in Coding, Studies

Writing an online scraper on Google App Engine (Python)

Sometimes you need to collect data – for visualization, data-mining, research or whatever you want. But collecting data takes time, especially when time is a major concern and data should be collected over a long period.
Typically you would use a dedicated machine (e.g. a server) to do this, rather then using your own laptop or PC to crawl the internet for weeks. But setting up a server can be complicated and time consuming – nothing you would do for a small private project.

A good and free alternative is the Google App Engine (GAE). The GAE is a web-hosting service of Google which offers a platform to upload Java and Python applications. It comes with its own user authentication system and its own database. If you already have a Google account, you can upload up to ten applications for free. However, the free version has some limitations, e.g. you only have a 1 GB database with a maximum of 50.000 write-operations per day (more details).

One big advantage of the GAE is the possibility to create cron-jobs. A cron-job is a task that is executed on fixed points in time, e.g. all 10 minutes. Exactly what you need to build a scraper!

But let’s do it step by step:

1. Registration

First of all, you need a Google account and you must be registered by the GAE. After your registration, you can create a new application (go to https://appengine.google.com and click on Create Application).

01_gae_registration

Choose the name for your application wisely, you can’t change it later on!

2. Install Python, GAE SDK and Google Eclipse plugin

To start programming for GAE, you need to set up some simple things. Since we want to develop an application in Python, Python (v. 2.7) must be installed on your computer. Also, you need to install the GAE SDK for Python. Optional, you can also install the Google plugin for Eclipse together wit PyDev which I would recommend, because it makes life much easier.

3. Create your application

Now you can start and develop your application! Open Eclipse and create a new PyDev Google App Engine Project. To make a GAE application, we need at least two files: a main Python script and the app.yaml (a configuration file). Since we want to create a cron-job, too, we need a third file (cron.yaml) to define this job. For reading a RSS stream we also use a third-party library called feedparser.py. Just download the ZIP-file and unpack the file feedparser.py to your project folder (this is ok for the beginning). A very simple scrawler could look like this:

Scrawler.py

app.yaml

Note: The application must be the same name as your registered on Google in the first step!

cron.yaml

Done! Your project should look like this now (including feedparser.py):

02_gae_project

4. Test it on your own machine

Before we deploy the application on GAE, we want to test it locally to see if it is really working. To do so, we have to make a new run-configuration in Eclipse. Click on the small arrow at the small green run button and choose “Run configurations…”. Then, create a new “Google App Engine” configuration and fill in the following parameters (see the pictures):

Name:
GAE (you can choose anything as name)

Project:
TechRepScrawler (your project in your Eclipse workspace)

Main Module:
C:Program Files (x86)Googlegoogle_appenginedev_appserver.py (dev_appserver.py in your GAE installation folder)

Program Arguments:
--admin_port=9000 "C:UsersThomasworkspace_pythonTechRepScrawler"

04_gae_run_config_1

04_gae_run_config_2

After starting GAE locally on your computer using the run configuration, just open your browser and go to http://localhost:8080/ to see the running application. You can also go to an admin perspective on http://localhost:9000/ to see, e.g. some information about your data.

5. Deploy your application to GAE

The next – and last step! – is to deploy the application on GAE. Using the Google Eclipse plugin, this is as easy as it can be. Just click right on your project, go to PyDev: Google App Engine and click upload. Now your app will be upload on GAE! On the first time, you will be asked for your credentials, that’s all.

05_gae_upload

06_gae_deployed

Now your app is online and available for every one! The cron-job will refresh it every 10 minutes (which just means, it will surf on your site like every other user would do it). Here’s how it should look:

07_last

Best regards,
Thomas Uhrig