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 you 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 lazy service facilities depend on ProxyManager; you will need to install that package before using the feature:

$ composer require ocramius/proxy-manager

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:

use MyApp\Buzzer;
use Zend\ServiceManager\Factory\InvokableFactory;
use Zend\ServiceManager\Proxy\LazyServiceFactory;
use Zend\ServiceManager\ServiceManager;

$serviceManager = new \Zend\ServiceManager\ServiceManager([
    'factories' => [
        Buzzer::class             => InvokableFactory::class,
    ],
    'lazy_services' => [
         // Mapping services to their class names is required
         // since the ServiceManager is not a declarative DIC.
         'class_map' => [
             Buzzer::class => Buzzer::class,
         ],
    ],
    'delegators' => [
        Buzzer::class => [
            LazyServiceFactory::class,
        ],
    ],
]);

This configuration tells the service manager to add the add LazyServiceFactory as a delegator for Buzzer.

You can now retrieve the buzzer:

use MyApp\Buzzer;

$buzzer = $serviceManager->get(Buzzer::class);
echo $buzzer->buzz();

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

use MyApp\Buzzer;

for ($i = 0; $i < 100; $i += 1) {
    $buzzer = $serviceManager->get(Buzzer::class);
    echo "created buzzer $i\n";
}

echo $buzzer->buzz();

Configuration

This is the config structure expected by Zend\ServiceManager\Proxy\LazyServiceFactory, in the lazy_services key passed in the service manager configuration:

[
    // map of service names and their relative class names - this
    // is required since the service manager cannot know the
    // class name of defined services up front
    'class_map' => [
        // '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,
];

After you have an instance, you can map lazy service/class pairs using mapLazyService():

$container->mapLazyService('foo', \MyApplication\Foo::class);