Architecture:MediaWiki/handler pattern

MediaWiki Architecture
Structure
Technical layers Domain modules Pattern language Subsystems Flows Practices
Pattern language
entry point pattern wiring pattern (including hook handlers) view pattern cursor pattern (iterators, etc) batch pattern pager pattern input form pattern service object pattern stateless service pattern service container pattern data access object pattern lookup pattern factory pattern registry pattern handler pattern content handler pattern API module pattern action pattern special page pattern media handler pattern page pattern (page types?) provider pattern (vague, could be a factory) codec pattern (serializer/deserializer) command pattern (aka business action) validator pattern formatter pattern skin pattern renderer pattern localizer pattern builder pattern event pattern listener pattern job pattern data update pattern output aggregator pattern data object pattern record pattern (has ID) immutable value pattern exception pattern active record anti-pattern content pattern (deprecate?) parser pattern (?) cache pattern writer pattern hook handler pattern client object pattern adapter pattern maintenance script pattern

Handlers are objects that provide a specialized implementation of a general interface, selected by name. The concrete implementation is chosen at runtime by a registry, among several available choices.

The handler pattern can be interpreted as a special case of the well known strategy pattern, in which the strategy is chosen at runtime based on a single parameter, a name. A typical example would be a set of classes that provide specialized handling for different types of media files.

Instantiation:
Handlers are typically obtained from specialized factories called registries. They should not be constructed directly. Handlers that are defined by an extension will typically make use of the ObjectFactory mechanism.

Subtypes:
Some well known types of handlers in MediaWiki include:

• content handlers (ContentHandler)
• API modules (ApiBase)
• page actions (Action)
• special pages (SpecialPage)
• media handlers (MediaHandler)

Naming Convention:
Handlers often have their name end in "Handler", though some types of handlers use their own separate naming convention.

Layer constraint:
The handler pattern may be useful in all layers, but the associated registries logic may be considered part of the wiring layer.

Status:
As of August 2020, several instances of the Handler pattern exist in the code base, but the application of the pattern is somewhat post-hoc, and the mechanisms for registering and obtaining handlers are not uniform.

Examples:
Good examples of concrete handlers used by MediaWiki are CssContentHandler and JpegHandler.