Google App Engine

        Google App Engine lets you run web applications on Google's infrastructure. With App Engine, you only pay for what you use. There are no set-up costs and no recurring fees. App Engine applications are easy to build, easy to maintain. Different data storage options it can be provided. In App Engine there is no servers to maintain an application. The applications are generally build and test on your local host, after the successful test it will upload  to Google, then it's ready to serve users.

         You can build your app using  Python , Java  , Go runtime  environments. These runtime environments are built to ensure that your application runs quickly, securely, and without interference from other apps on the system. App Engine can deal with heavy amount of data and it provide following features.

• Dynamic web serving, with full support for common web technologies
• Persistent storage with queries, sorting and transactions
• Automatic scaling and load balancing
• APIs for authenticating users and sending email using Google Accounts
• A fully featured local development environment that simulates Google App Engine on your computer
• Task queues for performing work outside of the scope of a web request
• Scheduled tasks for triggering events at specified times and regular intervals

         This post will consider only the App engine with Python environment. Using these environment you can implement your app in Python programming language and run it on an optimized Python interpreter. The Python environment provides rich Python API's for the data store, Google accounts,  URL fetch and email services.   

 
         For our app engine implementation  use Python 2.7(download ) version, and  use the Python software development kit(download). The SDK versions are available for Linux, Mac-Os and Windows. Download  SDK for Linux and unzip it on your home, we will only handle  Linux Development environment.

Basic application development

          Let's begin by implementing a tiny application that displays a short message "Hello World". Create a directory helloworld. All the files for this application inside in these directory. Inside the helloworld directory, create a file name helloworld.py  and copy  the following contents

import webapp2

class MainPage(webapp2.RequestHandler):
  def get(self)
      self.response.headers['Content-Type'] = 'text/plain'
      self.response.write('Hello, World!')

app = webapp2.WSGIApplication([('/', MainPage)],
                              debug=True)

         This Python script responds to request with an HTTP header that describes the content and message hello, world!, the webapp2, it is a light weighted web frame work for Python. The webapp2 application has two parts a request handler and a WSGIApplication. The requestHandler classes that process request and build response. The WSGIApplication instance that route incoming request to handle based on URLs.

        Next step to create a configuration file. An App Engine  applications has configuration file called app.yaml, this file describes which handler scripts should used for which URLs. Inside the helloworld directory, create a file named app.yaml and copy  the following contents.

application: helloworld
version: 1
runtime: python27
api_version: 1
threadsafe: true

handlers:
- url: /stylesheets
  static_dir: stylesheets

- url: /.*
  script: helloworld.app

Now you can test your first app using SDK using the command 

google_appengine/dev_appserver.py helloword/

The web server is now running visit http://localhost:8080/ you can see the message Hello, world!

Implementing a Student application

         Considering an example student information system.The application will provide you enter details of  students and  will get back all data by sorting order by mark and age, also providing delete option and search option.

Source code is available in github, you can clone it into the your local machine by running the code

git clone git@github.com:rahulthrissur/studentapp.git

Now you can see a folder in your local machine as student.

student/
 app.yaml
 test.html
 test.py

Just go through the some parts of code, for these application changed the app.yaml file as 

- url: /.*
  script: <python file name>.app

From python script test.py

app = webapp2.WSGIApplication([('/', MainPage),('/details', details), ('/sort1', sort1), ('/sort2', sort2),('/search',search), ('/remove', remove), ('/searchresult', searchresult)],
 debug=True)

The WSGIApplication that use to route incoming request to handle based on URLs. Here define all URLs and corresponding routing. 

Using class define all the pages.

class <page name>(webapp2.RequestHandler):
  def get(self):
    self.response.headers['Content-Type'] = 'html'
    self.response.write("html contents")

You can write simple html file  inside the python script but it is not a good practice, use always templates, will discuss later.

Designing of html page refer  the links

http://www.htmldog.com/

http://www.w3schools.com/html/default.asp

Consider the next page 'details'.

class details(webapp2.RequestHandler):
 def get(self):
  self.response.out.write(template.render("test.html",{}));
 def post(self):
  name=self.request.get('student_name')
  sex=self.request.get('sex')
  age=self.request.get('age')
  mark=self.request.get('mark')
  data=student(key_name =name, Name=name, Sex=sex, Age=age, Mark=mark)
  data.put()
  self.redirect("/")

The get part of the page use a template, test.html file to provide enter the details of student. The test.html file was defined separate and rendered to python script. To use templat in python script import:

from google.appengine.ext.webapp import template

 The template is separate the presentation document from its data, also handle the variables in template files.
The  post part of the page get the values from the test.html and store it in variables. For handling data, define a class. Define the table name  and entry details.

from google.appengine.ext import db

class student(db.Model):
 Name = db.StringProperty()
 Age = db.StringProperty()
 Sex = db.StringProperty()
 Mark = db.StringProperty()

Selt.redirect use to redirect to main page

self.redirect("/")

Now retrieve the stored data using GQLQuery. Get data from the table student sort by age. Using a iteration function  separate all the data.

self.response.write("Details of all student sorted by Age")
  data = db.GqlQuery('SELECT * FROM student ORDER BY Age ASC')

  self.response.headers['Content-Type'] = 'text/plain'   
  for i in data:
      self.response.write('\n'+i.Name +' '+i.Sex+' '+i.Age+' '+i.Mark+ '\n')

For delete the details of student using key_name, was set when storing data.

address_k = db.Key.from_path('student', <key_name>)
db.delete(address_k)

For searching  detail of a student by name.

For running the application use the command

google_appengine/dev_appserver.py student/

The web server is now running visit http://localhost:8080/

Upload the application

You can it upload into Google server. Follow the steps:

First register the application using the link  https://appengine.google.com/

Edit the app.yaml file, then change the value of application: setting from helloworld to your registered application id(my id  myApp1) and using the command upload

google_appengine/appcfy.py update student/ 

Now run your application from Google server.

if you want to see full source code , here is the link:https://github.com/rahulthrissur/studentapp