Overview of common customizations

This is a list of interfaces or abstract classes in DD4T that are very commonly implemented cq. extended.

Interface	Rationale	Examples	Known customers	Priority	Notes
ILogger	Implement a different logging mechanism	Serilog, MEF	Essity, Ecclesiastical	High
IDependencyMapper	Add dependency mapping independently from any DI implementation	For custom DI implementations: read dependency mappings For any other customization: write dependency mappings	Essity, Ecclesiastical	Very High
DD4T ViewModel base attributes	See ViewModel base attributes for a complete overview	Various extensions to the ViewModel attributes	Fujitsu, Essity, Ricoh	High
ICacheAgent	Implement a different caching mechanism	Redis, serialized cache agent	Essity, Ecclesiastical	High
IPublicationResolver	Implement a custom publication resolving mechanism	Common practice is to create a publication resolver wrapped around the DynamicMappingsRetriever (which isn't out of the box in DD4T). Other use cases would be to use browser settings or Geolocation.	Many	Low	In Web 8 one could argue that the DynamicMappingsRetriever is always an improvement, and DXA 2.0 does not support older Tridion versions.
ILogger	Critical, Error, Warning, Information, Debug	Log through the mechanism provided by DD4T	Many	High
ILinkResolver / DefaultLinkResolver	ResolveUrl (2 overloads)	Extend link resolving mechanism	Essity, Fujitsu	High	Examples: to remove file extensions, always link to canonical url, cross-site scripting (Fujitsu)
IRichTextResolver / DefaultRichTextResolver	Resolve	Extend rich text resolving mechanism	Essity	High	Examples: select correct CDN url out of a set (used with ECL), custom logic depending on the schema of the target component
IMessageProvider	Start	Create custom cache invalidation mechanism	Vasco, Fujitsu	Medium	Mostly used (I think) because of problems in the official DD4T version (see https://github.com/dd4t/DD4T.Caching.ApacheMQ).
ILogger	Critical, Error, Warning, Information, Debug	Log through the mechanism provided by DD4T	Many	High
ILinkResolver / DefaultLinkResolver	ResolveUrl (2 overloads)	Extend link resolving mechanism	Essity, Fujitsu	High	Examples: to remove file extensions, always link to canonical url, cross-site scripting (Fujitsu)
IRichTextResolver / DefaultRichTextResolver	Resolve		Essity		Select correct CDN url out of a set (used with ECL)
IPageProvider / IComponentPresentationProvider / IBinaryProvider /	Various	Support different Tridion version or load data from a different source, e.g. for unit testing.			Extremely powerful because it allows customers on older Tridion versions to use DD4T.