Coyote Point Systems Equalizer GX Series, Administration Manual

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Most client requests are a mix of requests for text and graphics. Layer 7 processing without Match
Rules balances requests across the specified server pool so that each server instance in the
server pool will see a mix of text and graphics requests. This means that all text and graphics
must be available on each server pool.
Some sites may want to have one system serve only requests for graphics, and one system serve
only text requests.
By adding appropriate Match Rules, Equalizer can examine each request to determine if the
content requested is Text or Graphics, and send the request to the appropriate server pool. In this
example, the servers need only hold the content they are serving, text or graphics.
How Match Rules are Processed
A match rule is like an if-then statement: an expression is evaluated and if it evaluates to true the
body of the match rule applies to the request.
A match expression is a combination of match functions with logical operators, and can be
arbitrarily complex. This allows for matching requests that have, for example:
(attribute A) AND NOT (attribute B)
If a match expression evaluates to true , then the data in the request has selected the match rule,
and the match body applies. The match body contains statements that affect the subsequent
handling of the request.
Multiple match rules are checked in order. Once the data in the request selects a match rule --
that is, the match rule expression evaluates to true -- no further match rules are checked against
the request.
Equalizer makes a load balancing decision as follows:
1. If the request headers contain a cookie that specifies a server pool for the match rule, Equal-
izer sends the request to the server in the cookie.
Otherwise:
2. Equalizer sends the request to the server pool specified in the match rule that is selected by
the load balancing policy in effect for the match rule.
This process applies even if all the servers selected for the match rule are unavailable. In this
case, when the match rule expression matches the request and all the servers in the match rule
server list are unavailable, no reply is sent to the client. Eventually, the client sees a connection
timeout.
If the match expression evaluates to false , then each subsequent match rule in the list of match
rules for the virtual cluster is processed until a match occurs. All virtual clusters have a Default
Match rule, which always evaluates to
true and which will use the entire set of servers for load
balancing. The
Default Match
rule is always processed last.
Each virtual cluster can have any number of match rules, and each match rule can have arbitrarily
complex match expressions. Keep in mind that Equalizer interprets match rules for every Layer 7
cluster connection, so it is a good idea to keep match rules as simple as possible.
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Equalizer Administration Guide