MOSS 2007 High Availability
Server Web Farm
The multiple servers in the Web infrastructure must appear as one Web server serving the DNS or the company URL. Web farming (clustering a group of Web servers) facilitates load sharing internally, while appearing to be a single server to users. Before designing a Web server farm or upgrading an existing Web server to a Web server farm, the architect must carefully analyze the requirements and study the design issues involved. The design areas include network infrastructure, load balancing, capacity planning, performance, fault-tolerance, availability, and security considerations. Web farms with 20 to 30 servers are not uncommon in real-world applications.
Network infrastructure
As the number of servers increases, the network infrastructure has to manage the Web servers. The basic network infrastructure that connects Web servers in a Web farm to the Internet includes network interface cards, hubs, layer-2 switches, and routers. The network design must ensure that the Web server is always available to its clients. The network infrastructure should be able to detect server and connection failures and take appropriate measures to keep the Web site running. It should be able to handle large increases in server capacity and protect the servers from hackers.Routing server
In a server farm, one server is used to route HTTP requests to different Web servers. Figure 1 shows routing to a Web server in a Web server farm. The routing server usually has a well-known DNS name to which the clients connect and a dedicated IP address. Each server in the farm has its own dedicated IP address as well. The routing server receives the client's requests and redirects them to one of the servers in the farm.In addition to network infrastructure, hardware and software solutions are necessary to route the server traffic to appropriate Web servers based on request type, priority, security level, and server availability. Furthermore, client sessions can be maintained by sending all requests coming from a client to the same server. By using appropriate load-balancing techniques within the routing server, the Web servers can actually share the Web traffic.
Load-balancing software and hardware
Round-robin DNS is one of the common load-balancing techniques; each logical DNS name maps to several IP addresses. When a client attempts to resolve the DNS name, the DNS server sends back one of the addresses from the list. The DNS server shuffles or rotates the addresses to distribute the Web traffic across a set of servers. In order to preserve user sessions, once a client resolves a DNS name into an IP address, the DNS server caches the IP address for the duration of the client session. Round-robin DNS is a simple and fast load distribution technique in which different users are given different IP addresses to balance the load.Instead of merely redirecting the traffic to one of the Web servers, you can use advanced load balancing techniques that consider the Web server's availability, the current load on the Web server, and the type of HTTP request before distributing the Web traffic. Such advanced methods let the administrator bring down a Web server for maintenance without affecting the overall availability of the Web farm.
Cisco Systems' LocalDirector is one such product with proprietary hardware that probes the status of Web servers in order to determine their availability. The Web servers can also send their load and availability information to LocalDirector. Microsoft's Windows NT Load Balancing Service is another load-balancing software solution, in which the Web servers constantly exchange performance statistics and divide the responsibilities of handling incoming requests using a proprietary algorithm.
Redundancy
Redundancy is required at every level of the Web farm design, from the ISP connection at the front end to the back-end infrastructure that accesses the Web servers. In order to eliminate single points of failure, you must have a backup for every network device placed between the ISP and the Web farm. You must also provide fail-over strategies to transfer control to the "healthy" devices; the fail-over process should take less than a few seconds and should be transparent to the clients.Security
A server farm is usually divided into secured and unsecured zones. Secured servers are placed behind a firewall. Web traffic initially goes through the unsecured servers and secure transactions go through the servers behind the firewall. Firewalls provide site security by monitoring Web traffic, network address translation, port-translation, and access-control lists. Secure socket layer (SSL) accelerators help speed up SSL transactions. SSL off-loading devices function as discrete network devices that store keys and certificates and perform all SSL processing - freeing secure servers from processing these functions.Configuring a Web Server Farm
A Web farm requires some type of switch or load balancer (Web switch). The primary purpose of the Web switch is to load balance the incoming IP traffic to multiple Web servers. It provides high availability, improved performance, fault tolerance, easier manageability, and proxy functionality as well.A Web switch must have two network interfaces. On one end, it must connect to the Internet via a router. On the other end, it must connect to the server network that connects the Web servers.
Let's look at an example to configure a Web server farm comprising four Web servers to the server www.myCompany.com This configuration uses two Web switches to make the Web server farm fault-tolerant to network failures (see Figure 2). This activity requires both a primary and a secondary Web switch. A special network cable connects the two switches together, which lets each switch check the status of the other. Both Web switches connect to the router LAN and to the server LAN. Each switch interface has a unique IP address. The IP address of the Web switch interface (connected to the router LAN) is in the same network as the Internet router.
Availability Equals Revenue
E-business enterprises depend on their Web sites to generate revenue, satisfy customers, and optimize business processes. The enterprise Web site must respond quickly and be available 24/7 to service customers, business partners, and employees. The Web infrastructure has to be designed, maintained, and scaled to achieve this high availability.
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