What is Network Load Balancing ?

The Network Load Balancing (NLB) feature in Windows Server 2008 R2 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. A single computer running Windows Server 2008 R2 provides 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.

The diagram above depicts two connected Network Load Balancing clusters. The first cluster consists of two hosts and the second cluster consists of four hosts. This is one example of how you can use NLB.

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.

The hosts in an NLB cluster exchange heartbeat messages to maintain consistent data about the cluster’s membership. By default, when a host fails to send heartbeat messages within five seconds, it has failed. When a host has failed, the remaining hosts in the cluster converge and do the following:

• Establish which hosts are still active members of the cluster.
  
• Elect the host with the highest priority as the new default host.
  
• Ensure that all new client requests are handled by the surviving hosts.
  

During a convergence, the surviving hosts look for consistent heartbeats. If the host that failed to send heartbeats begins to provides heartbeats consistently, it rejoins the cluster in the course of the convergence. When a new host attempts to join the cluster, it sends heartbeat messages that also trigger a convergence. After all cluster hosts agree on the current cluster membership, the client load is redistributed to the remaining hosts, and the convergence completes.

Convergence generally takes only a few seconds, so interruption in client service by the cluster is minimal. During convergence, hosts that are still active continue handling client requests without affecting existing connections. Convergence ends when all hosts report a consistent view of the cluster membership and distribution map for several heartbeat periods.