Microsoft Cluster Service or network load balancing?

Tags:
SQL,
VPN,
POP3,
IMAP

Carol Bailey MCSE+I Tech Republic

Published: 07 Nov 2002 16:28 GMT

The first of my recent articles on Windows clustering prompted a member to ask about the differences between the Microsoft Cluster Service (MSCS) and Microsoft's other clustering technology, network load balancing (NLB). This is an important topic, and to do it justice, I decided to dedicate an entire article to it.

I'll compare MSCS and NLB, both in terms of how they work and what solutions they offer. Along the way, I will point out some typical scenarios where the technologies can be used together to provide a high availability solution.

Cluster Service explained
I've previously explained how and why you might use the Cluster Service in Windows 2000 Advanced Server for achieving high availability with services like virtual servers, file shares and dynamic shares, Dfs roots, printers, WINS, DHCP, and IIS--in addition to the more traditional clustering services, such as Exchange and SQL.

With this service, typically, two servers (up to four, if you're running Windows 2000 Datacenter Server) share a data store so that another server can continue to offer the same data and service if the original fails (or if any dependent resource fails and can't be restarted). Benefits include not just fault tolerance to mitigate server or service failure, but also accommodating planned maintenance downtime for upgrades and security patches. It's important to reiterate that the Cluster Service does not protect the data. A shared data store simply means that more than one server can access it. The Cluster Service always assumes the data it offers is good, so you must ensure its integrity independently with hardware RAID (software RAID is not supported), UPS, good quality hardware, and so on.

The monitoring of availability is very granular. It's not just the clustered service itself that is monitored, but all its dependencies. So, for example, a clustered printer would monitor not just the spooler service but also the disk it was using for spooling and the virtual server's IP address and name. Any of these resources can be configured to automatically attempt to restart if it fails. You can set it up so that after a specified number of failures, it fails over to another server. Should the original server be able to offer the service again (e.g., the operating system disk has been repaired), the clustered service can be failed back to the original server.

You administer the Cluster Service with the Cluster Administrator MMC or with the command-line utility Cluster.exe. In addition to ensuring stable, reliable storage hardware for the shared data store, the servers in a Cluster Service cluster must be in an NT4 or Windows 2000 domain.