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Identity Map

Read-Models uses an identity map to ensure that you only ever have one instance, and the same instance of the object. Right now this is a very simple implementation that does not deal with updating the shared instance, so once loaded it is whatever the original data is. A future update may change this.

The identity map is used to resolve and track relationships between models and provides an aliasing system to match foreign key names to a class name.

Because of the identity map, it means you can perform object comparisons without needing to use an equality method; User object with id 1 will be the same as a reloaded User object with id 1.

There is a slight penalty during hydration for the lookups, however this does result in far fewer objects in memory at any one time (for example a User with permissions).

The identity map does need clearing at the end of a request and if running in a long running process be sure to periodically call Manager::instance()->map()->clear().

The Identity Map can be accessed from the Manager.

For Symfony based projects there are 2 kernel event listeners that can be subscribed:

  • IdentityMapClearerSubscriber clears the map on request / error
  • IdentityMapClearerMessengerSubscriber clears the map after handling messages in Messenger

IdentityMap Test Listener

When working with PHPUnit the identity map will preserve loading entities across tests unless cleared. A listener is included that will clear the identity map before and after every test case for you.

Add the following to your phpunit.xml config file to enable the automatic clearing:

<phpunit>
    <!-- other unit config excluded -->
    
    <extensions>
        <extension class="Somnambulist\Components\ReadModels\PHPUnit\PHPUnitListener"/>
    </extensions>
</phpunit>

In addition: if you do not use a dependency injection container in your tests, you may need to setup a bootstrap that will configure the Manager with suitable defaults. Remember: the Manager must be instantiated before it can be accessed statically.