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.. role:: ref(emphasis)

=============
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VMOD selector
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=============

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------------------------------------------------------------------------------------
Varnish Module for matching strings associated with backends, regexen and other data
------------------------------------------------------------------------------------
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:Manual section: 3

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SYNOPSIS
========

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::

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  import selector;

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  # Set creation
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  new <obj> = selector.set([BOOL case_sensitive])
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  VOID <obj>.add(STRING [, STRING string] [, STRING regex]
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		 [, BACKEND backend] [, INT integer])
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  VOID <obj>.create_stats()

  # Matching
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  BOOL <obj>.match(STRING)
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  BOOL <obj>.hasprefix(STRING)

  # Match properties
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  INT <obj>.nmatches()
  BOOL <obj>.matched(INT)
  INT <obj>.which([ENUM select])
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  # Retrieving objects after match
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  STRING  <obj>.element([INT n] [, ENUM select])
  STRING  <obj>.string([INT n]  [, ENUM select])
  INT     <obj>.integer([INT n] [, ENUM select])
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  BACKEND <obj>.backend([INT n] [, ENUM select])
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  BOOL    <obj>.re_match(STRING [, INT n] [, ENUM select])
  STRING  <obj>.sub(STRING text, STRING rewrite [, BOOL all] [, INT n]
                    [, ENUM select])
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  # VMOD version
  STRING selector.version()

DESCRIPTION
===========

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.. _VMOD re2: https://code.uplex.de/uplex-varnish/libvmod-re2

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Varnish Module (VMOD) for matching strings against sets of fixed
strings, and optionally associating the matched string with a backend,
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another string, an integer, or a regular expression.
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The VMOD is intended to support a variety of use cases that are
typical for VCL deployments, such as:

* 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.

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
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with the number of matches to be performed.
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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. For example::

  import selector;

  sub vcl_init {

      # Requests for URLs with these prefixes will be sent to the
      # associated backend.
      new url_prefix = selector.set();
      url_prefix.add("/foo/", backend=foo_backend);
      url_prefix.add("/bar/", backend=bar_backend);
      url_prefix.add("/baz/", backend=baz_backend);

      # For requests with these Host headers, generate a redirect
      # response, using the associated string to construct the
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      # Location header, and the integer to set the response code.
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      new redirect = selector.set();
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      redirect.add("www.foo.com", string="/foo", integer=301);
      redirect.add("www.bar.com", string="/bar", integer=302);
      redirect.add("www.baz.com", string="/baz", integer=303);
      redirect.add("www.quux.com", string="/quux", integer=307);
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      # Requests for these URLs are rewritten by altering the
      # query string, using the associated regex for a
      # substitution operation, each of which removes a
      # parameter.
      new rewrite = selector.set();
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      rewrite.add("/alpha/beta", regex="(\?.*)\bfoo=[^&]+&?(.*)$");
      rewrite.add("/delta/gamma", regex="(\?.*)\bbar=[^&]+&?(.*)$");
      rewrite.add("/epsilon/zeta", regex="(\?.*)\bbaz=[^&]+&?(.*)$");
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  }

  sub vcl_recv {

      # .match() returns true if the Host header exactly matches
      # one of the strings in the set.
      if (redirect.match(req.http.Host)) {
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	  # .string() returns the string added to the set above with
	  # the 'string' parameter, for the string that was
	  # matched. We use it to construct a Location header, which
	  # will be retrieved in vcl_synth below to construct the
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	  # redirect response.
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	  #
	  # .integer() returns the integer added to the set with the
	  # 'integer' parameter, for the string that was matched. We
	  # use it as the argument of synth() to set the response
	  # status (one of the redirect status codes).
	  set req.http.Location
		= "http://other.com" + redirect.string() + req.url;
	  return (synth(redirect.integer()));
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      }

      # If the URL matches the rewrite set, change the query string by
      # applying a substitution using the associated regex (removing a
      # query parameter).
      if (rewrite.match(req.req.url)) {
          set req.url = rewrite.sub(req.url, "\1\2");
      }
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  }
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  sub vcl_synth {

      # We come here when Host matched the redirect set in vcl_recv
      # above. Set the Location response header from the request header
      # set in vcl_recv.
      if (req.http.Location && resp.status >= 301 && resp.status <= 307) {
	  set resp.http.Location = req.http.Location;
	  return (deliver);
      }
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  }
  
  sub vcl_backend_fetch {

      # The .hasprefix() method returns true if the URL has a prefix
      # in the set.
      if (url_prefix.hasprefix(bereq.url)) {
          # .backend() returns the backend associated with the
	  # string in the set that was matched as a prefix.
	  set bereq.backend = url_prefix.backend();
      }
  }

Matches with the ``.match()`` and ``.hasprefix()`` methods scale well
as the number of strings in the set increases. The time needed for
``.match()`` is proportional to the length of the string to be
matched; for ``.hasprefix()``, it is proportional to the length of the
longest string in the set that forms a prefix of the string to be
matched. In both cases, the time for execution is independent of the
number of strings in the set, and is predictable and fast for large
sets of strings.

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In the case of non-matches, the search for a match stops as soon as it
encounters a character in the string such that no string in the set
can match. Thus if a set contains the strings ``foo``, ``bar`` and
``baz``, then the search stops after the first character if it is
neither of ``f`` or ``b``.

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When new strings are added to a set (with new ``.add()`` statements in
``vcl_init``), the VCL code that executes the various operations
(rewrites, backend assignment and so forth) can remain unchanged. So
the VMOD can contribute to better code maintainability.

Matches with ``.match()`` and ``.hasprefix()`` are fixed string
matches; characters such as wildcards and regex metacharacters are
matched literally, and have no special meaning. Regex operations
such as matching or substitution can be performed after set matches,
using the regex saved with the ``regex`` parameter. But if you need
to match against sets of patterns, consider using the set interface
of `VMOD re2`_, which provides techniques similar to the present VMOD.

Selecting matched elements of a set
-----------------------------------

The ``.match()`` operation is an exact, fixed string match, and hence
always matches exactly one string in the set if it succeeds. With
``.hasprefix()``, more than one string in the set may be matched, if
the set includes strings that are prefixes of other strings in the
same set::

  sub vcl_init {
      new myset = selector.set();
      myset.add("/foo/");		# element 1
      myset.add("/foo/bar/");		# element 2
      myset.add("/foo/bar/baz/");	# element 3
  }

  sub vcl_recv {
      # With .hasprefix(), a URL such as /foo/bar/baz/quux matches all
      # 3 elements in the set.
      if (myset.hasprefix(req.url)) {
          # ...
      }
  }

Just calling ``.hasprefix()`` may be sufficient if all that matters is
whether a string has any prefix that appears in the set. But for some
uses it may be necessary to identify one matching element of the set;
this is done in particular for the ``.element()``, ``.backend()``,
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``.string()``, ``.integer()``, ``.re_match()`` and ``.sub()`` methods,
which retrieve data associated with a specific set element. For such
cases, the method parameters ``INT n`` and ``ENUM select`` are used to
choose a matched element.
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As indicated in the example, elements of a set are implicitly numbered
in the order in which they were added to the set using the ``.add()``
method, starting from 1. In all of the following, the ``n`` and
``select`` parameters for a method call are evaluated as follows:

* If ``n`` >= 1, then the ``n``-th element of the set is chosen, and
  the ``select`` parameter has no effect. A method with ``n`` >= 1 can
  be called in any context, and does not depend on prior match
  operations.

* If ``n`` is greater than the number of elements in the set, the
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  method fails, with an error message written to the Varnish
  log. See `ERRORS`_ below for details about method failure.
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* If ``n`` <= 0, then the ``select`` parameter is used to choose an
  element based on the most recent ``.match()`` or ``.hasprefix()``
  call for the same set object in the same task scope; that is, the
  most recent call in the same client or backend context. Thus a
  method call in one of the ``vcl_backend_*`` subroutines refers back
  to the most recent ``.match()`` or ``.hasprefix()`` invocation in
  the same backend context.

  ``n`` is 0 by default, so it can be left out of the method call for
  this purpose.

* If ``n`` <= 0 and neither of ``.match()`` or ``.hasprefix()`` has
  been called for the same set object in the same task scope, or if
  the most recent call resulted in a failed match, then the method
  fails.

* When ``n`` <= 0 after a successful ``.match()`` call, then for any
  value of ``select``, the element chosen is the one that matched.

* When ``n`` <= 0 after a successful ``.hasprefix()`` call, then the
  value of ``select`` determines the element chosen, as follows:

  * ``UNIQUE`` (default): if exactly one element of the set matched,
    choose that element. The method fails in this case if more than
    one element matched.

    Since the defaults for ``n`` and ``select`` are 0 and ``UNIQUE``,
    ``select=UNIQUE`` is in effect if both parameters are left out of
    the method call.

  * ``EXACT``: if one of the elements in the set matched exactly
    (even if other prefixes in the set matched as well), choose
    that element. The method fails if there was no exact match.

    Thus if a prefix match for ``/foo/bar`` is run against a set
    containing ``/foo`` and ``/foo/bar``, the latter element is chosen
    with ``select=EXACT``.

  * ``FIRST``: choose the first element in the set that matched
    (in the order in which they were added with ``.add()``).

  * ``LAST``: choose the last element in the set that matched.

  * ``SHORTEST``: choose the shortest element in the set that matched.

  * ``LONGEST``: choose the longest element in the set that matched.

So for sets of strings with common prefixes, a strategy for selecting
the matched element after a prefix match can be implemented by
ordering the strings added to the set, by choosing only an exact match
or the longest match, and so on::

  # In this example, we set the backend for a fetch based on the most
  # specific matching prefix of the URL, i.e. the longest prefix in
  # the URL that appears in the set.

  sub vcl_init {
      new myset = selector.set();
      myset.add("/foo/",		backend=foo_backend);
      myset.add("/foo/bar/",		backend=bar_backend);
      myset.add("/foo/bar/baz/",	backend=baz_backend);
  }

  sub vcl_backend_fetch {
      if (myset.hasprefix(bereq.url)) {
          set bereq.backend = myset.backend(select=LONGEST);
      }
  }

  # This sets baz_backend for /foo/bar/baz/quux
  # bar_backend for /foo/bar/quux
  # foo_backend for /foo/quux
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.. _vmod_selector.set:
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new xset = selector.set(BOOL case_sensitive=1)
----------------------------------------------
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Create a set object. When ``case_sensitive`` is ``false``, matches
using the ``.match()`` and ``.hasprefix()`` methods are
case-insensitive.  By default, ``case_sensitive`` is ``true``.
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Example::

  sub vcl_init {
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      new myset = selector.set();
      # ...

      # For case-insensitive matching.
      new caseless = selector.set(case_sensitive=false);
      # ...
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  }

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.. _vmod_selector.set.add:
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VOID xset.add(STRING, [STRING string], [STRING regex], [BACKEND backend], [INT integer])
----------------------------------------------------------------------------------------
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::

      VOID xset.add(
            STRING,
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            [STRING string],
            [STRING regex],
            [BACKEND backend],
            [INT integer]
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      )

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Add the given string to the set. As indicated above, elements added to
the set are implicitly numbered in the order in which they are added
with ``.add()``, starting with 1.

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If values are set for the optional parameters ``string``, ``regex``,
``backend`` or ``integer``, then those values are associated with this
element, and can be retrieved with the ``.string()``, ``.backend()``,
``.integer()``, ``.re_match()`` or ``.sub()`` methods, as described
below.
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A regular expression in the ``regex`` parameter is compiled at VCL load
time. If the compile fails, then the VCL load fails with an error message.
Regular expressions are evaluated exactly as native regexen in VCL.

``.add()`` fails and invokes VCL failure (see `ERRORS`_) under the
following conditions:

* ``.add()`` is called any subroutine besides ``vcl_init``.

* The string to be added is NULL.

* The same string is added to the same set more than once.

* A regular expression in the ``regex`` parameter fails to compile.

Example::

  sub vcl_init {
      new myset = selector.set();
      myset.add("www.foo.com");
      myset.add("www.bar.com", string="/bar");
      myset.add("www.baz.com", string="/baz", backend=baz_backend);
      myset.add("www.quux.com", string="/quux", backend=quux_backend,
      		regex="^/quux/([^/]+)/");
  }

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.. _vmod_selector.set.create_stats:
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VOID xset.create_stats()
------------------------

Creates statistics counters for this object that are displayed by
tools such as ``varnishstat(1)``. See `STATISTICS`_ below for details.
It should be called in ``vcl_init`` after all strings have been added
to the set. No statistics are created for a set object if
``.create_stats()`` is not invoked.

Unlike the matching operations, the time needed for this method
increases as the number of strings in the set increases, since it
traverses the entire internal data structure. For large sets, the time
needed for a VCL load can become noticeably longer. If that is a
problem, consider using this method during development and testing,
and removing it for production deployments (since the stats values are
always the same for sets with the same strings).

``.create_stats()`` fails and invokes VCL failure if it is called in
any VCL subroutine besides ``vcl_init``.

Example::

  sub vcl_init {
      new myset = selector.set();
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      myset.add("foo");
      myset.add("bar");
      myset.add("baz");
      myset.create_stats();
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  }
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.. _vmod_selector.set.match:
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BOOL xset.match(STRING)
-----------------------

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Returns ``true`` if the given STRING exactly matches one of the
strings in the set. The match is case insensitive if and only if the
parameter ``case_sensitive`` was set to ``false`` in the set
constructor (matches are case sensitive by default).

``.match()`` fails and returns ``false`` under the following conditions:

* The string to be matched is NULL (such as an unset header).

* No strings were added to the set.

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* There is insufficient workspace for internal operations.
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Example::

  if (myset.match(req.http.Host)) {
     call do_on_match;
  }

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.. _vmod_selector.set.hasprefix:
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BOOL xset.hasprefix(STRING)
---------------------------

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Returns ``true`` if the STRING to be matched has a prefix that is in
the set. The match is case insensitive if ``case_sensitive`` was set
to ``false`` in the constructor.

``.hasprefix()`` fails and returns ``false`` under the same conditions
given for ``.match()`` above.
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Example::

  if (myset.hasprefix(req.url)) {
     call do_if_prefix_matched;
  }

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.. _vmod_selector.set.nmatches:
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INT xset.nmatches()
-------------------

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Returns the number of elements that were matched by the most recent
successful invocation of ``.match()`` or ``.hasprefix()`` for the same
set object in the same task scope (that is, in the same client or
backend context).

``.nmatches()`` returns 0 after either of ``.match()`` or
``.hasprefix()`` returned ``false``, and it returns 1 after
``.match()`` returned ``true``.  After a successful ``.hasprefix()``
call, it returns the number of strings in the set that are prefixes of
the string that was matched.

``.nmatches()`` returns 0 and writes an error message to the log if
there was no prior invocation of ``.match()`` or ``.hasprefix()`` in
the same task scope.
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Example::

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  # For a use case that requires a unique prefix match, use
  # .nmatches() to ensure that there was exactly one match, and fail
  # fast with VCL failure otherwise.
  if (myset.hasprefix(bereq.url)) {
     if (myset.nmatches() != 1) {
         std.log(bereq.url + " matched > 1 prefix in the set");
	 return (fail);
     }
     set bereq.backend = myset.backend(select=UNIQUE);
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  }

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.. _vmod_selector.set.matched:
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BOOL xset.matched(INT)
----------------------

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After a successful ``.match()`` or ``.hasprefix()`` call for the same
set object in the same task scope, return ``true`` if the element
indicated by the INT parameter was matched. The numbering corresponds
to the order of ``.add()`` invocations in ``vcl_init`` (starting from
1).

``.matched()`` always returns ``false`` if the most recent
``.match()`` or ``.hasprefix()`` call returned ``false``.

``.matched()`` fails and returns ``false`` if:

* The parameter is out of range -- it is less than 1 or greater than
  the number of elements in the set.

* There was no prior invocation of ``.match()`` or ``.hasprefix()`` in
  the same task scope.
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Example::

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  if (myset.match(req.http.Host)) {
     if (myset.matched(1)) {
         call do_if_the_first_element_matched;
     }
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  }

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.. _vmod_selector.set.which:
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INT xset.which(ENUM select)
---------------------------

::

      INT xset.which(
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            ENUM {UNIQUE, EXACT, FIRST, LAST, SHORTEST, LONGEST} select=UNIQUE
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      )

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After a successful ``.match()`` or ``.hasprefix()`` call for the same
set object in the same task scope, return the index of the matching
element indicated by ``select``. The numbering corresponds to the
order of ``.add()`` calls in ``vcl_init``.

Return 0 if the most recent ``.match()`` or ``.hasprefix()`` call
returned ``false``.

If more than one element matched after calling ``.hasprefix()``, the
index is chosen with the ``select`` parameter, according to the rules
given above. By default, ``select`` is ``UNIQUE``.

``.which()`` fails and returns 0 if:

* The choice of ``select`` indicates failure, as documented above; that
  is, if ``select`` is ``UNIQUE`` or ``EXACT``, but there was no unique
  or exact match, respectively.

* There was no prior invocation of ``.match()`` or ``.hasprefix()`` in
  the same task scope.
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Example::

  if (myset.hasprefix(req.url)) {
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     if (myset.which(select=SHORTEST) > 1) {
         call do_if_the_shortest_match_was_not_the_first_element;
     }
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  }

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.. _vmod_selector.set.element:
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STRING xset.element(INT n, ENUM select)
---------------------------------------

::

      STRING xset.element(
            INT n=0,
            ENUM {UNIQUE, EXACT, FIRST, LAST, SHORTEST, LONGEST} select=UNIQUE
      )

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Returns the element of the set indicated by the ``n`` and ``select``
parameters as described above. Thus if ``n`` >= 1, the ``n``-th
element of the set is returned; otherwise the matched element
indicated by ``select`` is returned after calling ``.match()`` or
``.hasprefix()``.

The string returned is the same as it was added to the set; even if a
prior match was case insensitive, and the matched string differs in
case, the string with the case as added to the set is returned.

``.element()`` fails and returns NULL if the rules for ``n`` and
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``select`` indicate failure; that is:

* ``n`` is out of range (greater than the number of elements in the
  set)

* ``n`` < 1 and ``select`` fails for ``UNIQUE`` or ``EXACT``

* ``n`` < 1 and there was no prior invocation of ``.match()`` or
  ``.hasprefix()``.
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Example::

  if (myset.hasprefix(req.url)) {
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     # Construct a redirect response for another host, using the
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     # matching prefix in the request URL as the new URL path.
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     set resp.http.Location = "http://other.com" + myset.element();
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  }

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.. _vmod_selector.set.backend:
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BACKEND xset.backend(INT n, ENUM select)
----------------------------------------

::

      BACKEND xset.backend(
            INT n=0,
            ENUM {UNIQUE, EXACT, FIRST, LAST, SHORTEST, LONGEST} select=UNIQUE
      )

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Returns the backend associated with the element of the set indicated
by ``n`` and ``select``, according to the rules given above; that is,
it returns the backend that was set via the ``backend`` parameter in
``.add()``.

``.backend()`` fails and returns NULL if:

* The rules for ``n`` and ``select`` indicate failure.

* No backend was set with the ``backend`` parameter in the ``.add()``
  call corresponding to the selected element.
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Example::

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  if (myset.hasprefix(bereq.url)) {
     # Set the backend associated with the string in the set that
     # forms the longest prefix of the URL
     set bereq.backend = myset.backend(select=LONGEST);
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  }

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.. _vmod_selector.set.string:
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STRING xset.string(INT n, ENUM select)
--------------------------------------

::

      STRING xset.string(
            INT n=0,
            ENUM {UNIQUE, EXACT, FIRST, LAST, SHORTEST, LONGEST} select=UNIQUE
      )

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Returns the string set by the ``string`` parameter for the element of
the set indicated by ``n`` and ``select``, according to the rules
given above.

``.string()`` fails and returns NULL if:

* The rules for ``n`` and ``select`` indicate failure.

* No string was set with the ``string`` parameter in ``.add()``.
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Example::

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  # Rewrite the URL if it matches one of the strings in the set.
  if (myset.match(req.url)) {
     set req.url = myset.string();
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  }

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.. _vmod_selector.set.integer:

INT xset.integer(INT n, ENUM select)
------------------------------------

::

      INT xset.integer(
            INT n=0,
            ENUM {UNIQUE, EXACT, FIRST, LAST, SHORTEST, LONGEST} select=UNIQUE
      )

Returns the integer set by the ``integer`` parameter for the element of
the set indicated by ``n`` and ``select``, according to the rules
given above.

``.integer()`` invokes VCL failure (see `ERRORS`_) if:

* The rules for ``n`` and ``select`` indicate failure.

* No integer was set with the ``integer`` parameter in ``.add()``.

Note that VCL failure for ``.integer()`` differs from the failure mode
for other methods that retrieve data associated with the selected set
element, such as ``.string()`` and ``.backend()``. Since there is no
distinguished "error value" for an INT, the VMOD does not return one
that can be detected in VCL, so that processing could continue without
failure.

So you may want to check more carefully in VCL for possible errors
that may cause ``.integer()`` to fail; for example, by checking
whether ``.nmatches() == 1`` before calling ``.integer()`` with
``select=UNIQUE`` or ``select=EXACT``, if the previous match operation
was ``.hasprefix()`` and the set contains overlapping prefixes.

Example::

  # Send a synthetic response if the URL has a prefix in the set,
  # using the response code set in .add().
  if (myset.hasprefix(req.url)) {
     # Check .nmatches() to ensure that select=UNIQUE can be used
     # without risk of VCL failure.
     if (myset.nmatches() == 1) {
	return( synth(myset.integer(select=UNIQUE)) );
     }
  }

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.. _vmod_selector.set.re_match:
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BOOL xset.re_match(STRING subject, INT n, ENUM select)
------------------------------------------------------

::

      BOOL xset.re_match(
            STRING subject,
            INT n=0,
            ENUM {UNIQUE, EXACT, FIRST, LAST, SHORTEST, LONGEST} select=UNIQUE
      )

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Using the regular expression set by the ``regex`` parameter for the
element of the set indicated by ``n`` and ``select``, return the
result of matching the regex against ``subject``. The regex match is
the same operation performed for the native VCL ``~`` operator, see
vcl(7).

In other words, this method can be used to perform a second match with
the saved regular expression, after matching a fixed string against
the set.

The regex match is subject to the same conditions imposed for matching
in native VCL; in particular, it may be limited by the varnishd
parameters ``pcre_match_limit`` and ``pcre_match_limit_recursion``
(see varnishd(1)).

``.re_match()`` fails and returns ``false`` if:

* The rules for ``n`` and ``select`` indicate failure.

* No regular expression was set with the ``regex`` parameter in
  ``.add()``.

* The regex match fails, for any of the reasons that cause a native
  match to fail.
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Example::

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  # If the Host header exactly matches a string in the set, perform a
  # regex match against the URL.
  if (myset.match(req.http.Host)) {
     if (myset.re_match(req.url)) {
         call do_if_the_URL_matches_the_regex_for_Host;
     }
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  }

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.. _vmod_selector.set.sub:
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STRING xset.sub(STRING str, STRING sub, BOOL all, INT n, ENUM select)
---------------------------------------------------------------------
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::

      STRING xset.sub(
            STRING str,
            STRING sub,
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            BOOL all=0,
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            INT n=0,
            ENUM {UNIQUE, EXACT, FIRST, LAST, SHORTEST, LONGEST} select=UNIQUE
      )

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Using the regular expression set by the ``regex`` parameter for the
element of the set indicated by ``n`` and ``select``, return the
result of a substitution using ``str`` and ``sub``.
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If ``all`` is ``false``, then return the result of replacing the first
portion of ``str`` that matches the regex with ``sub``. ``sub`` may
contain backreferences ``\0`` through ``\9``, to include captured
substrings from ``str`` in the substitution. This is the same
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operation performed by the native VCL function
``regsub(str, regex, sub)`` (see vcl(7)). By default, ``all`` is false.
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If ``all`` is ``true``, return the result of replacing each
non-overlapping portion of ``str`` that matches the regex with ``sub``
(possibly with backreferences). This is the same operation as native
VCL's ``regsuball(str, regex, sub)``.
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``.sub()`` fails and returns NULL if:
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* The rules for ``n`` and ``select`` indicate failure.
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* No regular expression was set with the ``regex`` parameter in
  ``.add()``.
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* The substitution fails for any of the reasons that cause native
  ``regsub()`` or ``regsuball()`` to fail.
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Example::

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  # In this example we match the URL prefix, and if a match is found,
  # rewrite the URL by exchanging path components as indicated.
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  sub vcl_init {
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      new rewrite = selector.set();
      rewrite.add("/foo/", regex="^(/foo)/([^/]+)/([^/]+)/");
      rewrite.add("/foo/bar/", regex="^(/foo/bar)/([^/]+)/([^/]+)/");
      rewrite.add("/foo/bar/baz/", regex="^(/foo/bar/baz)/([^/]+)/([^/]+)/");
  }

  if (rewrite.hasprefix(req.url)) {
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     set req.url = rewrite.sub(req.url, "\1/\3/\2/", select=LAST);
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  }

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  # /foo/1/2/* is rewritten as /foo/2/1/*
  # /foo/bar/1/2/* is rewritten as /foo/bar/2/1/*
  # /foo/bar/baz/1/2/* is rewritten as /foo/bar/baz/2/1/*

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.. _vmod_selector.set.debug:
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STRING xset.debug()
-------------------

Intentionally not documented.

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.. _vmod_selector.version:
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STRING version()
----------------

Return the version string for this VMOD.

Example::

  std.log("Using VMOD selector version: " + selector.version());

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STATISTICS
==========

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When ``.create_stats()`` is invoked for a set object, statistics are
created that can be viewed with a tool like varnishstat(1).

*NOTE*: except for ``elements``, the stats refer to properties of a
set object's internal data structure, and hence depend on the internal
implementation. The implementation may be changed in any new version
of the VMOD, and hence the stats may change. If you install a new
version, check the new version of the present document to see if there
are new or different statistics.

The stats have the following naming schema::

  SELECTOR.<vcl>.<object>.<stat>

... where ``<vcl>`` is the VCL instance name, ``<object>`` is the
object name, and ``<stat>`` is the statistic. So the ``elements`` stat
of the ``myset`` object in the VCL instance ``boot`` is named::

  SELECTOR.boot.myset.elements

The VMOD currently provides the following statistics:

* ``elements``: the number of elements in the set (added via
  ``.add()``)

* ``nodes``: the total number of nodes in the internal data structure

* ``leaves``: the number of leaf nodes in the internal data structure.
  There may be fewer leaf nodes than elements of the set, if the set
  contains common prefixes.

* ``dmin``: the minimum depth of a terminating node in the internal
  data structure; that is, a node at which a matching string may be
  found (not necessarily a leaf node, if the set has common prefixes)

* ``dmax``: the maximum depth of a terminating node in the internal
  data structure

* ``davg``: the average depth of a terminating node in the internal
  data structure, rounded to the nearest integer

The values of the stats are constant; they do not change during the
lifetime of the VCL instance.

The stats for a VCL instance are removed from view when the instance is
set to the cold state, and become visible again when it set to the warm
state. They are removed permanently when the VCL instance is discarded
(see varnish-cli(7)).

ERRORS
======

The method documentation above refers to two kinds of failures: method
failure and VCL failure.

When a method fails, an error message is written to the Varnish log
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with the ``VCL_Error`` tag. If the method's return type allows for a
distinguished "error" value, such as NULL for ``.string()``, then that
value is returned. VCL processing can then continue (although your
code should of course check for the error).

For some return types, there is no such distinguished value; for
example for INT as returned by ``.integer()``. In such cases, VCL
failues is invoked.
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VCL failure has the same results as if ``return(fail)`` is called from
a VCL subroutine:

* If the failure occurs in ``vcl_init``, then the VCL load fails with
  an error message.

* If the failure occurs in any other subroutine besides ``vcl_synth``,
  then a ``VCL_Error`` message is written to the log, and control is
  directed immediately to ``vcl_synth``, with ``resp.status`` set to
  503 and ``resp.reason`` set to ``"VCL failed"``.

* If the failure occurs in ``vcl_synth``, then ``vcl_synth`` is
  aborted, and the response line "503 VCL failed" is sent.
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REQUIREMENTS
============

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The VMOD requires Varnish 6.2.
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INSTALLATION
============

See `INSTALL.rst <INSTALL.rst>`_ in the source repository.

LIMITATIONS
===========

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The VMOD uses workspace for two purposes:

* Saving task-scoped data about a match with ``.match()`` and
  ``.hasprefix()``, for use by the methods that retrieve information
  about the prior match.  This data is stored separately for each
  object for which a match is executed.

* A copy of the string to be matched for case insensitive matches (the
  copy is set to all one case).

If you find that methods are failing with ``VCL_Error`` messages
indicating "out of space", consider increasing the varnishd parameters
``workspace_client`` and/or ``workspace_backend`` (see varnishd(1)).

Set objects and their internal structures are allocated from the heap,
and hence are only limited by available RAM.

The regex methods ``.re_match()`` and ``.sub()`` use the same internal
mechanisms as native VCL's ``~`` operator and the ``regsub/all()``
functions, and are subject to the same limitations. In particular,
they may be limited by the varnishd parameters ``pcre_match_limit``
and ``pcre_match_limit_recursion``, in which case they emit the same
``VCL_Error`` messages as the native operations. If necessary, adjust
these parameters as advised in varnishd(1).
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AUTHOR
======

* Geoffrey Simmons <geoff@uplex.de>

UPLEX Nils Goroll Systemoptimierung

SEE ALSO
========

* varnishd(1)
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* vcl(7)
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* varnishstat(1)
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* varnish-cli(7)
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* VMOD source repository: https://code.uplex.de/uplex-varnish/libvmod-selector
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  * Gitlab mirror: https://gitlab.com/uplex/varnish/libvmod-selector

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* `VMOD re2`_: https://code.uplex.de/uplex-varnish/libvmod-re2
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COPYRIGHT
=========

::

  Copyright (c) 2018 UPLEX Nils Goroll Systemoptimierung
  All rights reserved
 
  Author: Geoffrey Simmons <geoffrey.simmons@uplex.de>
 
  See LICENSE