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<div id="page-content"><div id="preamble"><h1>Apache模块 mod_filter</h1>
<table border="1" cellpadding="0" cellspacing="0" bordercolor="#AAAAAA" class="module">
<tr><th><a href="module-dict.html#Description">说明</a></th><td>根据上下文实际情况对输出过滤器进行动态配置</td></tr>
<tr><th><a href="module-dict.html#Status">状态</a></th><td>基本(B)</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">模块名</a></th><td>filter_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">源文件</a></th><td>mod_filter.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">兼容性</a></th><td>Version 2.1 及以后的版本中可用</td></tr>
</table>
<h3>概述</h3>
<p>This module enables smart, context-sensitive configuration of
output content filters. For example, apache can be configured to
process different content-types through different filters, even
when the content-type is not known in advance (e.g. in a proxy).</p>
<p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> works by introducing indirection into
the filter chain. Instead of inserting filters in the chain, we insert
a filter harness which in turn dispatches conditionally
to a filter provider. Any content filter may be used as a provider
to <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>; no change to existing filter modules is
required (although it may be possible to simplify them).</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="smart" id="smart">Smart Filtering</a></h2>
<p>In the traditional filtering model, filters are inserted unconditionally
using <code class="directive"><a href="../mod/mod_mime.html#addoutputfilter">AddOutputFilter</a></code> and family.
Each filter then needs to determine whether to run, and there is little
flexibility available for server admins to allow the chain to be
configured dynamically.</p>
<p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> by contrast gives server administrators a
great deal of flexibility in configuring the filter chain. In fact,
filters can be inserted based on any Request Header, Response Header
or Environment Variable. This generalises the limited flexibility offered
by <code class="directive"><a href="../mod/core.html#addoutputfilterbytype">AddOutputFilterByType</a></code>, and fixes
it to work correctly with dynamic content, regardless of the
content generator. The ability to dispatch based on Environment
Variables offers the full flexibility of configuration with
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> to anyone who needs it.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="terms" id="terms">Filter Declarations, Providers and Chains</a></h2>
<p class="figure">
<img src="../images/mod_filter_old.gif" width="160" height="310" alt="[This image displays the traditional filter model]" /><br />
<dfn>Figure 1:</dfn> The traditional filter model</p>
<p>In the traditional model, output filters are a simple chain
from the content generator (handler) to the client. This works well
provided the filter chain can be correctly configured, but presents
problems when the filters need to be configured dynamically based on
the outcome of the handler.</p>
<p class="figure">
<img src="../images/mod_filter_new.gif" width="423" height="331" alt="[This image shows the mod_filter model]" /><br />
<dfn>Figure 2:</dfn> The <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> model</p>
<p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> works by introducing indirection into
the filter chain. Instead of inserting filters in the chain, we insert
a filter harness which in turn dispatches conditionally
to a filter provider. Any content filter may be used as a provider
to <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>; no change to existing filter modules
is required (although it may be possible to simplify them). There can be
multiple providers for one filter, but no more than one provider will
run for any single request.</p>
<p>A filter chain comprises any number of instances of the filter
harness, each of which may have any number of providers. A special
case is that of a single provider with unconditional dispatch: this
is equivalent to inserting the provider filter directly into the chain.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="config" id="config">Configuring the Chain</a></h2>
<p>There are three stages to configuring a filter chain with
<code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>. For details of the directives, see below.</p>
<dl>
<dt>Declare Filters</dt>
<dd>The <code class="directive"><a href="#filterdeclare">FilterDeclare</a></code> directive
declares a filter, assigning it a name and filter type. Required
only if the filter is not the default type AP_FTYPE_RESOURCE.</dd>
<dt>Register Providers</dt>
<dd>The <code class="directive"><a href="#filterprovider">FilterProvider</a></code>
directive registers a provider with a filter. The filter may have
been declared with <code class="directive"><a href="#filterdeclare">FilterDeclare</a></code>; if not, FilterProvider will implicitly
declare it with the default type AP_FTYPE_RESOURCE. The provider
must have been
registered with <code>ap_register_output_filter</code> by some module.
The remaining arguments to <code class="directive"><a href="#filterprovider">FilterProvider</a></code> are a dispatch criterion and a match string.
The former may be an HTTP request or response header, an environment
variable, or the Handler used by this request. The latter is matched
to it for each request, to determine whether this provider will be
used to implement the filter for this request.</dd>
<dt>Configure the Chain</dt>
<dd>The above directives build components of a smart filter chain,
but do not configure it to run. The <code class="directive"><a href="#filterchain">FilterChain</a></code> directive builds a filter chain from smart
filters declared, offering the flexibility to insert filters at the
beginning or end of the chain, remove a filter, or clear the chain.</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="examples" id="examples">Examples</a></h2>
<dl>
<dt>Server side Includes (SSI)</dt>
<dd>A simple case of using <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> in place of
<code class="directive"><a href="../mod/core.html#addoutputfilterbytype">AddOutputFilterByType</a></code>
<div class="example"><p><code>
FilterDeclare SSI<br />
FilterProvider SSI INCLUDES resp=Content-Type $text/html<br />
FilterChain SSI
</code></p></div>
</dd>
<dt>Server side Includes (SSI)</dt>
<dd>The same as the above but dispatching on handler (classic
SSI behaviour; .shtml files get processed).
<div class="example"><p><code>
FilterProvider SSI INCLUDES Handler server-parsed<br />
FilterChain SSI
</code></p></div>
</dd>
<dt>Emulating mod_gzip with mod_deflate</dt>
<dd>Insert INFLATE filter only if "gzip" is NOT in the
Accept-Encoding header. This filter runs with ftype CONTENT_SET.
<div class="example"><p><code>
FilterDeclare gzip CONTENT_SET<br />
FilterProvider gzip inflate req=Accept-Encoding !$gzip<br />
FilterChain gzip
</code></p></div>
</dd>
<dt>Image Downsampling</dt>
<dd>Suppose we want to downsample all web images, and have filters
for GIF, JPEG and PNG.
<div class="example"><p><code>
FilterProvider unpack jpeg_unpack Content-Type $image/jpeg<br />
FilterProvider unpack gif_unpack Content-Type $image/gif<br />
FilterProvider unpack png_unpack Content-Type $image/png<br />
<br />
FilterProvider downsample downsample_filter Content-Type $image<br />
FilterProtocol downsample "change=yes"<br />
<br />
FilterProvider repack jpeg_pack Content-Type $image/jpeg<br />
FilterProvider repack gif_pack Content-Type $image/gif<br />
FilterProvider repack png_pack Content-Type $image/png<br />
<Location /image-filter><br />
<span class="indent">
FilterChain unpack downsample repack<br />
</span>
</Location>
</code></p></div>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="protocol" id="protocol">Protocol Handling</a></h2>
<p>Historically, each filter is responsible for ensuring that whatever
changes it makes are correctly represented in the HTTP response headers,
and that it does not run when it would make an illegal change. This
imposes a burden on filter authors to re-implement some common
functionality in every filter:</p>
<ul>
<li>Many filters will change the content, invalidating existing content
tags, checksums, hashes, and lengths.</li>
<li>Filters that require an entire, unbroken response in input need to
ensure they don't get byteranges from a backend.</li>
<li>Filters that transform output in a filter need to ensure they don't
violate a <code>Cache-Control: no-transform</code> header from the
backend.</li>
<li>Filters may make responses uncacheable.</li>
</ul>
<p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> aims to offer generic handling of these
details of filter implementation, reducing the complexity required of
content filter modules. This is work-in-progress; the
<code class="directive"><a href="#filterprotocol">FilterProtocol</a></code> implements
some of this functionality for back-compatibility with Apache 2.0
modules. For httpd 2.1 and later, the
<code>ap_register_output_filter_protocol</code>和<code>ap_filter_protocol</code> API enables filter modules to
declare their own behaviour.</p>
<p>At the same time, <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> should not interfere
with a filter that wants to handle all aspects of the protocol. By
default (i.e. in the absence of any <code class="directive"><a href="#filterprotocol">FilterProtocol</a></code> directives), <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>
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