Getting started with webapp2 and GAE

By : Akshar Raaj

Deploying a webapp2 app on Google app engine

We want to setup our dev environment to write a web application using webapp2 which we will deploy to Google app engine.

Agenda

  • About GAE(Google App Engine)
  • Setting up dev environment for GAE
  • Running webapp2 locally
  • Deploying it on app engine

About GAE

They key points around GAE are:

  • It provides Platform as a Service. GAE is not Infrastructure as a service.
  • Fully managed environment. Your runtime, libraries and frameworks are managed, installed and updated behind the scene. You don't have to do sysadmin task. You don't have to provision the server.
  • GAE provides shell access when needed. App engine docs puts it as "Infrastructure when you need it."

Setting up dev environment for GAE

GAE is part of Google Cloud Platform(GCP), so first we need GCP SDK.

GCP SDK can be installed from here. Once GCP is installed, you should be able to run command gcloud.

If you are on Mac, you should see a directory called "google-cloud-sdk" in ~/Downloads. If you are on Ubuntu, you will find this directory "google-cloud-sdk" in whichever folder your downloads go.

Once GCP is installed, we need App engine extension for Python. App engine extension can be downloaded from here. The most important thing is:

gcloud components install app-engine-python

Running webapp2 locally

Once app engine extension for python is installed, a directory called google_appengine is added to platform directory of Google Cloud SDK installation path.

So you should be able to see ~/Downloads/google-cloud-sdk/platform/google_appengine/. This directory has a file called dev_appserver.py which we need for running the app locallly.

Let's write some code now. Create a file main.py with following content

import webapp2

class HelloWebapp2(webapp2.RequestHandler):
    def get(self):
        self.response.write('Hello, webapp2!')

app = webapp2.WSGIApplication([
    ('/', HelloWebapp2),
], debug=True)

def main():
    from paste import httpserver
    httpserver.serve(app, host='127.0.0.1', port='8080')

if __name__ == '__main__':
    main()

We need a yaml file called app.yaml which tells to app engine about runtime environment and other provisioning information. Add a file app.yaml at same level as main.py with following content.

runtime: python27
api_version: 1
threadsafe: true

handlers:
- url: /.**
  script: main.app

Start the dev server by issuing following command

./<path-to-dev_appserver.py> app.yaml

Deploying to GAE

Create or choose an exisiting project on GCP console from here.

I have a project with id "the-pentameter-845". Using gcloud config, I set this as my current project so that I can deploy my code under this project.

gcloud config set account akshar@agiliq.com
gcloud config set project the-pentameter-845

And then deploy your code using following command

gcloud app deploy

gcloud would prompt and ask you if you want to deploy to https://the-pentameter-845.appspot.com. Your url would be different depending on your project id.

With this deployment should be done and you should be able to view your project at https://.appspot.com.

Adding more routes

Add one more class.

class NedStark(webapp2.RequestHandler):
    def get(self):
        self.response.write('Hi! I am Ned. People considered me very brave.')

And add it as a route so that this class gets used when someone accesses url /ned

app = webapp2.WSGIApplication([
    ('/', HelloWebapp2),
    ('/ned', NedStark),
], debug=True)

After this you should be able to access localhost:8080/ned. No change is needed in app.yaml

Deploy the app again.

gcloud app deploy app.yaml

Now you should be able to access https://.appspot.com/ned

Installing libraries with pip

GAE runtime environment provides some third party libraries, so these need not be installed with pip. Similarly during development, dev_appserver.py provides these same libraries which need not be installed with pip. eg: One such library is webapp2 itself. You can see complete list of third party libraries available in runtime environment here

If you want to use a library which isn't provided by app engine runtime, you need to follow the steps mentioned here

Assuming you want to write a handler which has to fetch data from some database. Suppose you want to use SQLAlchemy for fethcing data, so we need SQLAlchemy in our application.

Add line import sqlalchemy in main.py and try to access the url. Your development server wouldn't respond because you don't have sqlalchemy. To add SQLAlchemy do the following.

mkdir lib
pip install -t lib/ SQLAlchemy

Add a file appengine_config.py with following content

from google.appengine.ext import vendor

vendor.add('lib')

Now access any url, and development server would again start serving the urls properly.


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