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Chapter 8: Using Match Rules

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Equalizer Installation and Administration Guide

makes sense to route the request to the (for example) down server, and have the client receive an appropriate error --
so that the request can be retried.

If we instead were to skip a match rule because, for example, the server selected by the match rule is down, the
request would be evaluated by the next match rule -- or the default match rule. The request, therefore, could
potentially be sent to a server in the cluster that does not have the requested content. This means that the client
would receive a “not found” error, instead of an error indicating that the appropriate server is not currently available.

Considering Case in String Comparisons

String comparisons performed by match functions honor the setting of the

ignore case

cluster parameter: if it is set

on the cluster (the default), then all match rule functions used for that cluster are case insensitive; that is, the case of
strings is ignored. For example, the string “ab” will match occurrences of “ab”, “Ab”, “aB”, and “AB”. If

ignore

case

not

set on the cluster, then all string comparisons are by default case sensitive (the string “ab” will match

only “ab”).

To override the

ignore case

flag setting on the cluster for a match function or block of functions, you must logically

AND the

observe_case()

ignore_case()

functions with the match function or block. For example, if

ignore case

is set on the cluster, you would use the following construct to force the

header_substr()

function to make case

sensitive string comparisons:

(observe_case() AND header_substr("host", "MySystem"))

Regular Expressions

Some match functions have

prefix

suffix

substr

, or

regex

variants. The

regex

variants interpret an argument as a

regular expression to match against requests. Regular expressions can be very costly to compute, so use the

prefix

suffix

, or

substr

variants of functions (or Boolean combinations of prefix and suffix testing), rather than the

regex

function variants, for best performance. For example, the following regular expression match:

dirname_regex("(two|four|six|eight)")

Can be replaced by the more efficient:

dirname_substr("two") OR

dirname_substr("four") OR

dirname_substr("six") OR

dirname_substr("eight")

Equalizer supports POSIX regular expression syntax only. See Appendix D, ”Regular Expression Format” for a
description.

Supported Headers

All of the

header_*(

header

string

)

match functions take a

header

argument, which selects the header of interest. If

this header is not present in the request, the match function evaluates to

false

. Otherwise, the text associated with the

header is examined depending on the particular function.

Although HTTP permits a header to span multiple request lines, none of the functions matches text on more than
one line. In addition, Equalizer will only parse the first instance of a header. If, for example, a request has multiple

cookie

headers, Equalizer will only match against the first

cookie

header in the request.

The list of supported headers for the

header

argument are as follows:

Accept

From

Referer

Accept-Charset

Host