Nortel Networks WEB OS 212777 - Manual (Page 132)

Web OS 10.0 Application Guide
132 n Chapter 6: Server Load Balancing
212777-A, February 2002
Hash
The hash metric uses IP address information in the client request to select a server. The spe-
cific IP address information used depends on the application:
n
For Application Redirection, the client destination IP address is used. All requests for a
specific IP destination address will be sent to the same server. This is particularly useful
for maximizing successful cache hits.
n
For SLB, the client source IP address is used. All requests from a specific client will be
sent to the same server. This option is useful for applications where client information
must be retained between sessions.
n
For FWLB, both the source and destination IP addresses are used to ensure that the two
unidirectional flows of a given session are redirected to the same firewall.
When selecting a server, a mathematical hash of the relevant IP address information is used as
an index into the list of currently available servers. Any given IP address information will
always have the same hash result, providing natural persistence, as long as the server list is sta-
ble. However, if a server is added to or leaves the mix, then a different server might be
assigned to a subsequent session with the same IP address information even though the original
server is still available. Open connections are not cleared.
N OTE – The hash metric provides more distributed load balancing than minmisses at any
given instant. It should be used if the statistical load balancing achieved using minmisses is
not as optimal as desired. If the load balancing statistics with minmisses indicate that one
server is processing significantly more requests over time than other servers, consider using
the
hash metric.
Least Connections
With the leastconns metric, the number of connections currently open on each real server
is measured in real time. The server with the fewest current connections is considered to be the
best choice for the next client connection request.
This option is the most self-regulating, with the fastest servers typically getting the most con-
nections over time.
Round Robin
With the roundrobin metric, new connections are issued to each server in turn; that is, the
first real server in the group gets the first connection, the second real server gets the next con-
nection, followed by the third real server, and so on. When all the real servers in this group
have received at least one connection, the issuing process starts over with the first real server.