Architecture:MediaWiki/data object 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

The data object pattern is often used when modelling domain entities, across all layers. Data objects are generally simple wrappers around a data structure that represents the properties of the object in question.

Data objects are ideally lightweight, serializable, and newable. Data objects may however also be mutable or perform lazy initialization by relying on persistence layer service objects. Objects that write themselves into the database ("active records") still fit the definition, though this approach is not recommended.

Instantiation:
Some kinds of data objects, in particular immutable values and exceptions, are typically newable - that is, they can be constructed directly. Other kinds, particularly ones that support lazy initialization or otherwise interact with the storage layer, should use dependency injection, and should be obtained from some kind of factory.

Subtypes:

• A record is a data object that is defined in the persistence layer and can be retrieved using a unique identifier.
• An immutable value is a data object that cannot be modified. It takes all information as constructor arguments, and provides getters to access this information. It does not perform I/O operations, and has no access to service objects.
• An exception is a data object that extends the pre-defined Extension class. Exceptions should be immutable value objects.
• An active record is a data object that implements its own persistence logic, it can load itself from storage, and update itself in storage. This pattern should generally be avoided, since it typically relies on global state.
• A content objects represents user editable content from the content representation domain.

Layer constraint:
Data objects can be useful in all layers.

Status:
As of June 2020, data objects are used throughout the MediaWiki codebase. However, they are often not entirely independent of global state, and some prominent examples like the Title and User objects are extremely heavy and belong to the active record subtype.

Examples:
An example of the data object pattern is the includes/Revision/RevisionRecord.php class.