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Introduction to Google
App Engine with Python
Brian Lyttle (http://brianlyttle.com)
@brianly on Twitter

What is Google App Engine?
? Google call it ��cloud development in a box��.

? It is a hosted development environment along the lines of Microsoft
Azure.
? Targets users building internet applications, but that is changing.

? Built on the same scalable platform used for other Google services.

? Development is done locally in Python (or Java) on Windows, Mac
OS X, or Linux.
? Push to the cloud and Google will manage everything.

? We��ll dig into the details shortly.

Who��s using App Engine?

? Case Studies
? http://code.google.com/appengine/casestudies.html

? Most often GAE is used as a utility alongside another platform.

? GAE for Business will increase uptake. Eventually.

How much does it cost?
? It is free to get started, or run small applications.
? ~5M page views per month

? Google post quotas and costs on their website
? http://code.google.com/appengine/docs/quotas.html

? Once you reach certain limits Google will start charging you based on your usage.

? Usage costs can be expensive depending on your requirements, or alternative deployment
platforms.
? Choice of datastore can even impact costs

? Calculating costs can be tricky, but this is the case with all usage-based cloud providers and
services.
? You need to know your application, and the how the platform can be best used applied to
your needs.
? Cheaper features might have potential for higher lock-in.

Mapping use cases to App Engine
? You can��t really choose a framework or tool without some experience. Take this advice with a
pinch of salt.

? Good
? You want to learn a Python web development framework.
? Managing servers is not something you want to do.
? Your application needs to support a ridiculous number of users, or giant fluctuations in usage.
? You want to quickly prototype a larger application that you may develop elsewhere.

? Maybe
? An existing application is written in Python (or Java) and you want to create a web front end.

? Bad
? Data sources for your application live inside your company.
? Other data security or privacy requirements.
? You need a version of Python newer than 2.5.
? Your Python library requires a C extension and does not have a Python fallback.

Learning Python
? Zed Shaw��s ��Learn Python the Hard Way�� is recommended.
? http://learnpythonthehardway.org/index

? Many other resources are available:
? http://diveintopython.org/toc/index.html
? http://docs.python.org/tutorial/

? Focus on learning Python 2.x if you want to use the broadest range
of libraries today.
? Python 3.2 is the latest but newbies will have issues with libraries.

? GAE makes the choice easier. Use CPython 2.5.

? Get familiar with REPL-style development in the shell.

Development tools
? Python 2.5
? Only this version is supported at present.
? Free from http://www.python.org/ftp/python/
? Get the latest 2.5.x release from http://www.python.org/ftp/python/2.5.5/

? Google App Engine SDK
? Download the latest version and it will auto-update.
? Free from http://code.google.com/appengine/downloads.html

? A suitable editor
? Most (good) text editors provide syntax highlighting.
? I like JetBrains�� PyCharm, but ActiveState Komodo is also good. A lot of
people like the Wingware IDE.
? You could use the latest Python support for Visual Studio 2010 if you want
but it does not have special support for App Engine.

Documentation and samples
? All docs can be downloaded as a ZIP
? http://code.google.com/appengine/downloads.html -
Download_the_Google_App_Engine_Documentation

? Documentation
? http://code.google.com/appengine/docs/python/overview.html

? Sample Code
? http://code.google.com/p/google-app-engine-samples/
? http://code.google.com/p/rietveld/
? https://github.com/search?langOverride=&language=python&q=google+app+engine&repo=
&start_value=1&type=Repositories&x=0&y=0
? http://brizzled.clapper.org/id/77/

? Videos
? Google IO Conference on YouTtube.
? PyCon: http://pycon.bliptv.com

Services provided by App Engine
Datastore (SQL Server)
? Services are accessed via ��webapp��.
Blobstore (File system)
Capabilities
? Most have equivalents in the .NET world
(shown in parentheses) Channel (comet?)
Images (ImageGlue.NET)
? Sites hosted at *.appspot.com Mail (BCL)
Memcache (AppFabric)
? Limitations: Multitenancy (SQL Schemas)
? SSL for custom domains OAuth (BCL)
? Quotas impacting cost are complex Prospective Search
? Users all have Gmail emails by default Task Queues (MSMQ)

? Python libraries which depend on C URL Fetch (BCL)
extensions cannot run on the platform. Users (BCL)

? Google-like search requires some effort. XMPP (Lync API)

Basic App Engine demo

? Bookmarkz
? Bookmark database
? URL shortener

? Using the default ��webapp�� framework

? Source code is available from
https://bitbucket.org/brianly/bookmarkz/src

Webapp framework

? Based on a basic framework called WebOb that runs atop
WSGI.
? WSGI is an interface to Python web servers.

? Whilst you have basic support for MVC, you don��t have the
support provided by a full framework like ASP.NET MVC.

? Convention or configuration? Neither.

? For a lot of applications you should be considering a
framework like Tipfy (more later).

Example handler
from google.appengine.ext import webapp
from google.appengine.ext.webapp.util import run_wsgi_app

class MainPage(webapp.RequestHandler):
def get(self):
self.response.headers['Content-Type'] = 'text/plain'
self.response.out.write('Hello, webapp World!')

application = webapp.WSGIApplication(
[('/', MainPage)],
debug=True)

def main():
run_wsgi_app(application)

if __name__ == "__main__":
main()

Datastore

? You Entities based on Classes which derive from a special base class.

? Google provide a range of data types for Entity properties.

? Entities are stored in a system based on Big Table (Master/Slave) or
Megastore (High Replication).
? You choose this when you create your application.

? Choice of storage affects costs and can��t be changed easily.

? Google recommend the High Replication datastore for better all
round performance.
? Fewer peaks and troughs compared to the Master/Slave store.

Working with Models
? Models are defined in Python in a similar fashion to other ORM tools.

? The challenges are a little different from your experiences using ORMs with
relational databases.

? Data types
? http://code.google.com/appengine/docs/python/datastore/typesandpropertycla
sses.html
Definition Query

class Song(db.Model): query = GqlQuery("SELECT * FROM
author = db.UserProperty() Song WHERE composer
composer = db.StringProperty() = :composer",
date = db.DateTimeProperty composer="Lennon, John")
(auto_now_add=True)
tags = db.StringListProperty() query = db.Query(Song)
description = db.TextProperty() query.filter('composer=', id)
result = query.get()

More Models

? Expando and PolyModel.

? More information can be found in the SDK documentation

class Person(db.Expando): class Contact(polymodel.PolyModel):
first_name = db.StringProperty() phone_number = db.PhoneNumberProperty()
last_name = db.StringProperty() address = db.PostalAddressProperty()
hobbies = db.StringListProperty()
class Person(Contact):
first_name = db.StringProperty()
last_name = db.StringProperty()
mobile_number = db.PhoneNumberProperty()
class Company(Contact):
name = db.StringProperty()
fax_number = db.PhoneNumberProperty()

Blobstore
? Limits apply to object size in the datastore.

? Store large objects e.g. images or documents in the Blobstore

? Exceptions might be small thumbnails where they are always
returned with related fields.
? Use the BlobstoreUploadHandler class to make uploading blob
objects less painful.
? BlobstoreDownloadHandler provides a way to specify byte ranges.

? BlobReader gives you a stream-like API.

? Plays well with the Images API.

URL Fetch
? Enables requests to HTTP and HTTPS resources.

? Use for calls to REST (and SOAP) web services.

? Supports calls to your intranet through the Google Secure Data
Connector (SDC).

from google.appengine.api import urlfetch

result = urlfetch.fetch(url="http://www.corp.example.com/sales.csv",
headers={'use_intranet': 'yes'})
if result.status_code == 200:
parseCSV(result.content)

? Download size is limited to 32 MB.

Task Queues

? Allows you to do work later such as send a notification, or
update a 3rd party system.
# Application queues a task
taskqueue.add(url='/worker', params={'key': key})

# Handler does the work
class CounterWorker(webapp.RequestHandler):
def post(self): # should run at most 1/s
key = self.request.get('key')
def txn():
counter = Counter.get_by_key_name(key)
if counter is None:
counter = Counter(key_name=key, count=1)
else:
counter.count += 1
counter.put()
db.run_in_transaction(txn)

Notes for Windows users
? Python works pretty well on Windows compared to other
languages.

? Google��s imaging library needs an extension called PIL.

? 64-bit versions of Python and the Python Imaging Library (PIL)
? Make sure that the architectures of extensions match up.
? Windows programs a 64-bit application cannot load a 32-bit DLL,
and vice versa.
? See my blog post on this topic.

? Windows Firewall and blocked ports
? No server == no fun

Health and uptime

? Check the status page if
you experience issues.

? Situations
? Read-only Datastore
? ��Regular�� failures

? The Capabilities API is
provided to enable you
to handle maintenance
periods.

Better frameworks for App Engine
? The ��webapp�� framework is bare bones and based on WSGI.

? Any WSGI framework can run on App Engine.

? Kay
? Components: Django, Werkzeug, Jinja2, babel
? http://code.google.com/p/kay-framework/

? Tipfy
? Custom framework with multiple template engines, and plugins.
? http://www.tipfy.org/

? Django-nonrel
? A NoSQL-optimized version of Django.
? Wesley Chun��s from Google presented on this at PyCon.
? http://www.allbuttonspressed.com/projects/django-nonrel

Run your own App Engine
? ��Platform as a Service�� can lock you in
? Data lock-in is probably a bigger problem than frameworks.

? Possible solutions
? Typhoon AE: http://code.google.com/p/typhoonae/
? AppScale: http://sites.google.com/site/gaeasaframework/appscale

? Essentially these are a lot of pain to setup, and you are probably
using the App Engine framework to solve the wrong problem.
? A better solution is to use a native Python framework:
? Django: http://www.djangoproject.com/
? Pyramid (Pylons): http://pylonsproject.org/
? Flask: http://flask.pocoo.org/

Thanks!

? Don��t forget to provide provide feedback on my talk and
others you attended today.

? �ݺ�ߣs and code will be posted to http://brianlyttle.com
shortly.

�ݺ�ߣ

Googleappengineintro 110410190620-phpapp01

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