Writing a Django e-commerce framework

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Writing a Django e-‐commerce framework OSCON 2012 Friday, 20 July 12

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Me •David Winterbottom / @codeinthehole •Tangent Labs, London •Head of E-‐commerce team •Python hacker •commandlinefu.com, django-‐oscar Friday, 20 July 12

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Synopsis •Motivation •Oscar -‐ what problem does it solve? •Design decisions -‐ customisation techniques •E-‐commerce tips/advice Friday, 20 July 12

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Motivation Friday, 20 July 12

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•5 years of e-‐commerce projects •PHP => Python / Django •Lots of war stories Tangent Labs Friday, 20 July 12

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Books Friday, 20 July 12

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Bookshop domain •Millions of products •ISBNs, BIC/BISAC categories, authors, publishers •Automated feed-‐driven processing •Catalogue •Stock •Rich data Friday, 20 July 12

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Two tier structure •Application tier -‐ serves HTTP requests •Processing tier: •Download and import feeds •Data cleaning and processing •Periodic jobs for fulﬁlment Friday, 20 July 12

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Variations •New fulﬁlment partners •Stock/availability logic •Multiple payment partners •Split-‐payment sources •Multibuy oﬀers, vouchers •eBooks! Friday, 20 July 12

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PHP framework •Home-‐rolled web framework •Customisation via include path overrides •Lots of duplication •Hard to upgrade Friday, 20 July 12

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Nailvarnish Friday, 20 July 12

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T-‐shirts Friday, 20 July 12

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Digital music Friday, 20 July 12

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Bar equipment Friday, 20 July 12

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New requirements •Product variations -‐ colours, sizes etc •New fulﬁlment processes: •webservices / warehouses •Per-‐customer prices Friday, 20 July 12

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B2B •Sales reps, customer hierarchies •Tax rules •Managed budgets •Integration with “enterprise” partners Friday, 20 July 12

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•Original assumptions all broken •Domains vary wildly •Lots of work-‐arounds Friday, 20 July 12

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Oscar Friday, 20 July 12

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Requirements •Lean -‐ as few assumptions as possible •Models domain •without duplication •without too much meta-‐data Friday, 20 July 12

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Domain modelling •Implementation deeply connected to core business concepts •“...where powerful new features unfold as corollaries to older features.” Friday, 20 July 12

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Lots of others •Satchmo •Lightning-‐Fast-‐Shop •Satchless •Django-‐shop •Plata, Mamona, Cartridge, ... •http://djangopackages.com/grids/g/ecommerce/ Friday, 20 July 12

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Design decisions Friday, 20 July 12

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Python •Decimal support •**kwargs •Mixins Friday, 20 July 12

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Web framework •Well-‐thought out structure •Templating, HTTP, security, caching, ... •Class-‐based views Friday, 20 July 12

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Models •Models drive the design •Capture domain logic •Foundation for rest of application Friday, 20 July 12

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Eco-‐system •Haystack (search) •South (database migrations) •Celery (job queue) •Internal libraries Friday, 20 July 12

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Customisation Friday, 20 July 12

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DEBUG = True settings.py Friday, 20 July 12

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AUTH_PROFILE_MODULE = ‘customer.Profile’ settings.py Friday, 20 July 12

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Diﬀerent approach •Key idea: •Having the same name/identiﬁer as your parent •Subclass and override Friday, 20 July 12

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Templates Friday, 20 July 12

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Friday, 20 July 12

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TEMPLATE_LOADERS = ( 'django.template.loaders.filesystem.Loader', 'django.template.loaders.app_directories.Loader', ) TEMPLATE_DIRS = ( '/var/www/project/templates/', ) Replace Friday, 20 July 12

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TEMPLATE_LOADERS = ( 'django.template.loaders.filesystem.Loader', 'django.template.loaders.app_directories.Loader', ) TEMPLATE_DIRS = ( '/var/www/project/templates/' '/path/to/oscar/', ) Override parent of Oscar’s template directory Friday, 20 July 12

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# base.html {% extends 'templates/base.html' %} {% block scripts %} {% endblock %} Override Friday, 20 July 12

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# base.html {% extends 'templates/base.html' %} {% block scripts %} {{ block.super }} {% endblock %} Extend Friday, 20 July 12

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Recap •Filename is the ID •Use include-‐path trick to “subclass” parent •Use blocks to provide hook points •...but, don’t go too far Friday, 20 July 12

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Apps Friday, 20 July 12

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from django.db.loading import get_model Line = get_model('basket', 'Line') Dynamic loading 1 INSTALLED_APPS = ( 'oscar.apps.basket' ) Friday, 20 July 12

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from django.db.loading import get_model Line = get_model('basket', 'Line') Dynamic loading 1 INSTALLED_APPS = ( 'oscar.apps.basket' ) ‘App label’ Friday, 20 July 12

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INSTALLED_APPS = ( 'oscar', 'oscar.apps.catalogue', 'oscar.apps.basket', 'oscar.apps.checkout', 'oscar.apps.order', ... ) Friday, 20 July 12

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INSTALLED_APPS = ( 'oscar', 'oscar.apps.catalogue', 'myproject.basket', 'oscar.apps.checkout', 'oscar.apps.order', ... ) Same app label as parent app Friday, 20 July 12

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# myproject/basket/models.py from django.db import models from oscar.apps.basket.abstract_models import \ AbstractLine class Line(AbstractLine): cost_centre = models.CharField(max_length=64) from oscar.apps.basket.models import * Friday, 20 July 12

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INSTALLED_APPS = ( 'oscar', 'oscar.apps.catalogue', 'oscar.apps.basket', 'oscar.apps.checkout', 'oscar.apps.order', ... ) templatetags management commands Friday, 20 July 12

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Recap •App label is the ID •Models can overridden and extended Friday, 20 July 12

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from oscar.core.loading import get_class OrderCreator = get_class('order.utils', 'OrderCreator') Dynamic loading 2 Friday, 20 July 12

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# order/utils.py from oscar.apps.order.utils import OrderCreator as \ CoreOrderCreator class OrderCreator(CoreOrderCreator): def allocate_stock(self, line): # Override/extend method ... Friday, 20 July 12

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Recap •(App module, class name) is the ID •Any class can be overridden or extended Friday, 20 July 12

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Tips and advice Friday, 20 July 12

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Log everything ...except sensitive customer details. Friday, 20 July 12

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Audit models Friday, 20 July 12

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from django.db import models class NielsenDataFile(models.Model): filepath = models.CharField(max_length=128) PENDING, FAILED, PROCESSED = range(3) status = models.IntegerField(default=PENDING) num_valid_records = models.IntegerField() num_invalid_records = models.IntegerField() date_downloaded = models.DateTimeField(null=True) date_processed = models.DateTimeField(null=True) Friday, 20 July 12

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•High and low thresholds •It’s always your fault when things go wrong Monitor everything Friday, 20 July 12

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Service layers •Avoid views talking to your models directly •See ‘Facade’ design pattern •Anti-‐corruption layers Friday, 20 July 12

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Generic vs bespoke •“If you’re using a framework, you aren’t doing good modelling” •Capture the domain correctly •It’s ok to throw away the framework Friday, 20 July 12

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Summary •Django is great for modelling complex domains •Writing customisable django apps isn’t easy •Oscar’s future: •NoSQL -‐ no more EAV Friday, 20 July 12

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Image credits Books: http://www.flickr.com/photos/th3ph17/3091294342/ Nail varnish: http://www.flickr.com/photos/kqedquest/831547339/ T-shirts: http://www.flickr.com/photos/blazerman/177165473/ Friday, 20 July 12