How do I code: Introducing OData

Last weekend I introduced myself to OData. Here are the findings :-)

Contents:

1. Scenarios
2. Approaches
3. OData
4. Using LINQPad to access NetFlix
5. Examples from the Real World (ecosystem)
6. Resources
7. Developing OData Services(WCF Data Service, ASP.NET Web API)
8. Accessing OData Services (.NET client, jQuery)
9. Limitations
10. And GData?

First let’s start with the scenarios or the domain where OData can be relevant.

Scenarios:

- Let’s say you are a company which runs buses. You are looking for ways to share the information about buses such as arrival time, frequency, operating hours, capacity, service closures etc. You want other web sites and smart phone applications to be able to access this data. How would you do it?
- Say you are working for an online video streaming company like NetFlix and you want to expose your data about movie titles, availability, reviews etc. You want to share this information over the web so that e-commerce web sites like Amazon can create a catalog of your products or you are writing a Windows 8 app to display data from NetFlix.
- Say you are manager of a government department and you have a legal requirement to expose the data about projects such as date of completion, cost, overruns, tenders etc. on a web site and also in a format which can be easily accessed by other govt. applications

Now for all these scenarios we could have created a web site and displayed data on it, thereby creating a data island. But how can we make the data “reusable”, so that other applications (web /mobile) could consume it?

Also, a common requirement among the use cases above is that they return a list of entries or items. It is not about a request-response where you build a request package with some parameters and the service sends back the response after doing some calculation. This is more about - I want to expose data on my company’s URL say www.example.com/movies to share all the movie titles or www.busservice.com/buses to expose information about all the buses I have and step deeper www.nybusservice.com/buses(11)/frequency to get the frequency of bus with ID of “11”.

Approaches:

1) SOAP: We could create a WCF service (which uses SOAP format), host it in IIS so that it could become part of the organization’s web infrastructure. Now SOAP enables us to use WS-* protocols, metadata (WSDL); we can define operations like GetMoviesTitles, GetMovieReviews and also give us security, transactions, reliability, interoperability over HTTP. It supports request-response pattern over HTTP Post. But that is a lot of stuff made available to us, which may not be useful to use in certain scenarios.

2) POX (Plain Old XML): We could expose a ASP.NET URL and return XML data (could be done by writing our own http handler). The problem here is the data structure. The clients will have to ‘know’ the schema before hand to access the data and also we need to write extra stuff to enable CRUD operations. The advantage here is we avoid all the SOAP overhead.

3) REST: REST is an architectural style using the concept of ‘resources’ and how we can identify them with a URI, along with a fixed verbs for actions – GET, POST, PUT and DELETE for the CRUD operations on those resources. By using these HTTP verbs, it is easy for consumers to realize what actions need to be performed for interacting with the data.

In the scenarios above, let’s add one more - assume you run a blogging engine like Blogger, WordPress etc. The posts that your users write on your blogging engine needs to be exposed in a way so that blog readers (like Google Reader) could read the posts. You could again send back XML containing a list of all the posts in your own XML format. But then what would happen if every blog engine out there created its own format. Of course as we know this problem has been solved – all the blog engines use a standard format called Atom or RSS. This makes it easier for feed readers to just point to a URL and get all the posts and because they know it’s in Atom format it makes it quite easy for them. The Atom format is only for getting resources, and AtomPub (used by OData) is for editing web resources.

If we come to think of it, wouldn’t it better if we could return an Atom/RSS feed from our services? The atom/rss feed XML is understood by modern day browsers so anyone with a browser could go to the URL and get the data. This is in fact possible today with WCF. We could create RESTful services and reply back syndication feeds in RSS or ATOM format, sending back a list of your business objects.

But there is still a need of some more functionality to make it useful. Functionality like metadata, paging and filtering the feeds, for this we use OData. So we can say “OData is exposing your business objects (as a list/collection) over HTTP URL in a RESTful manner, by returning data in XML-AtomPub format along with providing facilities like metadata, filtering and paging.”

OData:

The “O” in OData stands for “Open”, the full name is “Open Data Protocol”. It is REST based, using HTTP + AtomPub + JSON.

The definition from OData.org:
"The Open Data Protocol (OData) is a Web protocol for querying and updating data that provides a way to unlock your data and free it from silos that exist in applications today. OData does this by applying and building upon Web technologies such as HTTP, Atom Publishing Protocol (AtomPub) and JSON to provide access to information from a variety of applications, services, and stores."

Let’s explore the NetFlix OData a little. If we browse to the URL http://odata.netflix.com/v2/Catalog/, this is what we get:

Under the <service> we have various collections: Genres, Titles, TitleAwards so on. If want to get all the titles we browse to: http://odata.netflix.com/v2/Catalog/Titles, and we get

We can see here it starts with the tag <feed> and there is an <entry> tag for every Title returned by the NetFlix service. In the <entry> you will find an “id” which will point to the URL we can use to get details about that particular Title.

If we go to URL http://odata.netflix.com/v2/Catalog/Titles('13aly') we get:

Using LINQPad to access OData services:

In the previous section we saw how we can see the OData in browser, now we will see how we can use LINQPad to access it.

I am using LINQPad 4, but the steps should be same for other versions. The first step is to ‘Add connection’ in LINQPad, for this you can click the ‘Add Connection’ link given in the left pane.

Upon clicking we get this dialog:

Select “WCF Data Services 5.1..” and click Next and enter the URI.

After clicking OK, you should get the NetFlix in the connection list along with various collections it exposes.

We can now query the data with LINQ queries.

> Get 10 genres: Genres.Take(10)