Developing Applications for the Cloud on the Microsoft Windows Azure Platform : Accessing the Surveys Application - Geo-Location

Windows Azure allows you to select a geographic location for your Windows Azure services so that you can host your application close to your users. This section describes how Tailspin uses this feature in the Surveys application.

---

Note:

The Surveys application is a “geo-aware” service.

---

1. Goals and Requirements

Tailspin wants to allow subscribers to the Surveys service to be able to specify which geo-specific instance of the Surveys application they’d like to use. For example, a U.S.–based customer would probably want to choose a U.S.–based service, and a European customer would probably want to choose a European-based service. However, it’s possible that a subscriber might want to run a survey in a different geographic region than the one the subscriber is located in. Figure 1 shows how a U.S.–based subscriber might want to run a survey in Europe:

You can check the current status of any Windows Azure data center here: http://www.microsoft.com/windowsazure/support/status/servicedashboard.aspx.

Figure 1. A U.S.–based subscriber running a survey in Europe

2. Overview of the Solution

Hosting a survey in a web role in a different geographic location doesn’t, by itself, mean that people filling out the survey will see the best response times when they use the site. To render the survey, the application must retrieve the survey definition from storage, and the application must save the completed survey results to storage. If, in the example shown in Figure 3-1, the application storage is in the U.S. data center, there is little benefit to European customers accessing a website hosted in the European data center.

Figure 2 shows how Tailspin designed the application to handle this scenario and resolve the issue just described.

Figure 2. Hosting a survey in a different geographic location

The following describes the steps illustrated in Figure 2:

1. The subscriber designs the survey, and the application saves the definition in storage hosted in the U.S. data center.

2. The Surveys application pushes the survey definition to another application instance in a European data center. This needs to happen only once.

3. Survey respondents in Europe fill out the survey, and the application saves the data to storage hosted in the European data center.

4. The application transfers the survey results data back to storage in the U.S. data center, where it is available to the subscriber for analysis.

In some scenarios, it may make sense to pre-process or summarize the data in the region where it’s collected and transfer back only the summarized data to reduce bandwidth costs. For the Surveys application, Tailspin decided to move all the data back to the subscriber’s region; this simplifies the implementation, helps to optimize the paging feature, and ensures that each response is moved between data centers only once.

---

Note:

When you deploy a Windows Azure application, you can select the subregion (which, at the moment, determines the data center) where you want to host the application. You can also define affinity groups that you can use to group inter-dependent Windows Azure applications and storage accounts together in order to improve performance and reduce costs. Performance improves because Windows Azure co-locates members of the affinity group in the same data center. This reduces costs because data transfers within the same data center do not incur bandwidth charges. Affinity groups offer a small advantage over simply selecting the same subregion for your hosted services, because Windows Azure makes a “best effort” to optimize the location of those services.

---