Duplication in tests can be a good thing. The same can be said for production code as well in some cases. No. I'm not going mad. I probably wouldn't believe this if you showed me this several years ago either. More experience has shown me that loose coupling is often more desirable than removing duplication. In other words, the more duplication you remove, the more coupling you introduce inadvertently.
Duplication of logic is bad and will always be the case. I am not debating this. You should only have one logical place for any domain logic. As always follow the DRY principle. However just because two pieces of code look the same, does not mean there is duplication.
A system from my past had two places where an address was required for display and serialization. Billing and Delivery addresses.
My gut reaction was to introduce a common address model that could be used for serialization and display. After all this screams of duplication. However a billing address and delivery address are two conceptually different things despite appearing identical.
Given time the needs of the billing functionality may very well differ from the needs of the delivery domain. Duplication of models/contracts is weak duplication. There is no logic here.
In DDD each bounded context will have different needs. As it turned out the Billing Address began to have specific billing related functionality added such as "IsDefaultAddress" and "IsSameAsDelivery". At this point the two models are very different. This was a problem.
Sharing via a common library would have removed the total lines of code but increase the number of dependencies. The Address is now coupled to a single form meaning updates and new requirements are harder. Versioning and packaging are now a concern. Any updates would need to be coordinated across teams. Udi Dahan has warned about this previously in what is summarized as "Beware the Share".
This example makes inheritance look like a good fit. While the use of inheritance when applied correctly is not a bad thing, this scenario is not appropriate. Inheritance is one of the strongest forms of coupling. Applying inheritance across a type that we don't own is risky for the reasons detailed previously. Now change is not only harder, it would potentially be a breaking change. How would we model a delivery address with multiple addresses? Why should both the billing and delivery domain use the same terminology for its fields? If we accept that both addresses are conceptually different despite looking identical at present, we can side step these issues.
What to Share?
- Domain types should be shared. Using the previous example a PostalCode would make a good type to share. The functionality here is identical regardless of the type of address. PostalCode would likely have logic associated with the type which would not make sense to duplicate or implement in each sub system.
- Shared functionality that must be consistent makes a good candidate also. Examples such as UI widgets including headers and footers.
- Crossing cutting concerns such as logging, security and configuration can be shared when appropriate. A downside to this is you now force your consumers to take specific dependency versions which may or may not be acceptable.
DDD has the concept of a Shared Kernel. The dictionary definition of a kernel is "the central or most important part of something". Shared Kernel's make sense to share the common functionality previously. The name "common" is poorly thought out however. Most codebases will have a common or utility library but by there very nature these will grow into large components.
The reason for this growth is everything is common across applications. All applications need some sort of data access, so stick it in the common library. All applications need some sort of serialization mechanisms, so stick it in the common library. All applications need some sort of web technology, so stick it in the common library. You should be able to see where this is going.
As always when dealing with duplication apply the Rule of Three where appropriate. If you really must create a shared component, a small, concise library is better than a library that handles multiple concerns. This will allow consumers to adopt a "plug 'n play" approach with which components they require. Even then, try to fight removing duplication unless you can be really sure there is a good reason to increase coupling. That reuse you are striving for might not even come to fruition.