus\Serialization\Envelope\Serializer\MessageInEnvelopeSerializer;
use SimpleBus\Message\Bus\MessageBus;

// $messageSerializer is an instance of MessageInEnvelopeSerializer
$messageSerializer = ...;

// $messageBus is an instance of MessageBus
$messageBus = ...;

$consumer = StandardSerializedEnvelopeConsumer($messageSerializer, $messageBus);

// keep fetching serialized envelopes
while ($aSerializedEnvelope = ...) {
    // this causes $messageBus to handle the deserialized Message
    $consumer->consume($aSerializedEnvelope);
}

For more information about envelopes and serializing messages, take a look at the documentation of SimpleBus/Serialization.

Routing keys

A routing key is a concept that originates from RabbitMQ: it allows you to let particular groups of messages be routed to specific queues, which may then be consumed by dedicated consumers.

Whether or not you use RabbitMQ, you might need the concept of a routing key somewhere in your application. This library contains an interface RoutingKeyResolver and two very simple standard implementations of it:

  1. The ClassBasedRoutingKeyResolver: when asked to resolve a routing key for a given Message, it takes the full class name of it and replaces \ with ..
  2. The EmptyRoutingKeyResolver: it always returns an empty string as the routing key for a given Message.

Additional properties

"Additional properties" is a concept that originates from RabbitMQ: it allows you to add metadata or otherwise configure a message before it is sent to the server.

Whether or not you use RabbitMQ, you might need these additional (message) properties somewhere in your application. This library contains an interface AdditionalPropertiesResolver and one implementation of that interface, the DelegatingAdditionalPropertiesResolver which accepts an array of AdditionalPropertiesResolver instances. It lets them all step in and provide values:

use SimpleBus\Asynchronous\Properties\DelegatingAdditionalPropertiesResolver;
use SimpleBus\Asynchronous\Properties\AdditionalPropertiesResolver;

class MyPropertiesResolver implements AdditionalPropertiesResolver
{
    public function resolveAdditionalPropertiesFor($message)
    {
        // determine which properties to use

        return [
            'content-type' => 'application/xml'
        ];
    }
}

$delegatingResolver = new DelegatingAdditionalPropertiesResolver(
    [
        new MyPropertiesResolver(),
        ...
    ]
);

// $message is some message (e.g. a command or event)
$message = ...;

$properties = $delegatingResolver->resolveAdditionalPropertiesFor($message);