Launchpad is a Python-based web application with over 350k lines of code and 25k tests. It was developed primarily by Canonical using Zope 3, Twisted, and other libraries. The presentation discusses best practices from Launchpad's development including extensive testing, code reviews, use of branches, and interfaces. It also covers challenges like slow tests and database evolution.

1. Launchpad
The good, the bad, and
the OMG how does that work?
Tim Penhey
tim@penhey.net
IRC: thumper
#nzpug
Kiwi PyCon 2009

2. Launchpad? WTF is Launchpad?
● Python based web application
● ~350k lines of python
● ~25k tests
● Uses Zope 3 and Twisted libraries
● Developed primarily by Canonical
● AGPL3 now
● https://launchpad.net
Kiwi PyCon 2009

3. Kiwi PyCon 2009

4. Learn From Others
● Been in development for over five years
● Made lots of mistakes
● Found many ways not to do things
● Nuggets hidden in the depths
● Here to share
Kiwi PyCon 2009

5. Testing is Essential
● All types of tests throughout the code
● Unit tests
● Doc tests
● Acceptance level tests
● Windmill Javascript tests
● Run your tests automatically
Kiwi PyCon 2009

6. Slow Tests are Bad
● Complete test suite run takes around four hours
● Cannot run all tests for every landing now
● Buildbot, devel and stable branches
● Sample data for tests causes problems
● tearDown needs to reset DB state
● Zope test layers proliferate
● Run the test in the lowest layer possible
Kiwi PyCon 2009

7. Code Reviews Are Good
● All changes are reviewed by at least one other
developer
● New team members went through a mentoring
process to learn from other reviewers
● Extra eyes on code can spot issues that the
developer misses
● Reviewer makes sure new code has tests
● Have a coding standard
Kiwi PyCon 2009

8. Multiple Environments are Good
● Production – updated every four weeks
● https://launchpad.net
● Edge – updated nightly
● https://edge.launchpad.net
● Beta testers are automatically redirected here
● Staging – updated nightly with production copy
● https://staging.launchpad.net
● Test area for people to mess around with
Kiwi PyCon 2009

9. Can lead to many branches
● devel – primary development branch
● stable – devel with all tests passed
● Rolled to edge nightly
● db-devel – database patches + devel
● db-stable – db-devel with all the tests passed
● Rolled to staging nightly
● production – what we rolled out + cherry picks
Kiwi PyCon 2009

10. Bazaar makes it all workable
● Bazaar is a distributed revision control system
(DVCS)
● Merging just works
● Develop in branches
● Merge into devel (or db-devel)
Kiwi PyCon 2009

11. Object-Relational Mappers
● Mixed blessing
● Originally used SQLObject
● Moved to Storm
● http://storm.canonical.com
Kiwi PyCon 2009

12. Branches
● Represent bazaar branches in Launchpad
● Have owners
● Most belong to a project or source package
● Branches can be linked to be “official”
● lp:bzr
● lp:ubunutu/jaunty/gwibber
● lp:~thumper/launchpad/fix-branch-layout
Kiwi PyCon 2009

13. Branch Listings
● Shows up to 100 branches on a page
● Primary text is the “bazaar identity”
● For any single branch it can effectively traverse
across 8 tables
● Naïve approach would mean 800 queries to
show a listing
Kiwi PyCon 2009

14. Branch Listing Solution
● Utility class – BranchListingQueryOptimiser
● lazr.delegates
● Wraps real object but can override specific method
● View class determines visible batch
● Single queries executed for the “set” of visible
branches
Kiwi PyCon 2009

15. Branch Collection Idiom
● Global utility to get all branches
>>> collection = getUtility(IAllBranches)
● Filter methods return a new branch collection
>>> collection = collection.inProject(foo)
>>> collection = collection.ownedBy(eric)
● Subclasses handle visibility based on privacy
>>> collection = collection.visibleBy(user)
>>> collection = collection.visibleBy(None)
Kiwi PyCon 2009

16. Branch Collection Idiom
● Accessor methods return result sets
>>> branches = collection.getBranches()
>>> branches.order_by(Branch.name)
● Look at the code:
● lib/lp/code/model/branchcollection.py
● lib/lp/code/interfaces/branchcollection.py
Kiwi PyCon 2009

17. Interfaces are Good
● zope.interface and zope.schema
● schema contains field types
class IFruit(Interface):
name = Text(required=True)
def throw(target):
"""No self defined for methods."""
Kiwi PyCon 2009

18. Interfaces are Good
● Model classes implement interfaces
class Fruit(object):
implements(IFruit)
def __init__(self, name):
self.name = name
def throw(self, target):
return target.hit_with(self)
Kiwi PyCon 2009

19. Interfaces are Good
>>> apple = Fruit('apple')
>>> IFruit.providedBy(apple)
True
>>> from zope.interfaces.verify import (
... verifyObject)
>>> verifyObject(IFruit, apple)
True
Kiwi PyCon 2009

20. Zope Views and Pages
● Path traversal is mostly object traversal
● Views are associated with objects through
interfaces
● Resulting web pages are rendered views
● Most views use a page template
Kiwi PyCon 2009

21. Adapters Supercharge Interfaces
● Adapters allow an object to be converted to a
different interface in a defined way
def branch_collection_for_product(project):
"""Adapt a project to a branch collection."""
return getUtility(IAllBranches).inProject(project)
>>> collection = IBranchCollection(project)
Kiwi PyCon 2009

22. Be Smart About Pages
● Many different objects have branches
● Simple branch listing registered against
IHasBranches instead of individual interfaces
● View adapts the objects to IBranchCollection
● Each object that implements IHasBranches
also has an IBranchCollection adapter
Kiwi PyCon 2009

23. Launchpad API using lazr.restful
● Annotations to the interface class allow the
objects to be exposed over a ReST based API
● This is still magic to me
● lazr.restful and lazr.restfulclient are found on
Launchpad
● launchpadlib is a Python client API
Kiwi PyCon 2009

24. Modules Matter
● Initially all of Launchpad was in the
canonical.launchpad module
● .browser – contained the views
● .interfaces – contained all the interfaces
● .database – contained all the model code
● .templates – contained all the page templates
● .doc – documentation including doc tests
● .ftests, .pagetests, .webapp, …
● Became just too cumbersome
Kiwi PyCon 2009

25. The Module Move Apocalypse
● Each team was responsible for moving code
● New base module “lp” chosen
● lp.registry – people, projects, distributions
● lp.bugs – code related to the bug tracker
● lp.services – code used by other applications
● mail, jobs, testing, scripts
● lp.codehosting, lp.blueprints, lp.translations
Kiwi PyCon 2009

26. ZCML Is Not Fun
● Zope Configuration Markup Language is an
XML document
● Defines content classes, security, adapters,
utilities, views
● Registration away from the actual code hinders
discoverability
● Ideally we'd like to bring in some of the ideas
from Grok and Martian
Kiwi PyCon 2009

27. Learn to use TAGS
● TAGS are used describe the location of the
function definition
● Can be read by Emacs and Vi
● Have a make target (or equivalent) to build your
TAGS file
Kiwi PyCon 2009

28. Databases Evolve
● Your database schema will evolve
● New tables and columns
● Changing columns
● With time comes complexity
● Have a way to evolve your database schema
● Launchpad uses SQL patch files to describe
discrete changes
● Production database is updated every 4 weeks
Kiwi PyCon 2009

29. Share Common Components
● lazr-js ● lazr.publisher
● lazr.authentication ● lazr.restful
● lazr.batchnavigator ● lazr.restfulclient
● lazr.canonicalurl ● lazr.smtptest
● lazr.config ● lazr.testing
● lazr.delegates ● lazr.uri
● lazr.enum ● storm
● lazr.exportedfolder ● wadllib
● lazr.lifecycle
Kiwi PyCon 2009

30. Eat Your Own Dog Food
Kiwi PyCon 2009

31. That's It
Questions?
Kiwi PyCon 2009

32. Lunch!
Kiwi PyCon 2009