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The Legal and Regulatory Environment For certain applications, you may need to take into account specific regulatory or legal issues. This may require some differences in functionality, specific legal messages to be displayed in the UI, guaranteed separate databases for storage, or storage located in a specific region. This may again lead to having separate multi-tenant deployments for groups of customers, or it may lead to requiring a single-tenant architecture. Handling Authentication and Authorization You may want to provide your own authentication and authorization systems for your cloud application that require customers to set up accounts for the users who will interact with the application. However, customers may prefer to use an existing authentication system and avoid having to create a new set of credentials for your application. In a multi-tenant application, this would imply being able to support multiple authentication providers, and it may possibly require a custom mapping to your application s authorization scheme. For example, someone who is a Manager in Microsoft Active Directory directory service at Adatum might map to being an Administrator in Adatum s Tailspin Surveys application. Note: For more information about claims-based identity, see the book, A Guide to Claims-Based Identity and Access Control. You can download a PDF copy of this book from http://msdn. microsoft.com/en-us/library/ff423674.aspx.

Your choice of a single-tenant or multi-tenant architecture will affect how easy it is to develop, deploy, maintain, and monitor your application. Maintaining the Code Base Maintaining separate code bases for different customers will rapidly lead to escalating support and maintenance costs for an ISV because it becomes more difficult to track which customers are using which version. This will lead to costly mistakes being made. A multi-tenant system with a single, logical instance guarantees a single code base for the application. You can still maintain a single code base with a multi-instance, single-tenant model, but there could be a short-term temptation (with long-term consequences) to branch the code for

individual customers in order to meet specific customer requirements. In some scenarios, where there is a requirement for a high-degree of customization, multiple code bases may be a viable option, but you should explore how far you can get with custom configurations or custom business rule components before going down this route. If you do need multiple code bases, you should structure your application such that custom code is limited to as few components as possible. Handling Application Upgrades A multi-tenant application makes it easy to roll out application updates to all your customers at the same time. This approach means that you only have a single, logical instance to upgrade, which reduces the maintenance effort. In addition, you know that all your customers are using the latest version of the software, which makes the support job easier. Windows Azure upgrade domains facilitate this process by enabling you to roll out your upgrade across multiple role instances without stopping the application. If a client has operational procedures or software that are tied to a specific version of your application, any upgrades must be coordinated with that client. To mitigate the risks associated with upgrading the application, you can implement a rolling upgrade program that upgrades some users, monitors the new version, and when you are confident in the new version, rolls out the changes to the remainder of the user base. Note: For more information about how to handle application upgrades, see Appendix A, Updating a Windows Azure Service. Monitoring the Application Monitoring a single application instance is easier than monitoring multiple instances. In the multi-instance, single tenant model, any automated provisioning would need to include setting up the monitoring environment for the new instance, which will add to the complexity of the provisioning process for your application. Monitoring will also be more complex if you decide to use rolling upgrades because you must monitor two versions of the application simultaneously and use the monitoring data to evaluate the new version of the application. Using .NET Providers and Third-Party Components If you decide on a multi-tenant architecture, you must carefully evaluate how well any third-party components will work. You may need to take some additional steps ensure that a third-party component is multi-tenant aware. With a single-tenant, multi-instance deployment where you want to be able to scale out for large tenants, you