Hexagonal Design in Django

Maarten van Schaik

Django

• Rapid application development• ./manage.py startapp polls• Define some models, forms, views and we’re

in business!

Applications live and grow

• More features are added:– API access– Reporting– Command Line Interface– Integration with other applications– Who knows what else…

Connected Design

• Components can access all other components• When you need to access a piece of data or a

piece of logic, just import and use it• Development is fast

Modular design

• Access to other components goes through well-defined interfaces (API’s) using well-defined protocols

• Components have high cohesion• Components are loosely

coupled

Connected vs Modular

Ports and Adapters

Ports and Adapters

• Specific adapter for each use of the application, e.g. web view, command line, message queue, etc.

• Each adapter connects to a port of the application

• Mock adapters and test harnesses facilitate testing

• Testing becomes easier and faster

Typical Django app

• __init__.py• admin.py• forms.py• models.py• tests.py• urls.py• views.py

App

Uh oh…

• Where is my application?

??GU

I

ModelsViews

Forms

Persistence

Refactoring Django apps

• Rules– Core domain model cannot depend on Django– Tell objects, ask values

Implications

• Core model cannot depend on framework– Core model cannot derive from models.Model– Communication with Django goes through

adapters• Tell, don’t ask– Views should render from immutable values– So no vote.save() in views.py!

Example – Poll

def vote(request, poll_id): p = get_object_or_404(Poll, pk=poll_id) try: selected_choice = p.choice_set.get( pk=request.POST[‘choice’]) except (KeyError, Choice.DoesNotExist): return render(request, …, {error: …}) else: selected_choice.votes += 1 selected_choice.save() return HttpResponseRedirect(…)

Example – Poll

def vote(request, poll_id): p = get_object_or_404(Poll, pk=poll_id) try: selected_choice = p.choice_set.get( pk=request.POST[‘choice’]) except (KeyError, Choice.DoesNotExist): return render(request, …, {error: …}) else: selected_choice.votes += 1 selected_choice.save() return HttpResponseRedirect(…)

Mutating data!

Example – Poll (2)

def vote(request, poll_id): try: poll_engine.register_vote(poll_id, request.POST[‘choice’]) except Exception as e: return render(request, …, {error: …}) else: return HttpResponseRedirect(…)

Example – Poll (2)

## Poll engine

def register_vote(poll_id, choice_id): p = Poll.objects.get(pk=poll_id) selected_choice = p.choice_set.get(pk=choice_id) selected_choice.votes += 1 selected_choice.save()

Example – Poll (2)

## Poll engine

def register_vote(poll_id, choice_id): p = Poll.objects.get(pk=poll_id) selected_choice = p.choice_set.get(pk=choice_id) selected_choice.votes += 1 selected_choice.save()

Dependency on Django models

Example – Poll (3)

## Poll engine

def register_vote(poll_id, choice_id): if not poll_repository.choice_exists(poll_id, choice_id): raise PollException(…)

poll_repository.increment_vote_count(choice_id)

Example – Poll (3)

## Django model adapter

def choice_exists(poll_id, choice_id): return Choice.objects.filter( poll_id=poll_id, pk=choice_id).exists()

def increment_vote_count(choice_id) Choice.objects.filter(pk=choice_id).update( votes=F(‘votes’)+1)

Conclusions

• Hexagonal design will help keep speed of adding new features constant

• Encourages modularity and encapsulation• Encourages clean and well-organized

applications• Tests become faster when using plain objects

and data• Django models are not that useful without

coupling with them

That’s it

Thanks and references

• Matt Wynne: Hexagonal Rails• Kent Beck: To Design or Not To Design?• Alistair Cockburn: Hexagonal Architecture • Django tutorial