CODEWITHSUNDEEP
phpc++domain-driven-designrepository-patternobject-persistence
Magnus
Magnus

Reputation: 101

In Domain Driven Design, does the Repository pattern maintain a reference to the objects?

I understand that the repository pattern abstracts the persistence of domain objects, allowing a developer to read/write/delete objects from persistent storage without knowing how the object is stored (SQL, NoSQL, flat files, etc). I'm quite fond of the repository pattern and find it works well in many situtations, e.g., abstracting the business logic from the persistence logic, allowing lazy-loading of objects where appropriate, etc.

However, what I'm not clear on is whether or not the repository object maintains a reference to all objects or not? For example:

Repository repository;
std::shared_ptr<Person> pPerson = repository.retrievePersonById("bob");

p->updateDetails("Bob", "the Builder");
repository.savePerson(p);
  1. In the above hypothetical C++ example, should repository maintain a reference to the Person instance returned? Yes - thanks Guillaume31 for your in-memory analogy.
  2. If the answer to Q1 is "yes", when and how should repository remove that instance of Person? Presumably this is when the reference count reaches zero.
  3. If the answer to Q1 is "no", how do you handle situations when another area of wants access to the same object, but because repository no longer stores an internal reference to it, it hydrates a fresh copy from the database, and you effectively have two instances of Person when you really should be referring to the same instance? Given that the answer to Q1 was "Yes", then the repository always maintains a reference and so should always return the same object.

I'm actually writing a PHP application, despite the fact that the above example is in C++.

Upvotes: 0

Views: 960

Answers (1)

guillaume31
guillaume31

Reputation: 14080

A more appropriate metaphor for a repository might be that it is the illusion of an in-memory collection of objects. Take your basic collection type from any OO language. If you get an element from that collection and modify the element, you typically don't have to save it back to the collection afterwards, because it has never ceased being in the collection.

Same goes for a repository -- it serves objects, can add objects to itself, but doesn't expose any features for saving modifications to an underlying storage. Indeed, it's all about hiding the existence of an underlying storage. It doesn't expose any methods to "update" the state of an entity either, because the entity it served is in memory and you can modify it freely, it's never out of sync.

It's also better if the Repository keeps its hands off transaction management and committing units of work. You should delegate that to the client (see Domain Driven Design p. 156).

To answer your questions, inside a business transaction, you shouldn't assume anything about the freshness of objects returned by a Repository. They just reflect the states of some entities at some point in time, all you have to do is take them as they are and use them. At a more global level, some external mechanism (typically, an ORM tool) will provide you with the ability to manage how you isolate your little business transaction from others, usually in the form of a Unit of Work implementation. Flushing changes to the database and handling potential stale entities problems is not a per repository query decision, it's a more global, business transaction level decision that should happen only when you decide your use case is finished and you want to commit it.

Upvotes: 1

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