Difference between OData and REST web services

Question

UPDATE Warning, this answer is extremely out of date now that OData V4 is available.

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I wrote a post on the subject a while ago here.

As Franci said, OData is based on Atom Pub. However, they have layered some functionality on top and unfortunately have ignored some of the REST constraints in the process.

The querying capability of an OData service requires you to construct URIs based on information that is not available, or linked to in the response. It is what REST people call out-of-band information and introduces hidden coupling between the client and server.

The other coupling that is introduced is through the use of EDMX metadata to define the properties contained in the entry content. This metadata can be discovered at a fixed endpoint called $metadata. Again, the client needs to know this in advance, it cannot be discovered.

Unfortunately, Microsoft did not see fit to create media types to describe these key pieces of data, so any OData client has to make a bunch of assumptions about the service that it is talking to and the data it is receiving.

The OData protocol is built on top of the AtomPub protocol. The AtomPub protocol is one of the best examples of REST API design. So, in a sense you are right - the OData is just another REST API and each OData implementation is a REST-ful web service.

The difference is that OData is a specific protocol; REST is architecture style and design pattern.

REST is a generic design technique used to describe how a web service can be accessed. Using REST you can make http requests to get data. If you try it in your browser it would be just like going to a website except instead of returning a web page you would get back XML. Some services will also return data in JSON format which is easier to use with Javascript.

OData is a specific technology that exposes data through REST.

If you want to sum it up real quick, think of it as:

• REST - design pattern
• OData - enabling technology

In 2012 OData underwent standardization, so I'll just add an update here..

First the definitions:

REST - is an architecture of how to send messages over HTTP.

OData V4- is a specific implementation of REST, really defines the content of the messages in different formats (currently I think is AtomPub and JSON). ODataV4 follows rest principles.

For example, asp.net people will mostly use WebApi controller to serialize/deserialize objects into JSON and have javascript do something with it. The point of Odata is being able to query directly from the URL with out-of-the-box options.