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| 说明 | 根据上下文实际情况对输出过滤器进行动态配置 |
|---|---|
| 状态 | 基本(B) |
| 模块名 | filter_module |
| 源文件 | mod_filter.c |
| 兼容性 | Version 2.1 及以后的版本中可用 |
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).
mod_filter 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 mod_filter; no change to existing filter modules is
required (although it may be possible to simplify them).
In the traditional filtering model, filters are inserted unconditionally
using AddOutputFilter 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.
mod_filter 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 AddOutputFilterByType, 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
mod_rewrite to anyone who needs it.
![[This image displays the traditional filter model]](../images/mod_filter_old.gif)
Figure 1: The traditional filter model
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.
![[This image shows the mod_filter model]](../images/mod_filter_new.gif)
Figure 2: The mod_filter model
mod_filter 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 mod_filter; 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.
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.
There are three stages to configuring a filter chain with
mod_filter. For details of the directives, see below.
FilterDeclare directive
declares a filter, assigning it a name and filter type. Required
only if the filter is not the default type AP_FTYPE_RESOURCE.FilterProvider
directive registers a provider with a filter. The filter may have
been declared with FilterDeclare; if not, FilterProvider will implicitly
declare it with the default type AP_FTYPE_RESOURCE. The provider
must have been
registered with ap_register_output_filter by some module.
The remaining arguments to FilterProvider 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.FilterChain 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.mod_filter in place of
AddOutputFilterByType
FilterDeclare SSI
FilterProvider SSI INCLUDES resp=Content-Type $text/html
FilterChain SSI
FilterProvider SSI INCLUDES Handler server-parsed
FilterChain SSI
FilterDeclare gzip CONTENT_SET
FilterProvider gzip inflate req=Accept-Encoding !$gzip
FilterChain gzip
FilterProvider unpack jpeg_unpack Content-Type $image/jpeg
FilterProvider unpack gif_unpack Content-Type $image/gif
FilterProvider unpack png_unpack Content-Type $image/png
FilterProvider downsample downsample_filter Content-Type $image
FilterProtocol downsample "change=yes"
FilterProvider repack jpeg_pack Content-Type $image/jpeg
FilterProvider repack gif_pack Content-Type $image/gif
FilterProvider repack png_pack Content-Type $image/png
<Location /image-filter>
FilterChain unpack downsample repack
</Location>
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:
Cache-Control: no-transform header from the
backend.mod_filter 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
FilterProtocol implements
some of this functionality for back-compatibility with Apache 2.0
modules. For httpd 2.1 and later, the
ap_register_output_filter_protocol和ap_filter_protocol API enables filter modules to
declare their own behaviour.
At the same time, mod_filter should not interfere
with a filter that wants to handle all aspects of the protocol. By
default (i.e. in the absence of any FilterProtocol directives), mod_filter
will leave the headers untouched.
At the time of writing, this feature is largely untested, as modules in common use are designed to work with 2.0. Modules using it should test it carefully.
| 说明 | Configure the filter chain |
|---|---|
| 语法 | FilterChain [+=-@!]filter-name ... |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | Options |
| 状态 | 基本(B) |
| 模块 | mod_filter |
This configures an actual filter chain, from declared filters.
FilterChain takes any number of arguments,
each optionally preceded with a single-character control that
determines what to do:
+filter-name@filter-name-filter-name=filter-name!filter-name+filter-name
| 说明 | Declare a smart filter |
|---|---|
| 语法 | FilterDeclare filter-name [type] |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | Options |
| 状态 | 基本(B) |
| 模块 | mod_filter |
This directive declares an output filter together with a
header or environment variable that will determine runtime
configuration. The first argument is a filter-name
for use in FilterProvider, FilterChain和FilterProtocol directives.
The final (optional) argument
is the type of filter, and takes values of ap_filter_type
- namely RESOURCE (the default), CONTENT_SET, PROTOCOL, TRANSCODE, CONNECTION或NETWORK.
| 说明 | Deal with correct HTTP protocol handling |
|---|---|
| 语法 | FilterProtocol filter-name [provider-name]
proto-flags |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | Options |
| 状态 | 基本(B) |
| 模块 | mod_filter |
This directs mod_filter to deal with ensuring the
filter doesn't run when it shouldn't, and that the HTTP response
headers are correctly set taking into account the effects of the
filter.
There are two forms of this directive. With three arguments, it applies specifically to a filter-name and a provider-name for that filter. With two arguments it applies to a filter-name whenever the filter runs any provider.
proto-flags is one or more of
change=yeschange=1:1byteranges=noproxy=noproxy=transformCache-Control: no-transform header.cache=no| 说明 | Register a content filter |
|---|---|
| 语法 | FilterProvider filter-name provider-name
[req|resp|env]=dispatch match |
| 作用域 | server config, virtual host, directory, .htaccess |
| 覆盖项 | Options |
| 状态 | 基本(B) |
| 模块 | mod_filter |
This directive registers a provider for the smart filter. The provider will be called if and only if the match declared here matches the value of the header or environment variable declared as dispatch.
provider-name must have been registered by loading
a module that registers the name with
ap_register_output_filter.
dispatch argument is a string with optional
req=, resp=或env= prefix
causing it to dispatch on (respectively) the request header, response
header, or environment variable named. In the absence of a
prefix, it defaults to a response header. A special case is the
word handler, which causes mod_filter
to dispatch on the content handler.
match argument specifies a match that will be applied to the filter's dispatch criterion. The match may be a string match (exact match or substring), a regex, an integer (greater, lessthan or equals), or unconditional. The first characters of the match argument determines this:
First, if the first character is an exclamation mark
(!), this reverses the rule, so the provider will be used
if and only if the match fails.
Second, it interprets the first character excluding
any leading ! as follows:
| Character | Description |
|---|---|
| (none) | exact match |
$ |
substring match |
/ |
regex match (delimited by a second /) |
= |
integer equality |
< |
integer less-than |
<= |
integer less-than or equal |
> |
integer greater-than |
>= |
integer greater-than or equal |
* |
Unconditional match |
| 说明 | Get debug/diagnostic information from
mod_filter
|
|---|---|
| 语法 | FilterTrace filter-name level |
| 作用域 | server config, virtual host, directory |
| 状态 | 基本(B) |
| 模块 | mod_filter |
This directive generates debug information from
mod_filter.
It is designed to help test and debug providers (filter modules), although
it may also help with mod_filter itself.
The debug output depends on the level set:
0 (default)1mod_filter will record buckets and brigades
passing through the filter to the error log, before the provider has
processed them. This is similar to the information generated by
mod_diagnostics.
2 (not yet implemented)