AppEngine backends allow us to perform long term, memory and processor intensive work. As the name suggests these are for back-end type work loads. Say for example you are crawling and scraping websites to collect price points from multiple sources to serve later or just doing a report. These types of tasks can be done on the back end. AppEngine backends are separate instances from the front end instances. In terms of quotas, this means a lot. Instead of clubbing all your work on the front-end instances, you can do work on the back end and save hours on the front end.
An important thing to bear in mind is that AppEngine backends share URL and code with your main app. Backends are part of your app but seperate instances. They are accessible via their names plus the main front end app as shown in the screenshot below. All the urls that your front end app exposes are available to the backends too. For example if your AppName is backendstest2013, you access the app as backendtest2013.appspot.com. And, if you have a back end named backendone, you trigger/invoke the urls on the backend as backendone.backendtest2013.appspot.com/<same URL here>.
There are two types of back-ends dynamic and resident. Dynamic backends are started on the first request and stay alive for the request. After some idle time, they are taken down. Resident backends are always on.
Here is an example of a python backend for AppEngine. A zip of source code is available here. The folder structure for this example is shown below. Project folder is backend_trial.
The App takes two URLs "/" and "/scripttest". The handlers are defined in app.yaml as shown here
There is a controller servlet like handler defined in backendtrial.py. This handler for the main page or default app page just shows some settings. The handler for "/scripttest" inside backendtrial.py is shown below.
AppEngine backends for python are configured in backends.yaml. The file is shown below.
We declare one instance of class B1. It is dynamic and publicly available for access from outside the main App. Class puts limits on your CPU and memory power. B1 is least with 128MB and 600MHz. Class B8 has 1024MB and 4.2GHz.
The code for handling URL "/scripttest" is shown below. We will access this from both the app instance and back end instance. The code just prints the backend name from the backends.yaml file, if the code is executed from a backend.
Upload the app using command
$ appcfg.py update backend_trial
Upload backend info using command
$ appcfg.py backends backend_trial update
Test the main app and back end by Accessing the app using URLs
1) Main App
2) Backend call
Source for the project is available ->> here