Capacity Planning
Capacity Planning |
|
The Network Load Balancing feature of Windows 2000 Advanced Server allows you to create server clusters containing up to 32 machines. Network Load Balancing is fully distributed, and is entirely software-based; it does not use a centralized dispatcher, and does not require any specialized hardware.
This feature transparently distributes client requests among the hosts in the cluster, using virtual Internet Protocol (IP) addresses. You must run IIS 5.0 or another TCP/IP service on each host, and the hosts must serve the same content so that any of them can handle any request. You can copy updated pages to local disks on the hosts, or use commercial filereplication software to perform updates. Network Load Balancing allows you the option of specifying that all connections from the same client IP address be handled by a particular server (unless, of course, that server fails). It also permits you to allocate all requests from a Class C address range to a single server. In addition, Network Load Balancing supports SSL.
Domain Name System (DNS) round-robin distribution is an earlier and less sophisticated technique for allocating requests among servers in a group. Consider a scenario in which there are four IP address entries for the same host on a DNS server:
domain.com A 172.17.21.31
domain.com A 172.17.21.35
domain.com A 172.17.28.41
domain.com A 172.17.28.52
If a client sends a query, the DNS server returns all four IP addresses, but typically the client uses only the first one it receives. With round-robin distribution, the next time the DNS server receives a query for this host the order of the list is changed in a cyclic permutation or “round-robin” (the address that was first in the previous list is last in the new list). Thus, when the client chooses the first IP address in the list, it connects to a different server.
This technique distributes incoming requests evenly among the available IP addresses, but does not fully balance the load because it is not interactive. That is to say, the DNS server neither checks on the loading of an IP address, nor whether a particular server is currently running. Nonetheless, round-robin distribution can be a useful starting point or a low-cost alternative for small groups of servers. You should, if you are using round-robin distribution, keep close tabs on your servers so that you can quickly remove any failed machines from the distribution list.