Scaling Web Applications with Windows 2000 Advanced Server's Network Load Balancing (Cont.)

Getting started with Network Load Balancing

Network load balancing in Windows 2000 is fairly easy to set up and run, assuming that you can manage to decipher the horrible documentation in the online help. In this section, I'll take you through a configuration scenario that hopefully will make your installation and configuration much easier, by highlighting important aspects of installation and startup.

Let's start by discussing what you need in order to use NLB. You'll need at least two machines running

Windows 2000 Advanced Server or better. You'll need at least one network card in each machine. You can also use multiple network cards ? one for the cluster communication and one for the dedicated IP address for all directly accessed resources. For testing, it's a good idea to have another machine that can run a Web stress testing tool to let you see how the cluster works under load.

I'm going to use two machines here to demonstrate how to set up and run NLB. Assume the IP addresses for these machines are 111.111.111.1 and 111.111.111.2. I'm going to set up NLB on a 'virtual IP' (NLB calls this the 'primary IP address') at 111.111.111.10. In order to set up NLB, every machine in the cluster must set up this IP address in addition to its dedicated machine IP address(es). To do so, right click on Network Neighborhood on the desktop, click Properties, then Internet Protocol (TCP/IP). The machine must have at least one fixed IP address in order for NLB to work ? DHCP clients with automatically assigned addresses will not work, so if you're using DHCP make sure to add at least one physical IP address to your machine configuration. Once you have a primary IP address for your machine, click on the Advanced button and add 111.111.111.10 as a new IP address. Make sure that subnet masks are the same on all of these IP addresses (255.255.255.0). Figure 2 shows what you should see in the IP display dialog.

Note that even though 111.111.111.10 is a virtual IP address, you can tie domain names to it with DNS. So, your master domain name such as www.yoursite.com would point to this virtual IP address in the DNS record.

You now have an IP address that the virtual IP can bind to. Go back out to the Local Area Connection Properties and notice the Network Load Balancing option in the list of network services. Check the checkbox and click the entry to bring up the service properties.

Cluster parameters

The first page of this multi-tab dialog (see Figure 4) shows you the cluster parameters. Here you put information about the cluster, such as the virtual IP address, subnet mask, and whether to use multicast messages. There are also additional options that allow you control the cluster remotely via the command line tools that the service provides. Note that this applet has a number of user interface bugs and quirks that make using it a little less than optimal, so be sure you look at the values before moving on to the next page or clicking OK. In addition, the documentation and help file are very scattered, making it hard to find helpful information. The best help is the '?' help from the dialog's control box. Drop the question mark onto individual fields for the best documentation on individual items in each of the dialogs.

NLB calls the virtual IP the Primary IP address, which is very confusing to say the least, since primary IP can mean primary IP of your system or of the cluster. Virtual IP is the term most commonly used in other load balancing packages, and I think it describes the concept much better. This Primary IP is the cluster's IP address, which will be used to access all the sites in the cluster. Public DNS entries such as www.yoursite.com will be bound to this 'virtual' IP address. Set this IP address to the new IP we added in Figure 2 for all of our cluster machines ? in this case 111.111.111.10 ? and adjust the subnet mask to 255.255.255.0.

The full Internet name is used only for remote administration and is used as an identifier for the machine. If you're using a single network adapter you'll want to enable multicast support to allow the network card to handle traffic both for the cluster and dedicated IP address.

Host Parameters

The Host parameters (see Figure 5) configure the cluster machine's native IP settings and affect how the cluster loads. The Dedicated IP Address for is the main physical IP address for the machine that is used to access the machine without going through the cluster. This IP address can be accessed directly, or NLB can use it for routing virtual IP traffic. In other words, you can access the machine via 111.111.111.1 or 111.111.111.10. Only the latter runs through the Load Balancing Cluster.

Another important parameter is the Priority setting, which doubles as a unique ID for each server in the cluster. Each server must have a unique ID, and the server with the lowest priority value (which is really the highest priority!) handles all of the default network traffic that is not specifically configured in the port rules. I'm referring to default network traffic, such as files passed between servers and standard network messages that don't fall within the range of rules applied in the port rules. It doesn't affect the configured ports, which are handled on an equal (presumably on a first come, first served) basis between the cluster machines.

The server with lowest ID handles the default network traffic serviced by the virtual IP. If the first server in the cluster fails, the one with the next higher priority will become the controller. Note that actual load balancing percentage settings can be configured separately in the Port Rules tab, and are not affected by the priority setting. The important thing is that each server gets a unique ID, since the cluster manager reports on the servers using the priority ID (for example, when converging IP addresses on startup).

You can dynamically add and delete servers from the cluster with the Active flag, keeping the other parameters of the server intact.

Do you need Load Balancing?

Load balancing software is a great way to assure that you application will

scale well beyond a single machine, but realize that as you add machines your application administration becomes a lot more complex. Today's hardware makes it possible to run a tremendous amount of traffic on a single box, especially if the machine uses multiple processors. Advanced Server can use up to 8 processors and Data Center Server can use up to 16. All but the busiest applications can run comfortably on this type of hardware and adminstration of a single machine can often make up for the cost of the higher end hardware required. Keep in mind that in most cases the best application scalability in sheer performance terms is achieved by maximizing the performance of a single box. There are many good reasons to use Load Balancing software including employing it as a live backup solution, but make sure you are using it for the right reasons.