Question

As I understand it, a UnitOfWork class is meant to represent the concept of a business transaction in the domain. It’s not directly supposed to represent a database transaction, which is a detail of only one possible implementation.

Q: So why does so much documentation about the Unit of Work pattern refer to “Commit” and “Rollback” methods?

These concepts mean nothing to the domain, or to domain experts. A business transaction can be “completed”, and therefore the UnitOfWork should provide a “Complete” method. Likewise, instead of a “Rollback” method, shouldn’t it be modeled as “Clear”?

Update:

Answer: Both answers below are correct. Their are two variants of UoW: object registration and caller registration. In object registration, Rollback serves to undo changes to all in-memory objects. In caller registration, Rollback serves to clear all recorded changes such that subsequent call to Commit will do nothing.

Answer 1

The Unit of Work design pattern, at least as defined by Fowler in Patterns of Enterprise Application Architecture – is an implementation detail concerning object-relational persistence mapping. It is not an entity defined in Evans’ Domain Driven Design.

As such, it should neither be part of the business discussion, nor an entity that’s directly exposed in a domain model – perhaps excepting the commit() method. Instead its intent is tracking “clean” and “dirty” business entities – the objects from a domain model exposed to clients. The purpose is allowing multiple interactions – in web context requests – with a domain model without the need to read and write from persistence (usually a database) each time.

Business entities call it when their methods are called. When their state is altered, they register themselves as dirty with the Unit of Work. Then the Unit of Work’s commit() handles the entire persistence transaction in terms of writing out the object graph and rollback() means restoring the state of entities to what they were. So its very much the implementation leaking through to the “abstraction”, but its intent is very clear.

On the other hand, “Undo” and “Complete” don’t necessarily map one-to-one with this definition. An “Undo” or “Clear” may only rollback an object graph partially for instance depending on the business context. While “Complete” may well be altering state on some entity as well as committing the graph. As such I would put these methods, with business meaning, on a Service Layer or Aggregate Root object.