injection.txt

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.


