Flash talk given on 7 aug 2013 for the Amsterdam Python Meetup Group

1. Hexagonal Design in Django
Maarten van Schaik

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

3. Applications live and grow
• More features are added:
– API access
– Reporting
– Command Line Interface
– Integration with other applications
– Who knows what else…

4. 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

5. 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

6. Connected vs Modular

7. Ports and Adapters

8. 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

9. Typical Django app
• __init__.py
• admin.py
• forms.py
• models.py
• tests.py
• urls.py
• views.py

10. App
Uh oh…
• Where is my application?
??
Models
Views
Forms

11. Refactoring Django apps
• Rules
– Core domain model cannot depend on Django
– Tell objects, ask values

12. 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!

13. 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(…)

14. 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!

15. 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(…)

16. 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()

17. 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

18. 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)

19. 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)

20. 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

21. That’s it

22. Thanks and references
• Matt Wynne: Hexagonal Rails
• Kent Beck: To Design or Not To Design?
• Alistair Cockburn: Hexagonal Architecture
• Django tutorial