Wrote document resolving #148

doc/book/cookbook/factories-vs-abstract-factories.md

@@ -0,0 +1,89 @@
+# When To Use Factories vs Abstract Factories
+
+Starting with version 3, `Zend\ServiceManager\Factory\AbstractFactoryInterface`
+extends `Zend\ServiceManager\Factory\FactoryInterface`, meaning they may be used
+as either an abstract factory, or mapped to a specific service name as its
+factory.
+
+As an example:
+
+```php
+return [
+	'factories' => [
+		SomeService::class => AnAbstractFactory::class,
+	],
+];
+```
+
+Why would you choose one approach over the other?
+
+## Comparisons
+
+Approach         | Pros           | Cons
+---------------- | -------------- | ----
+Abstract factory | One-time setup | Performance; discovery of code responsible for creating instance
+Factory          | Performance; explicit mapping to factory responsible | Additional (duplicate) setup
+
+Essentially, it comes down to *convenience* versus *explicitness* and/or
+*performance*.
+
+## Conveneience
+
+Writing a factory per service is time consuming, and, particularly in early
+stages of an application, can distract from the actual business of writing the
+classes and implementations; in addition, since requirements are often changing
+regularly, this boiler-plate code can be a nuisance.
+
+In such situations, one or more abstract factories — such as the
+[ConfigAbstractFactory](../config-abstract-factory.md) or the
+[zend-mvc LazyControllerAbstractFactory](https://docs.zendframework.com/zend-mvc/cookbook/automating-controller-factories/)
+— that can handle the bulk of your needs are often worthwhile, saving you
+time and effort as you code.
+
+## Explicitness
+
+The drawback of abstract factories is that lookups by the service manager take
+longer, and increase based on the number of abstract factories in the system.
+The service manager is optimized to locate *factories*, as it can do an
+immediate hash table lookup; abstract factories involve:
+
+- Looping through each abstract factory
+    - invoking its method for service location
+    - if the service is located, using the factory
+
+This means, internally:
+
+- a hash table lookup (for the abstract factory)
+- invocation of 1:N methods for discovery
+    - which may contain additional lookups and/or retrievals in the container
+- invocation of a factory method (assuming succesful lookup)
+
+As such, having an explicit map can aid performance dramatically.
+
+Additionally, having an explicit map can aid in understanding what class is
+responsible for initializing a given service. Without an explicit map, you need
+to identify all possible abstract factories, and determine which one is capable
+of handling the specific service; in some cases, multiple factories might be
+able to, which means you additionally need to know the *order* in which they
+will be queried.
+
+The primary drawback is that you also end up with potentially duplicate
+information in your configuration:
+
+- Multiple services mapped to the same factory.
+- In cases such as the `ConfigAbstractFactory`, additional configuration
+  detailing how to create the service.
+
+## Tradeoffs
+
+What it comes down to is which development aspects your organization or project
+favor. Hopefully the above arguments detail what tradeoffs occur, so you may
+make an appropriate choice.
+
+> ### Tooling
+>
+> We will likely provide tooling in the future to convert
+> `ConfigAbstractFactory` configuration into discrete factory classes in the
+> future, allowing you to mitigate the performance issues of using an abstract
+> factory when in production. As such, tooling support should also be a
+> consideration when deciding on your project strategy.

mkdocs.yml

@@ -9,6 +9,8 @@ pages:
       - 'Lazy services': lazy-services.md
       - 'Plugin managers': plugin-managers.md
       - 'Configuration-based Abstract Factory': config-abstract-factory.md
+    - Cookbook:
+      - 'Factories vs Abstract Factories': cookbook/factories-vs-abstract-factories.md
     - 'Migration Guide': migration.md
 site_name: zend-servicemanager
 site_description: 'zend-servicemanager: factory-driven dependency injection container'