GAE_KCDC2011

Building webapps for the Cloudwith Python and Google App Engine Juan Gomez PythonKC Co-founder June 25th, 2011

Agenda1. Intro to Google App Engine (GAE for short) • Brief intro to Python • Structure of a Python webapp • Setting up your dev environment with GAE • App Engine Architecture • The App Engine datastore and GQL2. Brief intro to Django • Using templates with GAE3. Quick demo of a sample webapp4. Beyond the basics • Scalability & Security • Quotas • Using the Google Data Services API5. Summary • Where to go from here? • References 2

Why Google App Engine?Challenges building web apps 3

Run your web applicationson Google’s infrastructure

Easy to start.Easy to scale.

The Components

1. Scalable Serving Infrastructure 2. Python Runtime 3. Software Development Kit 4. Web based Admin Console 5. Datastore 7

4. Web-based Admin Console 8

Click to Deploy 9

Memcache Send E-Mail Make HTTP RequestsAuthenticate with Google Accounts Image Manipulation

Does one thing well: running web apps

A Python Code Sample x = 34 - 23 # A comment. y = “Hello” # Another one. z = 3.45 if z == 3.45 or y == “Hello”: x = x + 1 y = y + “ World” # String concat. print x print y 14

Enough to Understand the Code• First assignment to a variable creates it• Assignment is = and comparison is ==• For numbers + - * / % are as expected • Special use: • + for string concatenation • % for string formatting (as in C’s printf)• Logical operators are words (and, or, not) not symbols (&&, ||, !).• The basic printing command is print 15

Comments• Start comments with #, rest of line is ignored• Can include a “documentation string” as the first line of a new function or class you define• Development environments, debugger, and other tools use it: it’s good style to include one def my_function(x, y): “““This is the docstring. This function does blah blah blah.””” # The code would go here... 16

Python and Types• Everything is an object!• “Dynamic Typing”-> Data types determined automatically.• “Strong Typing” -> Enforces them after it figures them out.x = “the answer is ” # Decides x is string.y = 23 # Decides y is integer.print x + y # Python will complain about this. 17

Basic Datatypes• Integers (default for numbers) •z = 5 / 2 # Answer 2, integer division• Floats •x = 3.456• Strings • Can use “” or ‘’ to specify with “abc” == ‘abc’ • Unmatched can occur within the string: “matt’s” • Use triple double-quotes for multi-line strings or strings that contain both ‘ and “ inside of them: “““a‘b“c””” 18

WhitespaceWhitespace is meaningful in Python: especiallyindentation and placement of newlines•Use a newline to end a line of code Use when must go to next line prematurely•No braces {} to mark blocks of code, useconsistent indentation instead • First line with less indentation is outside of the block • First line with more indentation starts a nested block•Colons start of a new block in many constructs,e.g. function definitions, then clauses 19

Assignment• You can assign to multiple names at the same time >>> x, y = 2, 3 >>> x 2 >>> y 3This makes it easy to swap values >>> x, y = y, x• Assignments can be chained >>> a = b = x = 2 20

A Python Code Samplex = 34 - 23 # A comment.y = “Hello” # Another one.z = 3.45if z == 3.45 or y == “Hello”: x = x + 1 y = y + “ World” # String concat.print xprint y 21

Side by Side with Java Java (C#) Pythonpublic class Employee class Employee():{ private String myEmployeeName; private int myTaxDeductions = 1; private String myMaritalStatus = "single"; def __init__(self, employeeName public Employee(String EmployeName) { , taxDeductions=1 this(EmployeName, 1); } , maritalStatus="single" ): public Employee(String EmployeName, int taxDeductions) { this(EmployeName, taxDeductions, "single"); } self.employeeName = employeeName public Employee(String EmployeName, int taxDeductions, self.taxDeductions = taxDeductions String maritalStatus) self.maritalStatus = maritalStatus { this.myEmployeeName = EmployeName; this.myTaxDeductions = taxDeductions; this.myMaritalStatus = maritalStatus; }} 22

Life is Short (You Need Python) - Bruce Eckel (Thinking in C++) 23

Things to read through: The Quick Python Book, 2nd Ed http://amzn.to/lXKzH5 “Dive into Python” http://diveintopython.org/Python 101 – Beginning Pythonhttp://www.rexx.com/~dkuhlman/python_101/python_101.html 24

Things to refer to: Python in a Nutshell, 2nd Ed http://amzn.to/mrkvFX Programming Python, 4th Ed http://amzn.to/kWjaW2The Official Python Tutorialhttp://www.python.org/doc/current/tut/tut.htmlThe Python Quick Referencehttp://rgruet.free.fr/PQR2.3.html 25

Google App Engine

App Engine Does One Thing Well• App Engine handles HTTP(S) requests, nothing else – Think RPC: request in, processing, response out – Works well for the web and AJAX; also for other services• App configuration is dead simple – No performance tuning needed• Everything is built to scale – “infinite” number of apps, requests/sec, storage capacity – APIs are simple, stupid 28

And that allows it to:• Serve static files• Serve dynamic requests• Store data• Call web services• Authenticate against Google’s user database• Send e-mail, process images, use memcache 29

Scaling• 5 million page views a month at the free quota• Low-usage apps: many apps per physical host• High-usage apps: multiple physical hosts per app• Stateless APIs are trivial to replicate• Memcache is trivial to shard• Datastore built on top of Bigtable; designed to scale well – Abstraction on top of Bigtable – API influenced by scalability • No joins • Recommendations: denormalize schema; precompute joins 5

Python webapps• App Engine includes a simple web application framework called webapp.• webapp is a WSGI-compatible framework.• You can use webapp or any other WSGI framework with GAE (like web.py, CherryPy, Tornado, Django)• Basic apps need Config file + webapp CGI 31

Config file (No XML!)• A webapp specifies runtime configuration, including versions and URLs, in a file named app.yaml.application: myappversion: 1runtime: pythonapi_version: 1handlers:- url: /admin/.* script: admin.py login: admin- url: /index.html script: home.py- url: /(.*.(gif|png|jpg)) static_files: static/1 upload: static/(.*.(gif|png|jpg)) 32

Structure of a Python webappfrom google.appengine.ext import webappfrom google.appengine.ext.webapp.util import run_wsgi_appclass 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() 33

Setting up your dev environment with GAE• The dev environment is really, really nice• Download the (open source) SDK – http://code.google.com/appengine/downloads.html – Google_App_Engine_SDK_for_Python• a full simulation of the App Engine environment• dev_appserver.py myapp for a local webserver• appcfg.py update myapp to deploy to the cloud• You get a GUI on Mac OS and Windows. 34

App Engine Architecture req/resp stateless APIs R/O FS urlfech Python stdlib VM mail process app images stateful datastore APIs memcache 4

The Datastore• Based on “BigTable”• Schemaless• Scales infinitely• NoSQL, with SQL type queries (GQL) – No joins (they do have “reference fields”) – No aggregate queries - not even count()! – Hierarchy affects sharding and transactions – All queries must run against an existing index• Maybe the best part of App Engine! 36

Hierarchical Datastore• Entities have a Kind, a Key, and Properties – Entity -> Record -> Python dict -> Python class instance – Key -> structured foreign key; includes Kind – Kind -> Table -> Python class – Property -> Column or Field; has a type• Dynamically typed: Property types are recorded per Entity• Key has either id or name – the id is auto-assigned; alternatively, the name is set by app – A key can be a path including the parent key, and so on• Paths define entity groups which limit transactions – A transaction locks the root entity (parentless ancestor key) 10

Creating an Entity in the Datastorefrom google.appengine.ext import db...class FoursquareUser(db.Model): """Creation of an entity of the kind FoursquareUser""" created = db.DateTimeProperty(auto_now_add=True) name = db.TextProperty() email = db.TextProperty() description = db.StringProperty(multiline=True) 38

GQL• GQL is a SQL-like language for retrieving entities or keys from the datastore.• GQLs features are different from a query language for a traditional relational database.• GQL syntax is (very) similar to that of SQL• SELECT [* | __key__] FROM <kind> [WHERE <condition> [AND <condition> ...]] [ORDER BY <property> [ASC | DESC] [, <property> [ASC |DESC] ...]] [LIMIT [<offset>,]<count>] [OFFSET <offset>] <condition> := <property> {< | <= | > | >= | = | != } <value> <condition> := <property> IN <list> <condition> := ANCESTOR IS <entity or key> 39

Querying my FoursquareUser Entityfrom google.appengine.ext import db...# I can query the datastore using the Query APIquery = db.Query(FoursquareUser)# Or the methods inherited form db.Modelmodel_query = FoursquareUser.all()# Or using GQLGQL_query = db.GqlQuery("SELECT * FROM FoursquareUser")# I can print the results to the HTTP response objectfor user in query: self.response.out.write("User name: %s" % user["name"]) self.response.out.write("User email: %s" % user["email"]) 40

Creating the Views• HTML embedded in code is messy and difficult to maintain.• Its better to use a templating system. – HTML is kept in a separate files with special syntax to indicate where the data from the application appears in the view.• There are many templating systems for Python: EZT, Cheetah, ClearSilver, Quixote, and Django are just a few.• You can use your template engine of choice by bundling it with your application code.• Or you can use Django out of the box! 41

It’s a magical pony!

Why use Django over webapp?• Django templating is one of the best in its class• Django has easy cookies and custom 500 errors• Django is less verbose• Django middleware is really handy• URL patterns – mapping between URL patterns (RegEx) to callback functions (views). – URLconf 43

Using templates with GAEfrom django.conf.urls.defaults import *from django.http import HttpResponsedef hello(request): return HttpResponse("Hello, World!")urlpatterns = patterns(, (^$, hello),)# You have to write even less code! 44

Quick demo of a sample webapp 13

Automatic Scaling to Application Needs• You don’t need to configure your resource needs• One CPU can handle many requests per second• Apps are hashed (really mapped) onto CPUs: – One process per app, many apps per CPU – Creating a new process is a matter of cloning a generic “model” process and then loading the application code (in fact the clones are pre-created and sit in a queue) – The process hangs around to handle more requests (reuse) – Eventually old processes are killed (recycle)• Busy apps (many QPS) get assigned to multiple CPUs – This automatically adapts to the need • as long as CPUs are available 8

Security• Prevent the bad guys from breaking (into) your app• Constrain direct OS functionality – no processes, threads, dynamic library loading – no sockets (use urlfetch API) – can’t write files (use datastore) – disallow unsafe Python extensions (e.g. ctypes)• Limit resource usage – Limit 3000 files per app, each at most 1MB – Hard time limit of 30 seconds per request – Most requests must use less than 300 msec CPU time – Hard limit of 32MB/10MB on request/response size, API call size, etc. – Quota system for number of requests, API calls, emails sent, etc 6

Preserving Fairness Through Quotas• Everything an app does is limited by quotas.• If you run out of quota that particular operation is blocked• Free quotas are tuned so that a well-written app (light CPU/datastore use) can survive a moderate “slashdotting”• The point of quotas is to be able to support a very large number of small apps.• Large apps need raised quotas ($$$) 9

Quotas Costs 49

Quotas Quota’s 50

Quotas 51

Using the Google Data Service API import gdata.docs.service# Create a client class which will make HTTP requests withGoogle Docs server.client = gdata.docs.service.DocsService()# Authenticate using your Google Docs email address andpassword.client.ClientLogin(jo@gmail.com, password)# Query the server for an Atom feed containing a list of yourdocuments.documents_feed = client.GetDocumentListFeed()# Loop through the feed and extract each document entry.for document_entry in documents_feed.entry: # Display the title of the document on the command line. print document_entry.title.text 52

What about vendor Lock-in? 53

What about vendor Lock-in?• Use Django-nonrel – independent branch of Django that adds NoSQL database support to the ORM http://www.allbuttonspressed.com/projects/django- nonrel – No JOINs! :-(• Port existing Django projects with very few changes!• Switch from Bigtable to other NoSQL DBs – MongoDB, SimpleDB. (soon Cassandra, CouchDB)• Move freely between Django hosting providers – ep.io, Gondor, Webfaction or your own VPS 54

Where to go from here?• Learn Python!• Download the App Engine SDK• Build Apps...Many Apps• Learn Django! (is awesome)• Join PythonKC – http://groups.google.com/group/pythonkc – http://www.meetup.com/pythonkc/ 55

Resources• Read the Articles on the Google App Engine site. – http://code.google.com/appengine/docs/python/gettingstarted/ – http://code.google.com/appengine/docs/python/overview.html – http://appengine-cookbook.appspot.com/• Keep up with new releases through the Blog – http://googleappengine.blogspot.com/• Read the source code from the SDK – http://code.google.com/p/googleappengine/ – Maybe you will find an undocumented API. 56

Before the Q & A... 14

Yes! Slides can be downloaded here:http://www.slideshare.net/_juandg/gaekcdc2011 58

Q&A• Anything goes 14

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GAE_KCDC2011

by Juan Gomez

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