For Developers

The Windows Azure Platform

The Open Government Data Initiative is based on Microsoft's Windows Azure Platform, a cloud-computing platform that helps developers quickly and easily create, deploy, manage, and run Web applications and services at Internet scale. To learn more please visit the Windows Azure Platform home page.

Querying OGDI

OGDI exposes data through REST Web services. The basic format of an OGDI service call is http://ogdi.cloudapp.net/v1/container/dataset?query, where:

• container is the name of the container (for example, "dc" for the District of Columbia's data sets).
• dataset is the name of the data set (for example, "CrimeIncidents" for the Crime Incidents data set in the DC container).
• query is your set of query parameters, expressed using a subset of the ADO.NET Data Services query syntax.

Note that OGDI currently only suppports the $filter and $top query options in the ADO.NET Data Services query syntax. Example queries using the Crime Incidents data set are available on the Data Browser page of our Interactive SDK.

Also note that if a property has a null value for a particular entity in the data set, it will be omitted entirely from the result set returned by OGDI. For example, in the Crime Incidents data set, the "method" property is only returned for records that have a "method" value in the underlying data set. Your application design should take this into account and handle potentially missing properties.

Data Formats

AtomPub

By default, OGDI returns data in the Open Data Protocol (OData) format. This format extends the broadly adopted Atom Publishing Protocol and can be easily consumed by a variety of platforms, including Microsoft .NET, Java, Ruby, PHP, and Python. Refer to the code samples on the Data Browser page for examples.

JSON

OGDI can also return data in the JavaScript Object Notation (JSON) format, which can be conveniently consumed using JavaScript and other technologies. To return data in JSON format, simply append the format=json parameter to your query. For example, to retrieve crime incidents in Washington, DC that occured during the police department's evening shift in JSON format:

http://ogdi.cloudapp.net/v1/dc/CrimeIncidents?$filter=shift eq 'EVN'&format=json

JSONP

To mitigate security vulnerabilities associated with cross-site scripting attacks, Web browsers generally prevent client-side JavaScript applications originating in one network domain (for example, yourdomain.com) from making HTTP requests to other network domains (for example, the ogdi.cloudapp.net network domain that hosts the OGDI data services). This can prevent JavaScript applications hosted in another domain from making straight-forward calls to the OGDI data services, but there a variety of techniques that can be used, such as this widely-used IFRAMES-based technique described by Michael Mahemoff.

OGDI's data services also provide direct support for the JSONP technique. Using this technique, OGDI's data services will call a callback function that you specify, passing in the results of your query in JSON format as an input format. To use this technique, issue a query with the following additional parameters: format=json&callback=yourCallback, where yourCallback is the name of a JavaScript callback function defined on the Web page issuing the request.

Refer to the JavaScript sample on the Data Browser page for an example of using JSONP with OGDI. In that sample, the AdditionalDataLoaded() function is the JSONP callback function.

Geospatial Data

Many of the data sets in OGDI also include geospatial data, which is returned in the Keyhole Markup Language (KML) format. This format is compatible with popular desktop and Web-based mapping technologies including Microsoft Virtual Earth, Google Maps, Yahoo! Maps, and Google Earth.

To return geospatial data in KML format, append the format=kml parameter to your query. For example, to retrieve geospatial points in KML format for crime incidents in DC that occurred during the police department's evening shift:

http://ogdi.cloudapp.net/v1/dc/CrimeIncidents?$filter=shift eq 'EVN'&format=kml

Note that if the data set that you are using does not include any geospatial data, a KML query to OGDI will return an empty result set.

Client Libraries

Developers using Microsoft Visual Studio 2008, Service Pack 1 (or later) can use ADO.NET Data Services to access data from OGDI through easy-to-use .NET classes. Within Visual Studio, this is accomplished by using the Add Service Reference feature (see .NET samples on the Data Browser page). To make accessing OGDI data easier, Java developers can use Restlet Extension for ADO.NET Data Services. PHP developers can take advantage of the Toolkit for Toolkit for PHP with ADO.NET Data Services.

Paging

OGDI and the underlying Windows Azure Table Storage service support paging through large sets of query results. The documentation for Query Timeout and Pagination in the Windows Azure Table Storage service provides a complete description of how OGDI and the underlying Azure platform support paging. You can also refer to the "C#/ASP.NET Paging" sample on the Data Browser page, which demonstrates how to perform paged queries using the ADO.NET Data Services client library.