A Simple PHP Publish-Subscribe System

I've been playing a lot with Dojo lately, and have been very impressed by its elegant publish-subscribe system. Basically, any object can publish an event, and any other object can subscribe to it. This creates an incredibly flexible notification architecture that's completely opt-in.

The system has elements of Aspect Oriented Programming (AOP), as well as the Observer pattern. Its power, however, is in the fact that an individual object does not need to implement any specific interface in order to act as either a Subject or an Observer; the system is globally available.

Being a developer who recognizes good ideas when he sees them, of course I decided to port the idea to PHP. You can see the results on github.

Usage is incredibly simple: an object publishes an event, which triggers all subscribers.

Probably the most illustrative solution would be for optionally logging. Say for instance that you create a logger instance in your application bootstrap; you could then subscribe it to all "log" events:


$log = new Zend_Log(new Zend_Log_Writer_Stream('/tmp/application.log'));
Phly_PubSub::subscribe('log', $log, 'info');

Then, in your code, whenever you might want to log some information, simply publish to the "log" topic:


Phly_PubSub::publish('log', 'Log message...');

In production, you could simply comment out the log definition and subscription, disabling logging throughout the application. Events that publish to topics without subscribers simply return early -- meaning no ramifications for code that uses the system. You could then enable the logger at will when you need to debug or determine what events are triggering.

As another example, consider a model that has a save method. You may want to log the data sent to it, as well as the id returned. Additionally, you may want to update your search index and caches once the item has been saved to your persistence store.

Your model's save method might then look like this:


class Foo
{
    public function save(array $data)
    {
        Phly_PubSub::publish('Foo::save::start', $data, $this);

        // ...

        Phly_PubSub::publish('Foo::save::end', $id, $this);
        return $id;
    }
}

Elsewhere, you may have defined your logger, indexer, and cache. Where those are defined, you would tell them what topics you're subscribing each to.


Phly_PubSub::subscribe('Foo::save::start', $logger, 'logSaveData');
Phly_PubSub::subscribe('Foo::save::end', $logger, 'logSaveId');
Phly_PubSub::subscribe('Foo::save::end', $cache, 'updateFooItem');
Phly_PubSub::subscribe('Foo::save::end', $index, 'updateFooItem');

The beauty of the approach is the simplicity: Foo doesn't need to implement it's own pub/sub interface -- in fact, if Foo already existed in your application, you could drop in this functionality trivially. On the other side of the coin, if you have no subscribers to the events, there are no drawbacks.

Some places it could be improved:

  • The ability for return values could be useful, to allow interruption of method execution or to modify arguments sent by the publisher. However, since each topic may have multiple handlers, a simple interface would be difficult to achieve.
  • Exception handling. In most cases, you probably don't want method execution to halt due to a subscriber raising an exception. However, you still need some way to report such errors.

I'm excited to see what uses you may be able to put this to; drop me a line if you start using it!

Update (2008-12-30): Based on some of the comments to this post, I created Phly_PubSub_Provider, which is a non-static implementation that can be attached to individual classes -- basically providing a per-object plugin system. Usage is as follows:


class Foo
{
    protected $_plugins;

    public function __construct()
    {
        $this->_plugins = new Phly_PubSub_Provider();
    }

    public function getPluginProvider()
    {
        return $this->_plugins;
    }

    public function bar()
    {
        $this->_plugins->publish('bar');
    }
}

$foo = new Foo();

// Subscribe echo() to the 'bar' event:
$foo->getPluginProvider()->subscribe('bar', 'echo');

$foo->bar(); // echo's 'bar'
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