varnish-selector - 2.6.0-5.1 main

Varnish Module (VMOD) for matching strings against sets of fixed
strings. A VMOD object may also function as an associative array,
mapping the matched string to one or more of a backend, another
string, an integer, or a regular expression. The string may also map
to a subroutine that can be invoked.
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The VMOD is intended to support a variety of use cases that are
typical for VCL deployments, such as:
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* Determining the backend based on the Host header or the prefix of
the URL.
* Rewriting the URL or a header.
* Generating redirect responses, based on a header or the URL.
* Permitting or rejecting request methods.
* Matching the Basic Authentication credentials in an Authorization
request header.
* Matching media types in the Content-Type header of a backend
response to determine if the content is compressible.
* Accessing data by string match, as in an associative array, or by
numeric index, as in a standard array.
* Dispatching subroutine calls based on string matches.
* Executing conditional logic that depends on features of the request
or response that can be determined by matching headers or URLs.
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Operations such as these are commonly implemented in native VCL with
an if-elsif-elsif sequence of string comparisons or regex matches.
As the number of matches increases, such a sequence becomes cumbersome
and scales poorly -- the time needed to execute the sequence increases
with the number of matches to be performed.
With the VMOD, the strings to be matched are declared in a tabular
form in vcl_init, and the operation is executed in a few lines.