Once Exchange 2000 is installed, it is fully cluster aware and can take advantage of clustering technologies to provide high availability. The main components of Cluster Services that provide Exchange 2000 with this functionality are resource DLLs, resources, and cluster groups. Each of these components, in regards to Exchange 2000, is discussed below.

Resource DLLs
Resource DLLs are used by Cluster Service to communicate with the resources. Once Exchange is installed, it adds its own resource DLL called exres.dll, which is used by Cluster Service to monitor the status of Exchange resources. The DLL performs functions on Exchange resources such as checking their status through IsAlive calls, reporting any resource failures, and bringing resources online.

Note: Cluster aware applications have custom DLLs written specifically for use in cluster environments, this providing more functionality. Applications that are not cluster aware use a generic DLL.

Resources
Resources are the logical entities controlled by the cluster. Once Exchange 2000 is installed on a cluster node, it adds its own applications-specific resources to the Exchange Virtual Server (EVS). It also uses some of the Cluster Service resources, such as the IP Address resource, the Network Name resource, and the Disk resource.

Cluster Groups
Exchange 2000 uses virtual servers in a cluster environment. Virtual servers are equivalent to cluster resource groups. Resource groups basically define units of fail over. If a resource within a group fails and needs to be moved to the second node, all resources within the group failover as well. Clustering Exchange 2000 means that the virtual servers are no longer linked with a single computer. If a resource within the virtual server fails, the virtual server can fail over to the second node in the cluster, this providing minimal downtime for users. Each node in the cluster requires at least one virtual server. Each EVS has the following requirements.

• Static IP address

• Unique NetBIOS name
• Shared storage space
• Exchange System Attendant

Each virtual server is assigned its own IP address and unique network name (NetBIOS name). This is the name users will use when connecting to their mailboxes, regardless of which node the virtual server is running on. This means if the virtual server fails over to the second node in the cluster, users can still access it using the same network name. By setting up Exchange in an Active/Active configuration, you specify that each virtual server also requires its own disk volume on the shared storage space.

Exchange Virtual Servers also require an Exchange System Attendant resource. The System Attendant is one of the main components of Exchange and is responsible for tasks such as monitoring server services and messaging connectors. Once the Exchange System Attendant is added as a resource to the virtual server, it adds the remaining required resources.