One of the more common approaches is to cluster systems together to create what appears to be a single computing solution. Systems are connected together using some form of high speed interconnect . Optionally, the clustered systems may share network attached or local storage. Some approaches require shared network attached storage.We then went on to discuss the different types of clusters organizations deploy. Even though the hardware configuration can be identical, different types of software can be deployed based upon the organization's goals. Here are the software approaches that were touched on during the conversation:
Access virtualization clusters
Clusters can be created using access virtualization, application virtualization, or several different forms of processing virtualization. The goals for each type of cluster differ from the others. Let's examine each type separately.
Access virtualization clusters are designed to make access to individual applications survive the loss of a system. Applications are hosted on multiple systems. Access to those applications is made possible using an access virtualization product such as Microsoft Terminal Services, Citrix XenDesktop or VMware View.
Access virtualization clusters depend upon the fact that each application is replicated multiple times, once on each server in the cluster. Users access these applications through access virtualization technology. If an application fails, the user is prompted to log in again. When they do, they will be attached to another system. This implementation may require costly changes to be made to the application itself to make it cluster-aware.
Several industry suppliers looked at the various forms of HA software and thought that the approaches used in building fault tolerant systems and in continuous processing in general would allow high levels of availability to be offered using a much simpler configuration and operations. Furthermore, storage virtualization technology, such as network-attached storage, is required to make sure that applications have access to the same database. Shared storage is traditionally an expensive hardware purchase.
In the instance of hardware or system failure, incomplete transactions will be lost, but it will be possible for the user to still have access to needed applications.
Application virtualization clusters
Application virtualization clusters are designed to make individual applications survive the loss of a system. As with access virtualization clusters, applications are hosted on multiple systems. A workload management system accepts the user's request for an application and connects them to the system having the greatest available capacity.
As with access virtualization clusters, application virtualization clusters depend upon the fact that each application is replicated multiple times, once on each server in the cluster. Not all applications are able to be virtualized off-the-shelf, and may require additional application development for legacy applications.
What is different is that the clustering is managed at the application layer rather than at the access layer. Like access virtualization clusters, if an application fails, the user is automatically switched to the same application running on another system. Furthermore, storage virtualization technology, such as network attached storage, is required to make sure that applications have access to the same database.
Uncompleted transactions can still be lost, but it will be possible for the user to still have access to needed applications.