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Availability, scalability, and clustering technologies

Windows Server 2008 R2 provides two clustering technologies: failover clusters and Network Load Balancing (NLB). Failover clusters primarily provide high availability; Network Load Balancing provides scalability and at the same time helps increase availability of Web-based services.

Your choice of cluster technologies (failover clusters or Network Load Balancing) depends primarily on whether the applications you run have long-running in-memory state:

  • Failover clusters are designed for applications that have long-running in-memory state, or that have large, frequently updated data states. These are called stateful applications, and they include database applications and messaging applications. Typical uses for failover clusters include file servers, print servers, database servers, and messaging servers.

  • Network Load Balancing is intended for applications that do not have long-running in-memory state. These are called stateless applications. A stateless application treats each client request as an independent operation, and therefore it can load-balance each request independently. Stateless applications often have read-only data or data that changes infrequently. Front-end Web servers, virtual private networks (VPNs), File Transfer Protocol (FTP) servers, and firewall and proxy servers typically use Network Load Balancing. Network Load Balancing clusters can also support other TCP- or UDP-based services and applications.

Network Load Balancing overview

The Network Load Balancing (NLB) service enhances the availability and scalability of Internet server applications such as those used on Web, FTP, firewall, proxy, virtual private network (VPN), and other mission-critical servers.

What are NLB clusters?

A single computer running Windows can provide a limited level of server reliability and scalable performance. However, by combining the resources of two or more computers running one of the products in Windows Server 2008 R2 into a single virtual cluster, NLB can deliver the reliability and performance that Web servers and other mission-critical servers need.

Each host runs a separate copy of the desired server applications (such as applications for Web, FTP, and Telnet servers). NLB distributes incoming client requests across the hosts in the cluster. The load weight to be handled by each host can be configured as necessary. You can also add hosts dynamically to the cluster to handle increased load. In addition, NLB can direct all traffic to a designated single host, which is called the default host.

NLB allows all of the computers in the cluster to be addressed by the same set of cluster IP addresses, and it maintains a set of unique, dedicated IP addresses for each host. For load-balanced applications, when a host fails or goes offline, the load is automatically redistributed among the computers that are still operating. When a computer fails or goes offline unexpectedly, active connections to the failed or offline server are lost. However, if you bring a host down intentionally, you can use the drainstop command to service all active connections prior to bringing the computer offline. In any case, when it is ready, the offline computer can transparently rejoin the cluster and regain its share of the workload, which allows the other computers in the cluster to handle less traffic.

Hardware and software considerations for NLB clusters

  • NLB is installed as a standard Windows networking driver component.

  • NLB requires no hardware changes to enable and run.

  • NLB Manager enables you to create new NLB clusters and to configure and manage clusters and all of the cluster's hosts from a single remote or local computer.

  • NLB lets clients access the cluster by using a single, logical Internet name and virtual IP address—known as the cluster IP address (it retains individual names for each computer). NLB allows multiple virtual IP addresses for multihomed servers.

    Note

    In the case of virtual clusters, the servers do not need to be multihomed to have multiple virtual IP addresses.
  • NLB can be bound to multiple network adapters, which allows you to configure multiple independent clusters on each host. Support for multiple network adapters is different from virtual clusters in that virtual clusters allow you to configure multiple clusters on a single network adapter.

Installing the NLB feature

To use Network Load Balancing (NLB), a computer must have only TCP/IP on the adapter on which NLB is installed. Do not add any other protocols (for example, IPX) to this adapter. NLB can load balance any application or service that uses TCP/IP as its network protocol and is associated with a specific Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) port.

To install and configure NLB, you must use an account that is listed in the Administrators group on each host. If you are not using an account in the Administrators group as you install and configure each host, you will be prompted to provide the logon credentials for such an account. To set up an account that NLB Manager will use by default: in NLB Manager, expand the Options menu, and then click Credentials. We recommend that this account not be used for any other purpose.

You can use Initial Configuration Tasks or Server Manager to install NLB. To install NLB, in the list of tasks, click Add features and in the list of features in the wizard, click Network Load Balancing.

Managing NLB

Server roles and features are managed by using Microsoft Management Console (MMC) snap-ins. To open the Network Load Balancing Manager snap-in, click Start, click Administrative Tools, and then click Network Load Balancing Manager. You can also open Network Load Balancing Manager by typing Nlbmgr at a command prompt.

Additional references for NLB

To learn more about NLB, you can view the Help on your server. To do this, open Network Load Balancing Manager as described in the previous section and press F1.

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