Clustering in Exchange 2000

I have good news and bad news about Microsoft Exchange 2000 Server's clustering capabilities. The good news is that clustering in Exchange 2000 works better than it does in Exchange Server 5.5, and it's less expensive to implement. The bad news is that you still need to be aware of the technology's limitations. To decide whether clustering makes sense for your environment, you need to know what clustering is.

Understanding Exchange's clustering capabilities will help you decide whether clustering meets your business requirements for increased availability and maintainability.

Clustering Basics
Clustering has a distinct lingo and cool buzzwords. A word you've probably heard is failover. A failover occurs when a service running on a clustered node fails. The essence of the failover operation is that the failed service automatically restarts on a functional node in the cluster—without requiring you to do anything (and ideally without users noticing). Failback is the opposite of failover: When the failed node returns to service, the services that failed over to another node go back to the node they came from.

Other key terms to understand are the words that describe which nodes in a cluster are doing the work. Ideally, every node in the cluster is doing something; in Exchange, you want every node to simultaneously handle Exchange clients. Microsoft refers to such clusters as active/active—two nodes actively handling different clients. Active/active clustering stands in contrast with the Exchange 5.5 clustering model of one node talking to clients and the other node waiting for the first node to fail. This model is called active/passive clustering. If you have more than two nodes in a cluster and one of them is quietly awaiting a failure on another node, you have an N+1 node cluster.

Consider how users see a cluster—as a separate machine on the network. The cluster appears this way because clusters are built out of resources. A resource can be physical (e.g., a disk) or virtual (e.g., an IP address). If you throw together the right set of resources (e.g., an IP address, a NetBIOS name, and some Exchange services), you create an Exchange virtual server, which is an instance of Exchange that appears as a separate physical machine—even though it's not. Users can connect to this virtual server without regard to the server's physical location.

How many nodes make a cluster? Most Exchange administrators believe that clusters always contain two nodes. However, clusters on other OSs—and even on Windows, with non-Microsoft cluster software—can contain 16, 32, 64, or more nodes. Windows 2000 Server doesn't support clustering. Win2K Advanced Server supports two-node clusters, and Win2K Datacenter Server supports four-node clusters. Count on larger clusters from Microsoft in the future. Of course, Exchange supports as many nodes as the underlying clustering software does—as many as four if you run Exchange 2000 Service Pack 1 (SP1) or later on Datacenter.

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