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Prior to reading this page, it is important to have an understanding of the API from a non-technical point of view. First read the REST Web Services API For Clients page before going through this one.
These pages exist to help a developer walk through creating a new ws. They might be more helpful to some devs than this page:
Adding a Web Service Step by Step Guide for Core Developers
Adding a Web Service Step by Step Guide for Module Developers
REST URL request information flow
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