DDD, Anti Corruption layer, how-to?

Question

At the moment, we have to build an application which is based on a legacy one. Code for that old application should be thrown away and rewritten, but as it usually goes - instead of rewriting it, we need to base something new on it. Recently, we decided to go the DomainDrivenDesign path. So -- anti corruption layer could be a solution for our problems. As far as I understand, this way it should be possible to gradually rewrite the old application.

But -- I can't find any good example. I would appreciate ANY information.

Answer 1

From the DDD book (Domain-Driven Design: Tackling Complexity in the Heart of Software) by Eric Evans:

The public interface of the ANTICORRUPTION LAYER usually appears as a set of SERVICES, although occasionally it can take the form of an ENTITY.

and a bit later

One way of organizing the design of the ANTICORRUPTION LAYER is as a combination of FACADES, ADAPTERS (both from Gamma et al. 1995), and translators, along with the communication and transport mechanisms usually needed to talk between systems.

So, you might find examples by looking at the suggested adapter pattern and facade pattern.

I'll try to paraphrase what Eric Evans said, your anti-corruption layer will appear as services to the outside of your layer. So outside of the anti-corruption layer the other layers will not know they are "speaking" with a anti-corruption layer. Inside of the layer you would use adapters and facades to wrap your legacy information sources.

More information about the anti-corruption layer:

• Anatomy of an Anti-Corruption Layer, Part 1
• Building the often needed anti-corruption layer
• DDD - Anti corruption layer

Answer 2

In my particular implementation, EmployeeAccessService is called by a Repository. It's really a facade into the Anti-corruption layer. It delegates to the EmployeeAccessAdapter. The adapter fetches an object from the legacy model (which it gets from EmployeeAccessFacade),then passes it to the EmployeeAccessTranslator to transform the object from the legacy model to the domain object in my application's model.

See h t t p://www.troygeek.com/static/ac-layer.gif for a class diagram (sorry, new user so I couldn't include the image directly).

EmployeeAccessService
public Employee findEmployee(String empID){
return adapter.findEmployee(empID);
}

EmployeeAccessAdapter 
public Employee findEmployee(String empID){
EmployeeAccessContainer container = facade.findEmployeeAccess(empID);
return translator.translate(container);
}

EmployeeAccessTranslator
public Employee translate(EmployeeAccessContainer container){
     Employee emp = null;
     if (container != null) {
      employee = new Employee();
      employee.setEmpID(idPrefix + container.getEmployeeDTO().getEmpID());
      ...(more complex mappings)