OData
stands for Open Data protocol. It's a REST-based web protocol for
querying and updating data completely independent of the platform or
source. OData accomplishes this by utilizing and enhancing existing web
technologies such as HTTP, JavaScript Object Notation (JSON), and the
Atom Publishing Protocol (AtomPub). Through OData, you can gain access
to a multitude of different applications and services from a variety of
sources including relational databases, file systems, and even
content-management systems.
The OData
protocol came about from experiences implementing AtomPub clients and
servers in an assortment of products over the past few years. OData
relies on URIs for resource identification, which provides consistent
interoperation with the Web, committing to an HTTP-based and uniform
interface for interacting with the different sources. OData is committed
to the fundamental web principles; this gives OData its great ability
to integrate and interoperate with a plethora of services, clients,
tools, and servers.
It doesn't matter if you have a
basic set of reference data or are architecting an enterprise-size web
application: OData facilitates the exposure of your data and associated
logic as OData feeds, thus making the data available to be consumed by
any OData-aware consumers such as business intelligence tools and
products as well as developer tools and libraries.
1. OData Producers
An OData producer
is a service or application that exposes its data using the OData
protocol. This article —pertains to SQL Azure and OData, SQL Azure
can expose data as OData. But so can SQL Server Reporting Services and
SharePoint 2010, among other applications. L
Many public (or live)
OData services have been made available, which anyone can consume in an
application. For example, Stack Overflow, NerdDinner, and even Netflix
have partnered with Microsoft to create an OData API. You can view a
complete list of such OData producers, or services, at www.odata.org/producers.
In the browser, you see a list of the categories by which you can browse or search for a movie offered by Netflix, as shown in Figure 1. You probably look at Figure 1
and think, "This looks a lot like WCF Data Services." That is correct,
because, as stated earlier, OData facilitates the exposure of your data
and associated logic as OData feeds, making it much easier via a standardized method to consume data regardless of the source or consuming application.
Thus, in Figure 1
you can see the categories via which you can search Netflix movie
catalog. For example, you can see the different endpoints of the API
through which to find a movie, such as Titles, People, Languages, and
Genres.
You can begin navigating through the vast Netflix catalog by entering
your query as a URI. For example, let's look at all the different genres
offered by Netflix. The URI is http://OData.netflix.com/Catalog/Genres
You're given a list of genres, each of which is in an <entry> element with the name of the genre in the <Name> element in the feed, shown in Figure 2.
Figure 2
shows the Comedy genre. Additional information lets you know what you
need to add to the URI to drill down into more detail. For example, look
at the <id> element. If you copy the value of that element into your browser, you see the detailed information for that genre.
Continuing the Comedy example, let's return all the titles in the Comedy genre. To do that, you need to append the /Titles filter to the end of the URI:
http://OData.netflix.com/Catalog/Genres('Comedy')/Titles
The Netflix OData service returns all the information for the movies in the Comedy genre. Figure 3 shows one of the movies returned from the service, displayed in the browser.
At this point you're just
scratching the surface—you can go much further. Although this article isn't intended to be a complete OData tutorial, here are some basic
examples of queries you can execute:
The key to knowing what to add
to the URL to apply additional filters is in the information returned by
the service. For example, let's modify the previous example as follows:
http://OData.netflix.com/Catalog/People?$filter=Name%20eq%20'Brad%20Pitt'
On the resulting page, several <link> elements tell you what additional filters you can apply to your URI:
<link rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/Awards"...
<link rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/TitlesActedIn"...
<link rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/TitlesDirected"...
These links let you know the information by which you can filter the data.
2. OData Consumers
An OData consumer
is an application that consumes data exposed via the OData protocol. An
application that consumes OData can range from a simple web browser (as
you've just seen) to an enterprise custom application. The follow is a
list of the consumers that support the OData protocol:
Browsers. Most modern browsers allow you to browse OData services and Atom-based feeds.
OData Explorer. A Silverlight application that allows you to browse OData services.
Excel 2010. Via PowerPivot for Excel 2010. This is a plug-in to Excel that has OData support.
LINQPad. A tool for building interactive OData queries.
Client libraries. Programming libraries such as .NET, PHP, Java, and Windows Phone 7 that make it easy to consume OData services.
Sesame (an OData browser). A browser built by Fabrice Marguerie specifically for browsing OData.
OData Helper for webMatrix. Along with ASP.NET, let's you retrieve and update data from any service that exposes its data via the OData protocol.
There are several more supported client libraries. You can find a complete list of consumers at www.odata.org/consumers.
OK, enough about OData. If you want to learn more, the OData home page is www.OData.org/home.
You should spend some time
reading up on OData and start playing with some of the services provided
by the listed producers. When people began getting into web services
and WCF services, there was an obvious learning curve involved in
understanding and implementing these technologies. Not so much with
OData—it has the great benefit of using existing technologies to build
on, so understanding and implementing OData is much faster and simpler.
