Types of Dependencies¶

• [[Framework]]
• [[3rd party library]]
• [[External system]] like [[File System]], [[Database]], Any [[system resource]].
• Dependency on a custom type built on top of the .NET framework

Policy depends on Details¶

UI -> Person -> PersonRepository

• High-level objects of the domain layer directly depend on low-level objects of the infrastructural layer.
• It's hard to replace coupled dependencies.
• We can solve any problem by introducing an extra level of indirection

Policy doesn't depend on Details¶

UI -> Person -> <interface> IPersonRepository <- PersonRepository

• IPersonRepository is a seam which inverts the dependencies.

Volatile and Stable dependencies¶

• What dependencies should we abstract away?
  • Volatile dependencies
    • Dependency itself depends on the [[environment]] (web servers, db)
    • Dependency doesn't yet exist and is still under development
    • Dependency which is not installed on all machines of developers
    • Dependencies has a [[nondeterministic behaviour]] ([[randomizer]]s inside)
    • If none of these are true, a dependency is stable.
    • Volatile dependencies are those which we want to abstract away by introducing levels of indirection.