Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 24)

8 Brocade Virtual ADX Server Load Balancing Guide
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Overview
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In Figure 1 , an application administrator has established a web site www.example7.com. This web
site is mapped to the virtual server (VIP 10.95.55.1) that is hosted on the Brocade Virtual ADX. All
queries made to this web site arrive at the virtual server. The Brocade Virtual ADX then distributes
these queries among the four back-end application servers. The actual addresses of these four real
web servers remain unknown and unseen to the end users. They observe only one IP address,
which is the VIP address for the web service.
How SLB works
A Brocade Virtual ADX running SLB software establishes a virtual server that acts as a front end to
physical servers, distributing user service requests among active real servers. SLB packet
processing is based on the Network Address Translation (NAT) method. Packets received by the
virtual server IP address are translated into the real physical IP address based on the configured
distribution metric (for example, “round robin”) and sent to a real server. Packets returned by the
real server for the end user are translated by SLB so that the source address is that of the virtual
server instead of the real server.
Port translation is not performed for any virtual port that is bound to a default virtual port.
Load-balancing predictor
The load-balancing predictor is an algorithm that determines how traffic is distributed among the
application servers (real servers).
It is possible to fine-tune traffic distribution among servers by selecting one of the following
predictors:
Least connections predictor
Sends the request to the real server that currently has the fewest active connections with clients.
For sites where a number of servers have similar performance, the least connections option
smooths distribution if a server gets bogged down. For sites where the capacity of various servers
varies greatly, the least connections option maintains an equal number of connections among all
servers. Servers that are capable of processing and terminating connections faster then receive
more connections than slower servers over time.
NOTE
The Least Connections predictor does not depend on the number of connections to individual ports
on a real server but instead depends on the total number of active connections to the server.
The Least Connections predictor can be applied globally to the entire Brocade Virtual ADX or locally
per virtual server as described in
“Changing the Load-Balancing Predictor Method” on page 21.
Round Robin predictor
Directs the service request to the next server, and treats all servers equally regardless of the
number of connections. For example, in a configuration of four servers, the first request is sent to
Server1, the second request is sent to Server2, the third is sent to Server3, and so on. After all
servers in the list have received one request, assignment begins with Server1 again. If a server
fails, SLB avoids sending connections to that server and selects the next server instead. The Round
Robin predictor can be applied globally to apply for the entire Brocade Virtual ADX or locally
per-virtual server as described in
“Changing the Load-Balancing Predictor Method” on page 21.