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The number of companies running solutions in the cloud is increasing at an incredible rate. Their success encourages other organizations to take the same step. Some organizations will not be able to move all of their workloads into the cloud, either because of regulatory issues or because some workloads cannot run in a virtualized environment. In these cases, hybrid computing, in which a company runs part of its infrastructure in the cloud and part on-premises, will be an important strategy.

The Microsoft Azure platform provides a great hybrid computing story. There are multiple ways to connect an on-premises datacenter to one or more Azure regions. Azure provides both site-to-site and point-to-site virtual network connectivity. Either option provides a secure VPN connection between on-premises assets and resources hosted in Azure. An additional hybrid connectivity option is Azure ExpressRoute, which enables a private connection between Azure and your on-premises infrastructure or colocation facility, all without going over the public Internet.


Network connectivity
Regardless of the chosen option—site-to-site, point-to-site, or ExpressRoute—hybrid connectivity is a key scenario for the Azure platform. Creating a hybrid connection opens a wide range of possibilities to extend an on-premises infrastructure to the cloud. Two common scenarios for network-enabled hybrid connectivity are the following:

 Hosting a website in Azure but keeping the database on premises In an organization’s journey to the cloud, migrating the on-premises data to Azure can be one of the more difficult tasks. The difficulty usually comes in one of two forms: a technical issue or a compliance requirement. On the technical front, as an example, the application in question is designed to use a database that is not supported in Azure. On the compliance front, perhaps there is a regulatory requirement that cannot be met with Azure SQL Database or by running a database (SQL Server, MongoDB, and so on) on Azure Virtual Machines. In these cases, an organization might choose to host the website in Azure using Azure Web Apps or Azure Virtual Machines, with the database remaining on premises. Connectivity between the website and the database could then be established using one of the aforementioned technologies: a site-to-site connection, a point-to-site connection, or ExpressRoute.

 Accessing an on-premises service Sometimes, a website has a dependency on a particular service that cannot be moved to the cloud. Perhaps the website depends on an API that performs a crucial business calculation, and that API cannot be moved due to security because other on-premises services also depend on the service or because it is legacy technology that is not supported in Azure. In such a scenario, a hybrid connection is established between Azure and the on-premises infrastructure to allow the Azure-hosted website to freely communicate with the necessary API that continues to reside on-premises.

Besides using a network connection in this scenario, an Azure Service Bus Relay could be used to access an on-premises service. For information on how to use the Azure Service Bus Relay service, please refer to http://azure.microsoft.com/documentation/articles/service-bus-dotnet-how-to-use-relay/.


Internet connectivity
There are many scenarios in which all that is needed is an Internet connection rather than a special hybrid connectivity solution. After all, the ability to connect to Internet-accessible services is one of the attractive features of cloud computing. A few common scenarios include these:

 Storage of archival data Large amounts of data, especially archival data that is rarely accessed, can be very expensive to store on-premises. The cost in terms of infrastructure, people, software licenses, and physical space can quickly put a tremendous financial burden on an organization. Azure provides virtually limitless storage capacity at an incredibly low price. An organization might wish to use the scalable storage provided by Azure Blob storage as a data archival store. When the data is needed, the on-premises service(s) download the data from Azure Blob storage and perform the necessary processing. A basic Internet connection will often suffice, but an ExpressRoute connection could also be used for improved speed and security.
Another option for storage of archival data is Microsoft Azure StorSimple. StorSimple includes a hardware appliance that is installed on-premises. The appliance keeps frequently accessed data local (on the device). As data ages (is accessed less frequently), it is automatically moved to Azure Blob storage. For more information on StorSimple, please refer to http://azure.microsoft.com/documentation/services/storsimple/.

 Organizations can choose to synchronize their Azure AD users and groups with user and group information from their on-premises Active Directory. In doing so, they can use Azure Active Directory Connect to synchronize the user data and a password hash, making Azure AD the authority for user authentication. Alternatively, an organization might wish to synchronize the user data but require users to authenticate via an Active Directory Federation Services (AD FS) endpoint residing on-premises, effectively redirecting the user to an on-premises AD FS site for authentication before redirecting to the desired location.

 Burst to the cloud Sometimes, an organization’s on-premises infrastructure is not able to handle the required load. Maybe there is a holiday season rush or a government-mandated period to sign up for an important service. Instead of building the on-premises infrastructure to handle the temporary surge in demand, an organization might choose to leverage the elastic nature of the cloud to burst to the cloud when needed and scale back to only on-premises services when the load returns to normal. In this scenario, an organization could use Azure Web Apps or Azure Virtual Machines to host the service and could implement autoscale rules to ensure capacity keeps up with user demand.

Source of Information : Microsoft Azure Essentials Fundamentals of Azure Second Edition