tailieunhanh - Tony Bourke Server Load Balancing phần 3

Hãy lấy ví dụ của một trang web có hiện diện tại hai trung tâm dữ liệu khác nhau, một ở San Jose, California, và một ở thành phố New York (xem Hình 1-5): 1. GSLB mang nội dung gần gũi hơn với người sử dụng. Với độ trễ giữa các quốc gia khoảng 60 ms (mili giây) hoặc hơn, nó làm cho cảm giác | Other SLB Methods 29 Web Server IP Loopback alias MAC 00 00 00 00 00 bb Figure 3-4. The DSR traffic path complexity to a configuration and added complexity can make a network architecture more difficult to implement. Also any Layer 5-7 URL parsing or hashing would not work because that process requires a synchronous data path in and out of the load balancer. Cookie-based persistence would not work in most situations although it is possible. Other SLB Methods There are several other ways to perform network-based SLB. The way it is normally implemented is sometimes called half-NAT since either the source address or the destination address of a packet is rewritten but not both. A method known as full-NAT also exists. Full-NAT rewrites the source and destination addresses at the same time. A given scenario might look like the one in Table 3-3. Table 3-3. Full-NATSLB Step Source Destination 1 2 3 4 In this situation all source addresses regardless of where the requests come from are set to one IP address. The downside to this is that full-NAT renders web logs 30 Chapter 3 Anatomy of a Server Load Balancer useless since all traffic from the web server s point of view comes from one IP address. A situation like this has limited uses in SLB and won t be discussed beyond this chapter. It can sometimes be useful for features such as proxy serving and cache serving but for SLB full-NAT is not generally used. Under the Hood SLB devices usually take one of two basic incarnations the switch-based load balancer or the server-based load balancer. Each has its general advantages and drawbacks but these greatly depend on how the vendor has implemented the technology. Server-BasedLoadBalancers Server-based load balancers are usually PC-based units running a standard operating system. Cisco s LocalDirector and F5 s BIG-IP are both examples of serverbased load .