Lazy Services

Zend\ServiceManager can use delegator factories to generate “lazy” references to your services.

Lazy services are proxies that get lazily instantiated, and keep a reference to the real instance of the proxied service.

Use cases

You may want to lazily initialize a service when it is instantiated very often, but not always used.

A typical example is a database connection: it is a dependency to many other elements in your application, but that doesn’t mean that every request will execute queries through it.

Additionally, instantiating a connection to the database may require some time and eat up resources.

Proxying the database connection would allow to delay that overhead until the object is really needed.

Setup

Zend\ServiceManager\Proxy\LazyServiceFactory is a delegator factory capable of generating lazy loading proxies for your services.

The LazyServiceFactory depends on ProxyManager, so be sure to install it before going through the following steps:

php composer.phar require ocramius/proxy-manager:0.3.*

Practical example

To demonstrate how a lazy service works, you may use the following Buzzer example class, which is designed to be slow at instantiation time for demonstration purposes:

namespace MyApp;

class Buzzer
{
    public function __construct()
    {
        // deliberately halting the application for 5 seconds
        sleep(5);
    }

    public function buzz()
    {
        return 'Buzz!';
    }
}

You can then proceed and configure the service manager to generate proxies instead of real services:

$serviceManager = new \Zend\ServiceManager\ServiceManager();

$config = array(
    'lazy_services' => array(
         // mapping services to their class names is required
         // since the ServiceManager is not a declarative DIC
         'class_map' => array(
             'buzzer' => 'MyApp\Buzzer',
         ),
    ),
);

$serviceManager->setService('Config', $config);
$serviceManager->setInvokableClass('buzzer', 'MyApp\Buzzer');
$serviceManager->setFactory('LazyServiceFactory', 'Zend\ServiceManager\Proxy\LazyServiceFactoryFactory');
$serviceManager->addDelegator('buzzer', 'LazyServiceFactory');

This will tell the service manager to use the LazyServiceFactory delegator factory to instantiate the buzzer service.

As you may have noticed, the standard setup for the LazyServiceFactory requires you to define a Config service. That’s because the functionality was thought to be easily integrated into Zend\Mvc.

You can now simply retrieve the buzzer:

$buzzer = $serviceManager->get('buzzer');

echo $buzzer->buzz();

To verify that the proxying occurred correctly, you can simply run the following code, which should delay the 5 seconds wait time hardcoded in Buzzer::__construct until Buzzer::buzz is invoked:

for ($i = 0; $i < 100; $i += 1) {
    $buzzer = $serviceManager->create('buzzer');

    echo "created buzzer $i\n";
}

echo $buzzer->buzz();

The setup above can also be represented via configuration in an MVC application’s context:

return array(
    'service_manager' => array(
        'invokables' => array(
            'buzzer' => 'MyApp\Buzzer',
        ),
        'delegators' => array(
            'buzzer' => array(
                'LazyServiceFactory'
            ),
        ),
        'factories' => array(
            'LazyServiceFactory' => 'Zend\ServiceManager\Proxy\LazyServiceFactoryFactory',
        ),
    ),
    'lazy_services' => array(
        'class_map' => array(
            'buzzer' => 'MyApp\Buzzer',
        ),
    ),
);

Configuration

This is the config structure expected by Zend\ServiceManager\Proxy\LazyServiceFactoryFactory:

return array(
    'lazy_services' => array(

        // map of service names and their relative class names - this
        // is required since the service manager cannot know the
        // class name of defined services upfront
        'class_map' => array(
            // 'foo' => 'MyApplication\Foo',
        ),

        // directory where proxy classes will be written - default to system_get_tmp_dir()
        'proxies_target_dir' => null,

        // namespace of the generated proxies, default to "ProxyManagerGeneratedProxy"
        'proxies_namespace' => null,

         // whether the generated proxy classes should be written to disk or generated on-the-fly
         'write_proxy_files' => false,
    ),
);