Providing Data as a Service

Besides providing applications as a service, you can go a step further and provide data as a service. Here, we show how to use ADO.NET Data Service to make data in a database available as a service for different clients

Monday, December 01, 2008

Application programming has seen an interesting evolution. We first moved away from developing applications that were tightly coupled to the OS and programming language to decoupled ones. So users were no longer tied down to a single machine to access an application. They could access it from any platform. This has made the application into a service that can be accessed from anywhere. However, in both these cases, the data remains tightly coupled with the application, irrespective of where you access it from. So the next obvious step is to decouple the data from the application, so that it can be accessed by other applications as well. This is in essence what ADO.NET is all about. It allows various applications to access various data sources like SQL Server, XML, or even those exposed by OLE DB, and modify or manipulate the data in them. This makes data usage much more flexible. We'll take a hypothetical example to explain this benefit. Consider a school where the student attendance, once taken, is fed into a database. In the traditional way of working, one would run a query on this database and pull out various reports like how many days has a student been absent, which class has the best attendance, etc. This could then be shared with parents in a PTM. With ADO.NET, data service, this same data could be made available to parents over web on demand, or further, to their mobile devices if required. Sounds interesting? In this article, we'll tell you how to implement it.

Implementation
To implement ADO.NET Data Service, you need Visual Studio 2008 SP1, a database and a web browser client. We used the 'School' database that comes with SQL Server 2008. Start by creating an ASP.NET Web Application project in and name it ADODS_Sample1. We have used C# for creating this. The first step is to create a conceptual model known as the Entity Data Model (EDM) of the sample database. This can be done by clicking on Project>Add New Item and then selecting ADO.NET Entity Data Model. After this, simply follow the wizard (check out our Nov 2008 issue for in depth explanation of this). This EDM will act as layer between the database and client from which the data would be picked up. After creation of 'School.edmx' file, you have to create the data service that will expose data as service to be picked up by any client.

To create a data service, click on Project>Add New Item and this time add 'ADO.NET Data Service' and name it 'School.svc'. We now have to make changes to the 'School.svc.cs' file. Here is the code snipped to do that:

To create a data service, click on 'Project>Add New Item'. Select 'ADO.NET Data Service'. We have named it 'School.svc'.

using System.Data.Services;
using System.Linq;
using System.ServiceModel.Web;
using System.Web;
namespace ADODS_sample1
{
public class School : DataService< SchoolEntities >
{
public static void InitializeService(IDataServiceConfiguration config)
{
config.SetEntitySetAccessRule("*", EntitySetRights.AllRead);
}
}
}

To display output of the project in browser, click on 'Tools>Internet Options' and then go to 'Advanced' tab, uncheck 'Disable script debugging (Internet Explorer)'.

To display output of the project, one has to make certain changes in browser settings. In Internet Explorer, go to 'Options', click on 'Advanced' tab, and under settings go to 'Browsing'. Uncheck Disable script debugging. Next click on the 'Content' tab and then click on 'Settings'. Uncheck 'Turn on feed reading view'. Press 'F5' to start debugging, browser will open with following URL:

http://localhost:2995/School.svc/

Here you will find XML representation of the data that can be used by any client. Now to drill down further, one can find data related to 'Department' and 'OnlineCourse' tables using following URI's:

http://localhost:2995/School.svc/Dept
http://localhost:2995/School.svc/OnlineCourse

To get final output in XML format one needs to uncheck 'Turn on feed reading view'. This check box is in 'Content' tab (click on 'Settings') of Internet Options

Similarly, other URIs can be provided to clients so that they can look for data as per their needs. Moving one step ahead, one can also display data associated with 'DepartmentID' (valued 1) in other table named 'Course'. The URL for this is:

http://localhost:2995/School.svc/Department(1)/Course

Using ADO.NET Data Service, we have shown here is how data represented as EDM is exposed as a web service. The same data can be exposed via RSS feeds as well. Plus, apart from converting URIs to Entity Framework Object Services method calls, you
can even use XML documents, web service etc.

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