The Core Layer

The core layer contains actual business logic. First we start off with the entity, the rich business objects of the application. In core/entities/entity.go,


type Gopher struct {
    Name string
    Age int

Entities are completely invisible to the outside layers. Not any thing but the interactors know about them. Entities contain business logic, e.g., a Gopher entity can modify itself or contain functions related to it, but the distinction between entities and interactors is the following:

  • entities modify themselves vs.
  • interactors modify entities

An entity can contain other entities: a Gopher, could technically possess a Tail and two Eyes, and it can modify them at will. This hierarchy is strictly unidirectional: a Gopher doesn’t know about other gophers, more importantly, it doesn’t know about the interactor.


Interactors contain rich business logic. They can manipulate entities and they implement boundaries. Here, we have the Gophers boundary from above to implement, so we implement a smallish interactor that implements it.

type Gophers struct {
    burrow map[int]entities.Gopher

func NewGophers() *Gophers {
    return &Gophers{
        burrow: make(map[int]entities.Gopher),

It implements the three methods as defined by the Gophers boundary

// Find finds a gopher from storage.
func (g Gophers) Find(req requests.FindGopher) (responses.FindGopher, error) {
    gopher, exists := g.burrow[req.ID]
    if !exists {
        return responses.FindGopher{}, errors.New("Not found.")

    return gopher.ToFindGopher()

func (g Gophers) FindAll(req requests.FindGopher) ([]responses.FindGopher, error) {
    var resps []responses.FindGopher
    for _, gopher := range g.burrow {
        fg, err := gopher.ToFindGopher()
        if err != nil {
            return []responses.FindGopher{}, err
        resps = append(resps, fg)
    return resps, nil

// Create creates a gopher.
func (g Gophers) Create(req requests.CreateGopher) (responses.CreateGopher, error) {
    var gopher entities.Gopher
    if err := gopher.Validate(req); err != nil {
        return responses.CreateGopher{}, err

    gopher.ID = g.getFreeKey()
    gopher.Name = req.Name
    gopher.Age = req.Age
    g.burrow[gopher.ID] = gopher

    return responses.CreateGopher{ID: gopher.ID}, nil

As one can see, the interactor is completely unaware of any protocol dependencies. The relation to web applications is obvious: we are, after all, talking about requests and responses, and the DTOs translate very easily to JSON objects. But they can be used without JSON, in fact, the whole point is that even a GUI application will pass the same objects around.

The interactors (and by extension, entities) are completely oblivious to their environment: they don’t care whether they are running inside a GUI application, a system-level daemon, or a web server.

Beware of Behemoths

Interactors are business logic units. How much business logic is too much business logic? The best rule of thumb is the single responsibility principle: an interactor should only do one thing, and one thing only. I’m also going to address this below, but the most important thing to understand about interactors is that they should operate only one one aspect of the business logic.

What this means may not be immediately clear. If you are building a REST API, you will generally have some separation of concerns already going on at the external API level, in the form of URIs. To use a book catalogue as an ad hoc example, you could have a URI for book authors at /authors and /books, these clearly indicate—to the API user, anyway—what lies beneath.

At the code level, this distinction must be maintained. An author may contain a collection of books they have, but whose responsibility is modifying them? Obviously, since we have two URIs here, one for books, one for authors, we must decide which one handles the logic of modifying book entities. In this case, any internal modification logic of the book entities must reside underneath a single interactor. There can be two cases here:

  • One interactor does everything. The /books URI is just an alias underneath the Author interactor, or vice versa.
    • Pros: no overlap in logic, no conflicts, since everything is contained under one unit (a single interactor).
    • Cons: must be split eventually, since otherwise it will grow to monstrous proportions.
  • Two interactors, ``AuthorInteractor`` and ``BookInteractor``. The AuthorInteractor calls methods of the IBookService (which BookInteractor implements) to modify the Book entities contained (or owned) by an Author entity.
    • Pros: no chance of overlap since the responsibilities are split.
    • Cons: risk of introducing circular dependencies between boundaries (see below).

If you’re building a really simple service, you don’t have to split interactor duties, but it’s a good idea. Be careful of choosing short-term practicality in favor of long-term abstractions, it may bite you in the rear one day!

As a summary, in the presented example, the AuthorInteractor should only modify things related to Authors, and preferably only read data about Books, leaving modification and updates to the BookInteractor. There are two ways on how to implement the necessary communication, that is, how the AuthorInteractor calls the BookInteractor, and this will be resolved later, but now we have a small interlude about something equally vital: the external world.