Python Ireland Nov 2009 Talk - Appengine

Google App Engine

Writing a
Stack Overflow clone
on App Engine
in under an hour*

http://blog.bitquabit.com/2009/07/01/one-which-i-call-out-hacker-news/

Agenda

1. App Engine Introduction
2. Demo: Building an app
3. Demo: Deploying an app
4. Demo: Writing App Overflow
5. App Engine's Services
6. Questions

If I say "Cloud" at any point, Boo!

Building web applications is hard

Thinking about scalability

Just a few users....

....the tools, platform and design
don't matter too much

Thinking about scalability

Lots and lots of users...

....you must design for scalability

WhiteHouse.gov/openforquestions

Serving developers AND their customers
Google App Engine
Scalable

High Performance

Standards

Cost effective

1. Scalable Serving Architecture

creative commons licensed photograph from cote


Incoming Requests

App Engine App Engine App Engine
Front End Front End Front End
Load Balancer

AppServer AppServer AppServer


Incoming Requests

App Engine App Engine App Engine
Front End Front End Front End
Load Balancer

AppServer AppServer AppServer

Other Google
AppServer Infrastructure

API Layer - Bigtable

- Google Accounts

App App App - Memcache

- Image manipulation

2. Distributed Datastore

The Datastore is...

Distributed

Transactional

Natively Partitioned

Hierarchial Wow. That is
one big table.
Schemaless

Based on Bigtable

2. Distributed Datastore

The Datastore is not...

A relational database

A SQL engine

Just Bigtable
Wow. That is
one big table.

App configuration

application: appoverflow
version: 1
runtime: python
api_version: 1

handlers:
- url: /.*
script: /request.py

Request handling

class IndexHandler(webapp.RequestHandler):
def render_template(self, name, values):
path = os.path.join(os.path.dirname(__file__),
'templates', name)
self.response.out.write(template.render(path, values))

def get(self):
self.render_template('index.html', {})

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

def main():
run_wsgi_app(application)

Demo: Deploying an app
In 30 seconds or less. Time me!

Adding authentication

class IndexHandler(webapp.RequestHandler):
def __init__(self):
self.user = users.get_current_user()

def render_template(self, name, values):
url = self.request.url
template_values.update({
'user': self.user,
'login_url':users.create_login_url(url),
'logout_url':users.create_logout_url(url),
})
path = os.path.join(os.path.dirname(__file__),
'templates', template_name)
self.response.out.write(template.render(path,
template_values))

Adding authentication

<html>
<head>
<title>{%block title%}App Overflow{%endblock%}</title>
{% block head %}{% endblock %}
</head>
<body>
<div class="login">
{% if user %}
Logged in as {{user.nickname}} |
<a href="{{logout_url}}">Log Out</a>
{% else %}
<a href="{{login_url}}">Log In</a>
{% endif %}
</div>
{% block body %}{% endblock %}
</body>
</html>

Storing data

from google.appengine.ext import db

class Question(db.Model):
asker = db.UserProperty(required=True)
title = db.StringProperty(required=True, indexed=False)
body = db.TextProperty(required=True)
asked_at = db.DateTimeProperty(auto_now_add=True)

Storing data

class NewQuestionHandler(BaseHandler):
def get(self):
self.render_form(forms.QuestionForm())

def post(self):
form = forms.QuestionForm(self.request.POST)
if not form.is_valid():
self.render_form(form)
return

entity = models.Question(
asker=self.user,
**form.clean_data)
entity.put()
self.redirect('/questions/%d/' % (entity.key().id(),))

Fetching data

class QuestionHandler(BaseHandler):
def get_question(self, id):
question = models.Question.get_by_id(int(id))
return question

def render_page(self, question):
template_values = {
'question': question,
}
self.render_template('question.html', template_values)

def get(self, id):
question = self.get_question(id)
if question:
self.render_page(question)

Queries

class IndexHandler(BaseHandler):
def get_questions(self):
q = models.Question.all()
q.order('-asked_at')
return q.fetch(20)

def render_page(self):
template_values = {
'questions': self.get_questions(),
}
self.render_template('index.html', template_values)

def get(self):
self.render_page()

Precomputation

class Question(db.Model):
asker = db.UserProperty(required=True)
title = db.StringProperty(required=True, indexed=False)
asked_at = db.DateTimeProperty(auto_now_add=True)
answer_count = db.IntegerProperty(required=True,
default=0)

class Answer(db.Model):
answerer = db.UserProperty(required=True)
answered_at = db.DateTimeProperty(auto_now_add=True)

Transactions

answer = models.Answer(
parent=question,
answerer=self.user,
# ...
)

def save_answer(answer):
def _tx():
question = db.get(answer.parent().key())
question.answer_count += 1
db.put([answer, question])
return answer.key()
return db.run_in_transaction(_tx)

XMPP

class XmppHandler(xmpp_handlers.CommandHandler):
def ask_command(self, message=None):
question = models.Question(
title=message.arg,
body=message.arg,
sender=message.sender)
question.put()
message.reply('Your question has been received. You will be pinged when an answer
is submitted.')

# Elsewhere...

xmpp.send_message([question.sender], 'Answer: ' +
answer.body)

Additional services / APIs

URL Fetch
Memcache
Mail - incoming and outgoing
Images
Google Accounts
Cron support
Task Queue

Python Ireland Nov 2009 Talk - Appengine

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