wiki.techinc.nl/docs/Injection.md

Depencency Injection

This is an overview of how MediaWiki makes use of dependency injection. The design described here grew from the discussion of RFC T384.

The term "dependency injection" (DI) refers to a pattern on object oriented programming that tries to improve modularity by reducing strong coupling between classes. In practical terms, this means that anything an object needs to operate should be injected from the outside, the object itself should only know narrow interfaces, no concrete implementation of the logic it relies on.

The requirement to inject everything typically results in an architecture that based on two main types of objects: simple value objects with no business logic (and often immutable), and essentially stateless service objects that use other service objects to operate on the value objects.

As of the beginning of 2016 (MW version 1.27), MediaWiki is only starting to use the DI approach. Much of the code still relies on global state or direct instantiation, resulting in a highly cyclical dependency graph.

Overview

The heart of the DI in MediaWiki is the central service locator, MediaWikiServices, which acts as the top level factory for services in MediaWiki. MediaWikiServices::getInstance() returns the default service locator instance, which can be used to gain access to default instances of various services. MediaWikiServices however also allows new services to be defined and default services to be redefined. Services are defined or redefined by providing a callback function, the "instantiator" function, that will return a new instance of the service.

When MediaWikiServices::getInstance() is first called, it will create an instance of MediaWikiServices and populate it with the services defined in the files listed by $wgServiceWiringFiles, thereby "bootstrapping" the DI framework. Per default, $wgServiceWiringFiles lists includes/ServiceWiring.php, which defines all default service implementations, and specifies how they depend on each other ("wiring").

When a new service is added to MediaWiki core, an instantiator function that will create the appropriate default instance for that service must be added to ServiceWiring.php. This makes the service available through the generic getService() method on the service locator returned by MediaWikiServices::getInstance().

Extensions can add their own wiring files to $wgServiceWiringFiles, in order to define their own service. Extensions may also use the 'MediaWikiServices' hook to define or redefined services by calling methods on the default MediaWikiServices instance.

It should be noted that the term "service locator" is often used to refer to a top level factory that is accessed directly, throughout the code, to avoid explicit dependency injection. In contrast, the term "DI container" is often used to describe a top level factory that is only accessed when services are created. We use the term "service locator" for the top level factory because it is more descriptive than "DI container", even though application logic is strongly discouraged from accessing MediaWikiServices directly. MediaWikiServices::getInstance() should ideally be accessed only in "static entry points" such as hook handler functions. See "Migration" below.

Service Reset

Services get their configuration injected, and changes to global configuration variables will not have any effect on services that were already instantiated. This would typically be the case for low level services like the ConfigFactory or the ObjectCacheManager, which are used during extension registration. To address this issue, Setup.php resets the global service locator instance by calling MediaWikiServices::resetGlobalInstance() once configuration and extension registration is complete.

Note that "unmanaged" legacy services services that manage their own singleton must not keep references to services managed by MediaWikiServices, to allow a clean reset. After the global MediaWikiServices instance got reset, any such references would be stale, and using a stale service will result in an error.

Services should either have all dependencies injected and be themselves managed by MediaWikiServices, or they should use the Service Locator pattern, accessing service instances via the global MediaWikiServices instance state when needed. This ensures that no stale service references remain after a reset.

Configuration

When the default MediaWikiServices instance is created, a Config object is provided to the constructor. This Config object represents the "bootstrap" configuration which will become available as the 'BootstrapConfig' service. As of MW 1.27, the bootstrap config is a GlobalVarConfig object providing access to the $wgXxx configuration variables.

The bootstrap config is then used to construct a 'ConfigFactory' service, which in turn is used to construct the 'MainConfig' service. Application logic should use the 'MainConfig' service (or a more specific configuration object). 'BootstrapConfig' should only be used for bootstrapping basic services that are needed to load the 'MainConfig'.

Note: Several well known services in MediaWiki core act as factories themselves, e.g. ApiModuleManager, ObjectCache, SpecialPageFactory, etc. The registries these factories are based on are currently managed as part of the configuration. This may however change in the future.

Migration

This section provides some recipes for improving code modularity by reducing strong coupling. The dependency injection mechanism described above is an essential tool in this effort.

Migrate access to global service instances and config variables

Assume Foo is a class that uses the $wgScriptPath global and calls wfGetDB() to get a database connection, in non-static methods.

• Add $scriptPath as a constructor parameter and use $this->scriptPath instead of $wgScriptPath.
• Add LoadBalancer $dbLoadBalancer as a constructor parameter. Use $this->dbLoadBalancer->getConnection() instead of wfGetDB().
• Any code that calls Foo's constructor would now need to provide the $scriptPath and $dbLoadBalancer. To avoid this, avoid direct instantiation of services all together - see below.

Migrate class-level singleton getters

Assume class Foo has mostly non-static methods, and provides a static getInstance() method that returns a singleton (or default instance).

• Add an instantiator function for Foo into ServiceWiring.php. The instantiator would do exactly what Foo::getInstance() did. However, it should replace any access to global state with calls to $services->getXxx() to get a service, or $services->getMainConfig()->get() to get a configuration setting.
• Add a getFoo() method to MediaWikiServices. Don't forget to add the appropriate test cases in MediaWikiServicesTest.
• Turn Foo::getInstance() into a deprecated alias for MediaWikiServices::getInstance()->getFoo(). Change all calls to Foo::getInstance() to use injection (see above).

Migrate direct service instantiation

Assume class Bar calls new Foo().

• Add an instantiator function for Foo into ServiceWiring.php and add a getFoo() method to MediaWikiServices. Don't forget to add the appropriate test cases in MediaWikiServicesTest.
• In the instantiator, replace any access to global state with calls to $services->getXxx() to get a service, or $services->getMainConfig()->get() to get a configuration setting.
• The code in Bar that calls Foo's constructor should be changed to have a Foo instance injected; Eventually, the only code that instantiates Foo is the instantiator in ServiceWiring.php.
• As an intermediate step, Bar's constructor could initialize the $foo member variable by calling MediaWikiServices::getInstance()->getFoo(). This is acceptable as a stepping stone, but should be replaced by proper injection via a constructor argument. Do not however inject the MediaWikiServices object!

Migrate parameterized helper instantiation

Assume class Bar creates some helper object by calling new Foo( $x ), and Foo uses a global singleton of the Xyzzy service.

• Define a FooFactory class (or a FooFactory interface along with a MyFooFactory implementation). FooFactory defines the method newFoo( $x ) or getFoo( $x ), depending on the desired semantics (newFoo would guarantee a fresh instance). When Foo gets refactored to have Xyzzy injected, FooFactory will need a Xyzzy instance, so newFoo() can pass it to new Foo().
• Add an instantiator function for FooFactory into ServiceWiring.php and add a getFooFactory() method to MediaWikiServices. Don't forget to add the appropriate test cases in MediaWikiServicesTest.
• The code in Bar that calls Foo's constructor should be changed to have a FooFactory instance injected; Eventually, the only code that instantiates Foo are implementations of FooFactory, and the only code that instantiates FooFactory is the instantiator in ServiceWiring.php.
• As an intermediate step, Bar's constructor could initialize the $fooFactory member variable by calling MediaWikiServices::getInstance()->getFooFactory(). This is acceptable as a stepping stone, but should be replaced by proper injection via a constructor argument. Do not however inject the MediaWikiServices object!

Migrate a handler registry

Assume class Bar calls FooRegistry::getFoo( $x ) to get a specialized Foo instance for handling $x.

• Turn getFoo into a non-static method.
• Add an instantiator function for FooRegistry into ServiceWiring.php and add a getFooRegistry() method to MediaWikiServices. Don't forget to add the appropriate test cases in MediaWikiServicesTest.
• Change all code that calls FooRegistry::getFoo() statically to call this method on a FooRegistry instance. That is, Bar would have a $fooRegistry member, initialized from a constructor parameter.
• As an intermediate step, Bar's constructor could initialize the $fooRegistry member variable by calling MediaWikiServices::getInstance()->getFooRegistry(). This is acceptable as a stepping stone, but should be replaced by proper injection via a constructor argument. Do not however inject the MediaWikiServices object!

Migrate deferred service instantiation

Assume class Bar calls new Foo(), but only when needed, to avoid the cost of instantiating Foo().

• Define a FooFactory interface and a MyFooFactory implementation of that interface. FooFactory defines the method getFoo() with no parameters.
• Precede as for the "parameterized helper instantiation" case described above.

Migrate a class with only static methods

Assume Foo is a class with only static methods, such as frob(), which interacts with global state or system resources.

• Introduce a FooService interface and a DefaultFoo implementation of that interface. FooService contains the public methods defined by Foo.
• Add an instantiator function for FooService into ServiceWiring.php and add a getFooService() method to MediaWikiServices. Don't forget to add the appropriate test cases in MediaWikiServicesTest.
• Add a private static getFooService() method to Foo. That method just calls MediaWikiServices::getInstance()->getFooService().
• Make all methods in Foo delegate to the FooService returned by getFooService(). That is, Foo::frob() would do self::getFooService()->frob().
• Deprecate Foo. Inject a FooService into all code that calls methods on Foo, and change any calls to static methods in foo to the methods provided by the FooService interface.

Migrate static hook handler functions (to allow unit testing)

Assume MyExtHooks::onFoo is a static hook handler function that is called with the parameter $x; Further assume MyExt::onFoo needs service Bar, which is already known to MediaWikiServices (if not, see above).

• Create a non-static doFoo( $x ) method in MyExtHooks that has the same signature as onFoo( $x ). Move the code from onFoo() into doFoo(), replacing any access to global or static variables with access to instance member variables.
• Add a constructor to MyExtHooks that takes a Bar service as a parameter.
• Add a static method called newFromGlobalState() with no parameters. It should just return new MyExtHooks( MediaWikiServices::getInstance()->getBar() ).
• The original static handler method onFoo( $x ) is then implemented as self::newFromGlobalState()->doFoo( $x ).

Migrate a "smart record"

Assume Thingy is a "smart record" that "knows" how to load and store itself. For this purpose, Thingy uses wfGetDB().

• Create a "dumb" value class ThingyRecord that contains all the information that Thingy represents (e.g. the information from a database row). The value object should not know about any service.
• Create a DAO-style service for loading and storing ThingyRecords, called ThingyStore. It may be useful to split the interfaces for reading and writing, with a single class implementing both interfaces, so we in the end have the ThingyLookup and ThingyStore interfaces, and a SqlThingyStore implementation.
• Add instantiator functions for ThingyLookup and ThingyStore in ServiceWiring.php. Since we want to use the same instance for both service interfaces, the instantiator for ThingyLookup would return $services->getThingyStore().
• Add getThingyLookup() and getThingyStore() methods to MediaWikiServices. Don't forget to add the appropriate test cases in MediaWikiServicesTest.
• In the old Thingy class, replace all member variables that represent the record's data with a single ThingyRecord object.
• In the old Thingy class, replace all calls to static methods or functions, such as wfGetDB(), with calls to the appropriate services, such as LoadBalancer::getConnection().
• In Thingy's constructor, pull in any services needed, such as the LoadBalancer, by using MediaWikiServices::getInstance(). These services cannot be injected without changing the constructor signature, which is often impractical for "smart records" that get instantiated directly in many places in the code base.
• Deprecate the old Thingy class. Replace all usages of it with one of the three new classes: loading needs a ThingyLookup, storing needs a ThingyStore, and reading data needs a ThingyRecord.

Migrate lazy loading

Assume Thingy is a "smart record" as described above, but requires lazy loading of some or all the data it represents.

• Instead of a plain object, define ThingyRecord to be an interface. Provide a "simple" and "lazy" implementations, called SimpleThingyRecord and LazyThingyRecord. LazyThingyRecord knows about some lower level storage interface, like a LoadBalancer, and uses it to load information on demand.
• Any direct instantiation of a ThingyRecord would use the SimpleThingyRecord implementation.
• SqlThingyStore however creates instances of LazyThingyRecord, and injects whatever storage layer service LazyThingyRecord needs to perform lazy loading.