Google writes and supports client libraries for developers. This makes it easier to write applications using Google product data as well as accessing the corresponding Google product API’s.
Google Data Protocol Provides Client Libraries
*PHP client library is distributed as part of Zend, not as a Google Code Project.
All client libraries are hosted on code.google.com (Google Code) other than PHP. Separate documentation is posted for each library. You can view the open-source code and browse through any new feature requests. Read articles about how other developers have used client libraries. Each client library has its own FAQ section. File bug reports, if you find any! Google continues to actively support the Client Libraries, including bug fixes.
There is a single external client library, neither written nor supported by Google, the Force.com Toolkit for Google Data API’s. It is an Apex client library for Salesforce.com developers.
More details about Data API’s and client libraries
Google introduced many new products recently, both regular releases and Labs. Many have API’s. I counted twenty listings in the most recent official Google Data API Directory.
Not surprisingly, Google retains parity with its other products by Blogger coverage of the client libraries. The Google Apps Developer blog includes Data Protocol related posts.
Why is a Client Library helpful?
According to Google Data Protocol,
the library provides tools and an abstraction layer, letting you construct queries and use response data without having to create HTTP requests or process HTTP responses by hand.”
Each client library offers classes corresponding to elements and data types used by the API.
What are ETags?
Make sure to have a look at usage suggestions for ETags. ETags are identifiers that help specify entry version numbers, and are helpful for chronology and avoiding overwrites, They also facilitate conditional retrieval and updates. Only services running Google Data Protocol version 2.0 support ETags.