Domain-Driven Design: Tackling Complexity in the Heart of Software

Author: Eric Evans
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by jgrodziski   2017-10-19
Identifying changing "stuff" in the real world is for me a fundamental topic of any serious data modeling for any kind of software (be it an API, a traditional database stuff, etc). Identity is also at the center of the entity concept of Domain-Driven Design (see the seminal book of Eric Evans on that:

I started changing my way of looking at identity by reading the rationale of clojure (

More specifically concerning the article, I do agree with the point of view of the author distinguishing access by identifier and hierarchical compound name better represented as a search. On the id stuff, I find the amazon approach of using URN (in summary: a namespaced identifier) very appealing: And of course, performance matters concerning IDs and UUID:

Happy data modeling :)

EDIT: - add an excerpt from the clojure rationale

by anonymous   2017-10-01

Implement Repository pattern is pretty easy, you just need to create interface with CRUD methods and use it in your domain logic.

For example:

class CreateEntityException;
class ReadEntityException;
class UpdateEntityException;
class DeleteEntityException;

interface Repository<Entity> {
    Entity create(Entity entity) throws CreateEntityException;
    Entity read(long entityId) throws ReadEntityException;
    Entity update(Entity entity) throws UpdateEntityException;
    void delete(long entityId) throws DeleteEntityException;

Methods count and signature can be different in your own project but an approach is the same. After it you can create concrete implementation of repository that encapsulate one or another datasource - ContentProviderRepository, OrmLiteRepository, RealmRepository etc. Then by using Dependency Injection principle you should inject correct implementation.

There are few good books that covered Repository patterns. Pattern is independent from platform so it is easy to implement and use every platform.

by anonymous   2017-09-18
I agree that this is the best approach. For a very detailed overview of this design pattern, check out the great Eric Evans book, Domain Driven Design (
by galeaspablo   2017-08-20
> If you aren’t familiar with the database pattern known as event sourcing (don’t worry — it’s relatively new),

It's not relatively new. That “transaction file” thing in your database? Event Sourcing.


If that doesn't do it for you, please just remember the good old CAP theorem.

by jpalomaki   2017-08-20
Two books that affected my thinking on the subject were Domain Driven Design[1] by Eric Evans and Object Thinking[2] by David West. Many years since I read the books and I don't claim to have studied them in detail so I'm not saying if they were good or bad, but at least I got some ideas out of them.



by anonymous   2017-08-20

Active Record Pattern

Django is tailored towards the use of the Active Record Pattern as described on this Django Design Philosophy page.

Your second example follows this pattern - the model itself has its properties, behaviour and data access contained within.

You can still use this pattern in a more DDD-like way, if you push more behaviour onto the model. e.g. in your example, a more effective use of the pattern would be to wrap the line

poll.question += '?'

in an intention revealing method on the poll object, so that the update_poll method is:
def update_poll(poll_id):
    poll = models.Polls.objects.get(poll_id)

This has the advantage of separating the business logic (pushed into the model) from the application flow logic (the update_poll method)

Although I'd suggest using a name that actually illustrates the intent or purpose of the method rather than just add_question.

But even if you do this, you are still using the Active Record pattern, not pure DDD.

You ask "Is there a need for DDD with ORM?"

DDD and an ORM are attempting to solve different problems. ORMs provide a convenient way of abstracting the set-like record-oriented world of databases in a more object oriented fashion.

DDD is an approach to assist with modelling complex real world situations in code.

Many DDD systems use ORMs to solve the infrastructure concerns of retrieving and persisting from the database (sometimes wrapping the ORM in a repository for a variety of reasons), but the DDD focus is on the domain models and how suitably they model the domain under consideration.

So - in your example, the benefits of DDD are difficult to see, as the business logic is so relatively simple.

I recommend reading the authoritive source on DDD - Domain Driven Design by Eric Evans for a language agnostic overview of the approach and the situations where it adds value.


You ask:

Can you update me with one good example where using DDD with an ORM makes sense


If we use ORM I think there is no need for a DDD-repository

I think a better way to think about it is - when using an ORM, the ORM is the repository. You ask it for a model and it returns a model. That is the purpose of a repository. When people wrap it in a class called 'repository' it is usually because they want to do one of a few things:

  1. make it easier to inject a mock repository to simplify unit testing
  2. abstract the specific orm technology used to give flexibility to change the ORM later without having to touch the services or domain

This overview of the repository pattern provides another good writeup of the ddd repository pattern.

by anonymous   2017-08-20

Repository pattern emerges in the context of DDD, and the best book to read on that would be the original Domain-Driven Design: Tackling Complexity in the Heart of Software

One of the things that Evans keeps saying over and over is that all of the concepts of the domain must be explicit.

What does your menu represent? Does it have a name in the domain?

Lets say it does, and it is the concept of "weekly promoted products", then you could make this concept explicit - and introduce a separate class for that, with a separate repository, that could read data from products table, and from categories table to come up with the list of promoted products.

Please note that according to Evans our domain and persistence models do not have to match, and we don't need to have a repository per domain object (specifically, we should only have repositories for aggregate roots, and references all other entities should be obtained by traversing the association from the aggregate root)

Alternatively, in a design, somewhat departed from DDD, you could place the selection of those promoted products in the product repository, pre-load the categories and just have a TopCategory calculation property on a product

Please note, that if the new entity "promoted products" is purely virtual, and you are only planning to do read operations, without mutating its state, then it makes a lot of sense to apply Command Query Responsibility Seggregation, and have a separate read model for "promoted products" outside of your main domain model.

by anonymous   2017-08-20

The whole Chapter 6 of Eric Evans book is devoted to the problems you are describing.

First of all, Factory in DDD doesn't have to be a standalone service -

Evans DDD, p. 139:

There are many ways to design FACTORIES. Several special-purpose creation patterns - FACTORY METHOD, ABSTRACT FACTORY, and BUILDER - were thoroughly treated in Gamma et. al 1995. <...> The point here is not to delve deeply into designing factories, but rather to show the place of factories as important components of a domain design.

Each creation method in Evans FACTORY enforces all invariants of the created object, however, object reconstitution is a special case

Evans DDD, p. 145:

A FACTORY reconstituting an object will handle violation of an invariant differently. During creation of a new object, a FACTORY should simply balk when invariant isn't met, but a more flexible response may be necessary in reconstitution.

This is important, because it leads us to creating separate FACTORIES for creation and reconstitution. (in the diagram on page 155 TradeRepository uses a specialized SQL TradeOrderFactory, not a general general purpose TradeOrderFactory )

So you need to implement a separate logic for reconstitution, and there are several ways to do it (You can find the full theory in Martin J Fowler Patterns Of Enterprise Application Architecture, on page 169 there's a subheading Mapping Data to Domain Fields, but not all of methods described look suitable(for example making the object fields package-private in java is seems to be too intrusive) so I'd prefer only one of the following two options

  • You can create a separate FACTORY and document it so that developers should only use it only for persistence or testing.
  • You can set the private field values with reflection, as for example Hibernate does.

Regarding the anemic domain model with setters/and getters, the upcoming Vaughn Vernon book criticizes this approach a lot so I dare say it is an antipattern in DDD.

by anonymous   2017-08-20

As far as I can see contracts don't exist without cars hence cars is the aggregate root.

This is not necessarily true. 'Don't exist without' is not enough for an entity to become a part of an Aggregate Root. Consider classic order processing domain. You have an Order that is an Aggregate Root. You also have a Customer that is an Aggregate Root. Order can not exist without a Customer but it does not mean that Orders are part of the Customer Aggregate. In DDD entities inside one Aggregate can have references to other Aggregate Roots. From DDD book:

Objects within the AGGREGATE can hold references to other AGGREGATE roots.

Aggregate is a life cycle and data exchange unit. It is essentially a cluster of objects that enforces invariants. This is something you want to be locked if you have multiple users changing domain at the same time.

Back to your question, my understanding is that the domain is something like rent / lease a car / truck / limo / bulldozer. I think that HireContract may not be a part of Car aggregate because they may have different lifecycles and HireContract just makes sense on its own, without a Car. It seem to be more of a Order-Product relationship that is also a classic example of two different Aggregates referencing each other. This theory is also confirmed by the fact that business needs to see "All Contracts". They probably don't think of Car containing all Contracts. If this is true than you need to keep your ContractsRepository.

On an unrelated note, you might be interested in reading this answer about repository interface design.

by anonymous   2017-08-20

What you are looking for is what in xUnit Test Patterns is called Test-Specific Equality.

While you can sometimes choose to override the Equals method, this may lead to Equality Pollution because the implementation you need to the test may not be the correct one for the type in general.

For example, Domain-Driven Design distinguishes between Entities and Value Objects, and those have vastly different equality semantics.

When this is the case, you can write a custom comparison for the type in question.

If you get tired doing this, AutoFixture's Likeness class offers general-purpose Test-Specific Equality. With your Student class, this would allow you to write a test like this:

public void VerifyThatStudentAreEqual()
    Student st1 = new Student();
    st1.ID = 20;
    st1.Name = "ligaoren";

    Student st2 = new Student();
    st2.ID = 20;
    st2.Name = "ligaoren";

    var expectedStudent = new Likeness<Student, Student>(st1);

    Assert.AreEqual(expectedStudent, st2);

This doesn't require you to override Equals on Student.

Likeness performs a semantic comparison, so it can also compare two different types as long as they are semantically similar.