Using Google App Engine

Google App Engine (GAE) is a Platform as a Service (PaaS) cloud-based Web hosting service on Google's infrastructure.

This service allows developers to build and deploy Web applications and have Google manage all the infrastructure needs, such as monitoring, failover, clustering, machine instance management, and so forth. For an application to run on GAE, it must comply with Google's platform standards, which narrows the range of applications that can be run and severely limits those applications' portability.

GAE supports the following major features:
• Dynamic Web services based on common standards
• Automatic scaling and load balancing
• Authentication using Google's Accounts API
• Persistent storage, with query access sorting and transaction management features
• Task queues and task scheduling
• A client-side development environment for simulating GAE on your local system
• One of either two runtime environments: Java or Python

When you deploy an application on GAE, the application can be accessed using your own domain name or using the Google Apps for Business URL.

Google App Engine currently supports applications written in Java and in Python, although there are plans to extend support to more languages in the future. The service is meant to be language-agnostic. A number of Java Virtual Machine languages are compliant with GAE, as are several Python Web frameworks that support the Web Server Gateway Interface (WSGI) and CGI. Google has its own Webapp framework designed for use with GAE. The AppScale (http://appscale.cs.ucsb.edu/) open-source framework also may be used for running applications on GAE.

To encourage developers to write applications using GAE, Google allows for free application
development and deployment up to a certain level of resource consumption. Resource limits are described on Google's quota page at http://code.google.com/appengine/docs/quotas.html, and the quota changes from time to time.

When you enable billing for an application deployed to GAE, you pay for consumption of CPU, network I/O, and other usage above the level of the free quotas that GAE allows.

Applications running in GAE are isolated from the underlying operating system, which Google
describes as running in a sandbox. This allows GAE to optimize the system so Web requests can be matched to the current traffic load. It also allows applications to be more secure because applications can connect only to computers using the specified URLs for the e-mail and fetch services using HTTP or HTTPS over the standard well-known ports. URL fetch uses the same infrastructure that retrieves Web pages on Google. The mail service also supports Gmail's messaging system.

Applications also are limited in that they can only read files; they cannot write to the file system
directly. To access data, an application must use data stored in the memcache (memory cache), the datastore, or some other persistent service. Memcache is a fast in-memory key-value cache that can be used between application instances. For persistent data storage of transactional data, the datastore is used. Additionally, an application responds only to a specific HTTP request—in real-time, part of a queue, or scheduled—and any request is terminated if the response requires more than 30 seconds to complete.

The App Engine relies on the Google Accounts API for user authentication, the same system used when you log into a Google account. This provides access to e-mail and display names within your app, and it eliminates the need for an application to develop its own authentication system. Applications can use the User API to determine whether a user belongs to a specific group and even whether that person is an administrator for your application.