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CHAPTER 9 .NET REMOTING TIPS AND BEST PRACTICES
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Cluster: 192.168.0.40
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SERVER01 - 192.168.0.41
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SERVER02 - 192.168.0.42
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Figure 9-1. A cluster s IP addresses
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To leverage the load-balancing capabilities of Windows, you will now have to deploy the exact same remoting services on both machines. You can, for example, copy the same server-side configuration files and bin\ subdirectory to an IIS virtual directory on both machines. Instead of accessing this service using one of the two servers real IP address, you will then configure the clients to use the cluster s virtual IP address as the server s location. You can, for example, access a correctly configured service on the cluster presented earlier using the following line of code: ICustomerManager mgr = (ICustomerManager) Activator.GetObject( typeof(ICustomerManager), "http://192.168.0.40/Remoting/CustomerManager.rem"); Whenever a client establishes a TCP/IP connection to the cluster s virtual IP address, Windows NLB selects one of the cluster s nodes to handle the connection request.
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Note There are essentially two different kinds of load-balancing infrastructures. One is based on IP-level
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load-balancing devices that sit right in front of your network (from the client s point of view) and that distribute the load among the nodes in a cluster. In the second way, which is implemented by Windows Network Load Balancing, however, there is no such need for an additional device. Request distribution is instead handled by a distributed algorithm running on all nodes in a cluster.
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NLB for Throughput and High Availability
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Using Network Load Balancing as illustrated previously provides a great way to increase your application s throughput (if you reach CPU limits) and availability. Most people unfortunately look at NLB only if they need the additional throughput, which can be achieved by scaling out a solution onto a cluster of machines. However, I personally recommend the use of NLB clusters even for smaller setups in which the performance requirements could easily be fulfilled with a single server.
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CHAPTER 9 .NET REMOTING TIPS AND BEST PRACTICES
The reason for this is simple: increased availability. If you design your application correctly, then it is possible to take nodes offline from your cluster without affecting a single running client. All future requests will then be handled by the remaining node(s). This allows you to install new versions of your application or service packs or even update operating systems without taking your application offline. After you ve finished administrative work on one node, you allow it to join the cluster again; you can then proceed to take the next node offline.
Note Windows NLB is compatible across all supported operating systems. You could have some nodes
running on Windows NT 4.0, and a few on Windows 2000, while at the same time updating the remaining part of your cluster to Windows Server 2003. As I personally believe that this will also be true for future operating systems, it essentially means that you will never again have to take your application offline for scheduled maintenance even if you upgrade your operating system. Your system administrators will love you for not having to work during late night and weekend maintenance timeslots.
HTTP, TCP, Connections, and Sessions
I am sure that you are already looking forward to getting your hands dirty with your first NLB cluster configuration. But before I show you how to set up an NLB cluster (which shouldn t take much more than about 15 clicks), there is one more important technical peculiarity that I have to tell you: as briefly mentioned earlier, NLB actually only balances connection requests. As soon as a TCP connection has been established, all future communication using the specific socket link will take place between the client and the originally selected cluster node. If a single cluster node fails, all clients that currently have open communication links to it will therefore receive an exception, telling them that the connection has been dropped. This wouldn t be that big of a problem if it weren t for .NET Remoting reusing the underlying TCP connection. Remoting uses the so-called HTTP 1.1 KeepAlive functionality. This means that multiple HTTP requests will be sent using a single TCP/IP connection. The connection will be kept open for a certain period of time (usually around two minutes), and all further requests occurring during that time window will be sent over the same connection. This is usually a good thing because it eliminates the need for reestablishing dozens of TCP connections whenever you open a conventional Web page containing multiple images. Otherwise, the client would have to open separate TCP connections for each and every image file. In cases when you want to provide maximum availability for your application, this Weboriginating optimization in the HTTP protocol actually causes a number of problems. As soon as a node fails, all clients who currently have an open (cached) connection to this node will receive exceptions whenever they invoke a method on the server-side object. In this case, it would actually be beneficial if no connections were cached at all so that every single remoting request causes a new TCP connection that could then be load balanced on its own. Fortunately, this is easily possible when hosting your remoting components in IIS which is actually the only load-balancing configuration that is officially supported by Microsoft. You just have to open an IIS management console and navigate to the Web site that contains your remoting servers. After right-clicking, you will be presented with the dialog box shown in Figure 9-2 in which you can uncheck the checkbox option HTTP Keep-Alives Enabled.
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