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NAME ^

Apache2::Translation - Configuring Apache dynamically

SYNOPSIS ^

  LoadModule perl_module /path/to/mod_perl.so
  PerlLoadModule Apache2::Translation
  PerlTransHandler Apache2::Translation
  PerlMapToStorageHandler Apache2::Translation
  TranslationEvalCache 1000
  TranslationKey MyKey
  <TranslationProvider DB>
      Database  dbi:mysql:dbname:host
      User      username
      Password  password
      Singleton 1
      Table     tablename
      Key       keycolumn
      Uri       uricolumn
      Block     blockcolumn
      Order     ordercolumn
      Action    actioncolumn
      Cachetbl  cachetablename
      Cachecol  cachecolumn
      Cachesize 1000
  </TranslationProvider>

  # another provider
  <TranslationProvider File>
      Configfile /path/to/config
  </TranslationProvider>

  # export our provider parameters
  <Location /config>
    SetHandler modperl
    PerlResponseHandler Apache2::Translation::Config
  </Location>

  # configuring the WEB interface
  PerlModule Apache2::Translation::Admin
  <Location /-/transadm/>
    SetHandler modperl
    PerlResponseHandler Apache2::Translation::Admin
  </Location>

DESCRIPTION ^

As the name implies Apache2::Translation lives mostly in the URI Translation Phase. It is somehow similar to mod_rewrite but configuration statements are read at runtime, thus, allowing to reconfigure a server without restarting it.

The actual configuration statements are read by means of a Translation Provider, a Perl class offering a particular interface, see below. Currently there are 3 providers implemented, Apache2::Translation::DB, Apache2::Translation::BDB, and Apache2::Translation::File.

There is also a WEB interface (Apache2::Translation::Admin).

An Example

Let's begin with an example. Given some database table:

 id  key    uri      blk ord action
  1  front  :PRE:    0   0   Cond: $HOSTNAME !~ /^(?:www\.)xyz\.(?:com|de)$/
  2  front  :PRE:    0   1   Redirect: 'http://xyz.com'.$URI, 301
  3  front  :PRE:    1   0   Do: $CTX{lang}='en'
  4  front  :PRE:    1   1   Cond: $HOSTNAME =~ /de$/
  5  front  :PRE:    1   2   Do: $CTX{lang}='de'
  6  front  /static  0   0   File: $DOCROOT.'/'.$CTX{lang}.$MATCHED_PATH_INFO
  7  front  /appl1   0   0   Proxy: 'http://backend/'.$CTX{lang}.$URI
  8  front  /appl2   0   0   Proxy: 'http://backend/'.$URI.'?l='.$CTX{lang}
  9  front  /        0   0   Config: ['AuthName "secret"'], ['AuthType Basic']
 10  back   :PRE:    0   0   Cond: $r->connection->remote_ip ne '127.0.0.1'
 11  back   :PRE:    0   1   Error: 403, 'Forbidden by Apache2::Translation(11)'
 12  back   /appl1   0   0   PerlHandler: 'My::Application1'
 13  back   /appl2   0   0   PerlHandler: 'My::Application2'

The id column in this table is not really necessary for Apache2::Translation. But if you want to deploy Apache2::Translation::Admin you need it.

Well, here we have a frontend/backend configuration. The frontend records are labeled with the key front, the backend records with back.

When a request comes in first the records with :PRE: in the uri-field are examined. Suppose, a request for http://abc.com/static/img.png comes in. Record 1 (id=1) checks the Host header. The expression after Cond: is evaluated as Perl code. It obviously returns true. Cond stands for condition. But how does it affect the further workflow? Here blk and ord come in. All records with the same key, uri and blk form a block. ord gives an order within this block. Within a block all actions are executed up to the first condition that is false.

Now, because our condition in record 1 is true the action in record 2 (within the same block) is executed. It redirects the browser with a HTTP code of 301 (MOVED PERMANENTLY) to http://xyz.com/static/img.png.

When the redirected request comes back the condition in record 1 is false. Hence, the next block (key=front, uri=:PRE:, blk=1) is evaluated. First a lang member of a context hash is set to en. A Do action is similar to a condition, only its value is ignored. Record 4 then checks if the Host header matches /de$/. If so, then record 5 sets the language to de.

Now, the records labeled with :PRE: are finished. The handler starts looking for blocks labeled with the request uri. That is, it looks for a block with key=front, uri=/static/img.png. None is found.

Then it cuts off the last part of the uri (/img.png), repeats the lookup and finds record 6. The File action sets $r->filename to $DOCROOT/en/img.png. Apache2::Translation provides some convenience variables. They are tied to members of the request record. $MATCHED_PATH_INFO contains the uri part cut off (/img.png). More on them below.

Now another round is started and the next uri part is cut off. Record 9 matches. We see a Config action that sets AuthName and AuthType.

At the end the translation handler checks if $r->filename was set and returns Apache2::Const::OK or Apache2::Const::DECLINED respectively.

I think that example gives a general idea, what Apache2::Translation does.

Processing States

Internally Apache2::Translation is implemented as a state machine. It starts in the START state, where some variables are initialized. From there it shifts immediately to the PREPOC state. Here all :PRE: rules are evaluated. From PREPROC it shifts to PROC. Now the rules with real uris are examined. When the DONE state is reached processing is finished.

There is a special state named LOOKUPFILE. It is only used for subrequests that don't have an URI. For such requests the URI translation phase of the request cycle is skipped. Hence a PerlTransHandler would never be called. Such requests are results of calling $r->lookup_file for example.

To catch also such requests install Apache2::Translation both as PerlTransHandler as well as PerlMapToStorageHandler. Then if such a subrequest occures the handler enters the LOOKLUPFILE state instead of PREPROC. From LOOKLUPFILE it normally shifts to PROC unless it executes a Restart action. In that case it shifts to PREPROC.

You have to set $MATCHED_URI to some initial value if you want to hop through the PROC phase. A still empty $MATCHED_URI shifts from PROC immediately to DONE.

Note: The LOOKUPFILE stuff is still somewhat experimental.

You can control the current state by means of the State, Done and Restart actions.

Blocks and Lists of Blocks

Above, we have defined a block as all records with the same key, uri and block. The actions within a block are ordered by the order field.

A list of blocks is then an ordered list of all blocks with the same key and uri. The order is given by the block number.

Actions

An action starts with a key word optionally followed by a colon and some arguments. The key words are case insensitive.

Apache2::Translation provides some environment for code snippets in actions. They are compiled into perl functions. The compiled result is cached. 2 variables, $r and %CTX, are provided plus a few convenience variables. $r is the current Apache2::RequestRec. %CTX is a hash that can be used to store arbitrary data. This hash is not used by Apache2::Translation itself. It can be used to pass data between actions. But note, the hash is localized to the translation handler. So, it cannot be used to pass data between different phases of the apache request cycle. Use $r->notes or $r->pnotes for that.

Do: perl_code
Fixup: perl_code

Do is the simplest action. The Perl code is evaluated in scalar context. The return value is ignored. Fixup is just the same save it is run in the Fixup phase

Cond: perl_code

This is almost the same as Do. The return value is taken as boolean. If it is false, the current block is finished. Processing continues with the next block.

Done

Done finishes the current block list and transfers control to the next state. That means if encountered in PREPROC state it switches to PROC. If the current state is PROC then the translation handler ends here.

This action is a combination of State: next_state and Last, see below.

Don't try to use Done to return from a subroutine. Use Last instead.

File: string

This action sets $r->filename to string. It is equivalent to

 Do: $FILENAME=do{ string }
Doc: ?content_type?, string

string is evaluated as well as content_type if given. Then a special moperl handler is installed that simply sets the given content type and prints out the string to the client. content_type is text/plain if omitted.

Proxy: ?url?

This tells Apache to forward the request to url as a proxy. url is optional. If ommitted $r->unparsed_uri is used. That means Apache must be used as a proxy by the browser.

CgiScript: ?string?

is equivalent to

 Do: $r->handler( 'cgi-script' );
 FixupConfig: ['Options ExecCGI']

If string is given it is evaluated and the result is assigned to $r->filename.

PerlScript: ?string?

is equivalent to

 Do: $r->handler( 'perl-script' );
 FixupConfig: ['Options ExecCGI'], ['PerlOptions +ParseHeaders']

If string is given it is evaluated and the result is assigned to $r->filename.

PerlHandler: string

In short this action tries to figure out what string means and calls it as modperl handler.

In detail it installs a Apache2::Translation::response as PerlResponseHandler. When called the handler evaluates string which results either in a subroutine name, a package name, a subroutine reference or an object or class that implements the handler method. If a package name is given it must implement a handler subroutine.

If the given package is not yet loaded it is requireed.

Then the resulting subroutine or method is called and $r is passed.

Further, a PerlMapToStorageHandler is installed that skips the handling of Directory containers and .htaccess files. If not set, this handler also sets path_info. Assumed,

 #uri        blk ord action
 /some/path  0   0   PerlHandler: ...

and a request comes in for /some/path/foo/bar. Then path_info is set to /foo/bar.

Config: list_of_strings_or_arrays
FixupConfig: list_of_strings_or_arrays

Surprisingly, these are the most complex actions of all.

Config adds Apache configuration directives to the request in the Map To Storage phase before the default MapToStorage handler. Think of it as a kind of .htaccess. FixupConfig does the same in the Fixup phase. While Config is used quite often FixupConfig is seldom required. It is used mainly to mend configurations that are spoiled by the default MapToStorage handler.

Arguments to both actions are strings or arrays of one or two elements:

 Config: 'AuthName "secret"',
         ['AuthType Basic'],
         ['ProxyPassReverse http://...', '/path']

To understand the different meaning, you have to know about how Apache applies its configuration to a request. Hence, let's digress a little.

Each Apache directive is used in certain contexts. Some for example can occur only in server config context, that means outside any Directory, Location or even VirtualHost container. Listen or PidFile are examples. Other directives insist on being placed in a container.

Also, the point in time when a directive takes effect differs for different directives. PidFile is clearly applied during server startup before any request is processed. Hence, our Config action cannot apply PidFile. It's simply too late. AllowOverride is applied to single requests. But since it affects the processing of .htaccess files it must be applied before that processing takes place. To make things even more confusing some directives take effect at several points in time. Consider

 Options FollowSymLinks ExecCGI

FollowSymLinks is applied when Apache looks up a file in the file system, while ExecCGI influences the way the response is generated ages later.

Apache solves this complexity by computing a configuration for each single request. As a starting point it uses the server default configuration. That is the configuration outside any Location or Directory for a virtual host. This basic configuration is assigned to the request just between the Uri Translation Phase and Map to Storage. At the very end of Map to Storage Apache's core Map to Storage handler incorporates matching Directory containers and .htaccess files into the request's current configuration. Location containers are merged after Map to Storage is finished.

Our Config action is applied early in Map to Storage. That means it affects the way Apache maps the request file name computed to the file system, because that comes later. But it also means, your static configuration (config file based) overrides our Config actions. This limitation can be partly overcome using FixupConfig instead of Config.

Now, what does the various syntaxes mean? The simplest one:

 #uri      blk ord action
 /uri      0   0   Config: 'ProxyPassReverse http://my.backend.org'

is very close to

 <Location /uri>
   ProxyPassReverse http://my.backend.org
 </Location>

Only, it is applied before any Directory container takes effect. Note, the uri-argument to the Location container is the value of $MATCHED_URI, see below. This is also valid if the Config action is used from a Called block.

The location uri is sometimes important. ProxyPassReverse, for example, uses the path given to the location container for its own purpose.

All other forms of Config are not influenced by $MATCHED_URI.

These two:

 Config: ['ProxyPassReverse http://my.backend.org']
 Config: ['ProxyPassReverse /path http://my.backend.org', '']

are equivalent to

 <Location />
   ProxyPassReverse http://my.backend.org
 </Location>

Note, the location container uri differs.

The first one of them is also the only form of Config available with mod_perl before 2.0.3.

The next one:

 Config: ['ProxyPassReverse http://my.backend.org', '/path']

is equivalent to

 <Location /path>
   ProxyPassReverse http://my.backend.org
 </Location>

I have chosen ProxyPassReverse for this example because the Location container uri matters for this directive, see httpd docs. The following form of applying ProxyPassReverse outside of any container is not possible with Apache2::Translation:

 ProxyPassReverse /path http://my.backend.org

Now let's look at another example to see how Directory containers and .htaccess files are applied. AllowOverride controls which directives are allowed in .htaccess files. As said before Apache applies Directory containers and .htaccess files after our Config directives. Unfortunately, they are both applied in the same step. That means we can say:

 Config: 'AllowOverride Options'

But if at least one Directory container from our httpd.conf is applied that says for example AllowOverride AuthConfig it will override our Config statement. So, if you want to control which directives are allowed in .htaccess files with Apache2::Translation then avoid AllowOverride in your httpd.conf, especially the often seen:

 <Directory />
   AllowOverride None
 </Directory>

Put it instead in a PREPROC rule:

 #uri     blk ord action
 :PRE:    0   0   Config: 'AllowOverride None'

So subsequent rules can override it.

A similar problem exists with Options FollowSymlinks. This option affects directly the phase when Directory containers are applied. Hence, any such option from the httpd.conf cannot be overridden by a Config rule.

In Apache 2.2 at least up to 2.2.4 there is a bug that prevents Config: AllowOverride Options from working properly. The reason is an uninitialized variable that is by cause 0, see http://www.gossamer-threads.com/lists/apache/dev/327770#327770

Call: string, ?@params?

Well, the name suggests it is calling a subroutine. Assume you have several WEB applications running on the same server, say one application for each department. Each department needs of course some kind of authorization:

 #uri      blk ord action
 AUTH      0   0   Config: "AuthName \"$ARGV[0]\""
 AUTH      0   1   Config: 'AuthType Basic'
 AUTH      0   2   Config: 'AuthUserFile /etc/htaccess/user/'.$ARGV[1]
 /dep1     0   0   Call: qw/AUTH Department_1 dep1/
 /dep2     0   0   Call: qw/AUTH Department_2 dep2/

The AUTH in the Call actions refer to the AUTH block list in the uri column. An optional parameter list is passed via @ARGV.

Call fetches the block list for a given uri and processes it. If a Last action is executed the processing of that block list is finished.

Redirect: url, ?http_code?

The Redirect action sends a HTTP redirect response to the client and abort the current request. The optional http_code specifies the HTTP response code. Default is 302 (MOVED TEMPORARILY).

Redirect tries to make the outgoing Location header RFC2616 conform. That means if the schema part is ommitted it figures out if it has to be http or https. If a relative url is given an appropriate url is computed based on the current value of $URI.

If the current request is the result of an internal redirect the redirecting request's status is changed to http_code. Thus, Redirect works also for ErrorDocuments.

Error: ?http_code?, ?message?

Error aborts the entire request. A HTTP response is sent to the client. The optional http_code specifies the HTTP response code. The optional message is logged as reason to the error_log.

http_code defaults to 500 (INTERNAL SERVER ERROR), message to unspecified error.

Uri: string

This action sets $r->uri to string. It is equivalent to

 Do: $URI=do{ string }
Key: string

string is evaluated in scalar context. The result is assigned to the current key. The new key takes effect if the list of blocks matching the current uri is finished.

For example:

 id  key    uri      blk ord action
  1  dflt   :PRE:    0   0   Cond: $CLIENTIP eq '192.168.0.1'
  2  dflt   :PRE:    0   1   Key: 'spec'
  3  dflt   :PRE:    0   2   Do: $DEBUG=3
  4  dflt   :PRE:    1   0   Config: 'Options None'
  5  dflt   /        0   0   File: $DOCROOT.$URI
  6  spec   /        0   0   File: '/very/special'.$URI

Here an entirely different directory tree is shown to a client with the IP address 192.168.0.1. In record 2 the current key is set to spec if the condition in record 1 matches. Also, $DEBUG is set in this case (record 3).

The next block in record 4 is executed for all clients, because the key change is not in effect, yet.

Records 5 and 6 are new lists of blocks. Hence, record 6 is executed only for 192.168.0.1 and record 5 for the rest.

The action Key: 'string' is equivalent to Do: $KEY='string'.

Restart: ?newuri?, ?newkey?, ?newpathinfo?

Restart restarts the processing from the PREPROC phase. The optional arguments ar evaluated and assumed to result in strings. newuri is then assigned to $r->uri and $MATCHED_URI. newkey is assigned to $KEY and newpathinfo to $MATCHED_PATH_INFO.

State: string

If you look for a premature exit from the current block list take the Done action.

This action affects the current state directly. Thus, you can loop back to the PREPROC state from PROC. It is mostly used the prematurely finish the translation handler from the PREPROC state. As the Key action it takes effect, when the current list of blocks is finished.

string is evaluated as perl code. It is expected to result in one of the following strings. If not, a warning is printed in the error_log. State names are case insensitive:

    start
    preproc
    proc
    done

The State action is similar to setting the convenience variable $STATE. Only in the latter case you must use the state constants, e.g. $STATE=DONE.

Last

If you look for a premature exit from the current block list take the Done action.

This action finishes the current list of blocks (just like a false condition finishes the current block). It is used together with State to finish the translation handler from a conditional block in the PREPROC state:

 :PRE:  0 0 Cond: $finish
 :PRE:  0 1 State: 'done'
 :PRE:  0 2 Last

Another application of Last is to return from a Call action.

Convenience Variables and Data Structures

$URI

tied to $r->uri

$REAL_URI

tied to $r->unparsed_uri

$METHOD

tied to $r->method

$QUERY_STRING

tied to $r->args

$FILENAME

tied to $r->filename

$DOCROOT

tied to $r->document_root

$HOSTNAME

tied to $r->hostname

$PATH_INFO

tied to $r->path_info

$REQUEST

tied to $r->the_request

$HEADERS

tied to $r->headers_in

$C

tied to $r->connection

$CLIENTIP

tied to $r->connection->remote_ip

$KEEPALIVE

tied to $r->connection->keepalive

For more information see Apache2::RequestRec.

$MATCHED_URI

tied to $r->notes('Apache2::Translation::n::uri')

$MATCHED_PATH_INFO

tied to $r->notes('Apache2::Translation::n::pathinfo')

While in PROC state the incoming uri is split in 2 parts. The first part is matching the uri field of a database record. The second part is the rest. They can be accessed as $MATCHED_URI and $MATCHED_PATH_INFO.

$KEY

the current key.

Tied to $r->notes('Apache2::Translation::n::key')

$STATE

the current processing state.

$RC

Normally, Apache2::Translation checks at the end if $r->filename is set. If so, it returns Apache2::Const::OK to its caller. If not, Apache2::Const::DECLINED is returned. The first alternative signals that the Uri Translation Phase is done and no further handlers are to be called in this phase. The second alternative says that subsequent handlers are to be called. Thus, mod_alias or the core translation handler see the request.

Setting $RC your action decide what is returned.

$RC is also set by the PerlHandler action. Modperl generated responses are normally not associated with a single file on disk.

$DEBUG

tied to $r->notes('Apache2::Translation::n::debug')

If set to 1 or 2 debugging output is sent to the error_log.

%CTX

a hash to store arbitrary data. It can be used to pass data between action blocks. But note, it is localized to the translation handler. So, it cannot be used to pass data between different phases of the apache request cycle. Use $r->notes or $r->pnotes for that.

APACHE CONFIGURATION DIRECTIVES ^

After installed and loaded by

  PerlLoadModule Apache2::Translation

in your httpd.conf Apache2::Translation is configured with the following directives:

<TranslationProvider class> ... </TranslationProvider>

Currently there are 3 provider classes implemented, Apache2::Translation::DB, Apache2::Translation::File and Apache2::Translation::BDB.

The ellipsis represents configuration lines formatted as

 NAME   VALUE

These lines are passed as parameters to the provider. NAME is case insensitive and is converted to lowercase before passed to the provider object. Spaces round VALUE are stripped off. If VALUE begins and ends with the same quotation character (double quote or single quote) they are also stripped off.

If VALUE is not quoted or is quoted with double quote characters then it is subject to environment variable expansion. All substrings that match ${VAR} are replaced by the environment variable VAR.

The provider object is then created by:

 $Apache2::Translation::class->new( NAME1=>VALUE1, NAME2=>VALUE2, ... );

where class is exchanged by the actual provider name.

TranslationProvider class param1 param2 ...

This is an alternative way to specify translation provider parameters.

Each parameter is expected to be a string formatted as

 NAME=VALUE

There must be no spaces around the equal sign. The list is passed to the constructor of the provider class as named parameters:

 $Apache2::Translation::class->new( NAME1=>VALUE1, NAME2=>VALUE2, ... );

If class is literally inherit the provider of the base server is used. This is obviously valid only in a VHost configuration.

TranslationKey initial-key

This sets the initial value for the key. Default is the string default.

TranslationEvalCache number

Apache2::Translation compiles all code snippets into functions and caches these functions. Normally, an ordinary hash is used for this. Strictly speaking this is a memory hole if your translation table changes. I think that can be ignored, if the number of requests per worker is limited, see MaxRequestsPerChild. If you think this is too lax, put a number here.

If set the cache is tied to Tie::Cache::LRU. The number of cached code snippets will then be limited by number.

VHost merging

If in a VHost configuration any of the above directives is ommitted it is inherited from the base server.

WHICH PROVIDER TO CHOOSE ^

Unless you want to implement your own provider you can choose from these 3:

EXPORTING OUR PROVIDER PARAMETERS ^

A WEB server can export its provider parameters by means of the Apache2::Translation::Config module. That can then be used by the admin interface to connect to that provider.

THE WEB ADMINISTRATION INTERFACE ^

The simplest way to configure the WEB interface is this:

  PerlModule Apache2::Translation::Admin
  <Location /-/transadm/>
    SetHandler modperl
    PerlResponseHandler Apache2::Translation::Admin
  </Location>

Note, here an extra PerlModule statement is necessary. If nothing else specified the provider that has handled the current request is used.

Note, there is a slash at the end of the location statement. It is necessary to be specified. Also, the URL given to the browser to reach the WEB interface must end with a slash or with /index.html.

Another provider is given by creating an Apache2::Translation::Admin object:

  <Perl>
    $My::Transadmin=Apache2::Translation::Admin->new
         (provider_spec=>[File,
                          ConfigFile=>'/path/to/config']);
  </Perl>

  <Location /-/transadm/>
    SetHandler modperl
    PerlResponseHandler $My::Transadmin->handler
  </Location>

Here the provider is specified in a way similar to the TranslationProvider statement above.

Also, an URL can be given that links to an exported parameter set:

  <Perl>
    $My::Transadmin=Apache2::Translation::Admin->new
         (provider_url=>'http://host/config');
  </Perl>

In this case LWP::UserAgent is used to fetch the parameters.

Or you can create the provider object by yourself and pass it:

  <Perl>
    use Apache2::Translation::File;
    $My::Transadmin=Apache2::Translation::Admin->new
        (provider=>Apache2::Translation::File->new
                      (configfile=>'/path/to/config'));
  </Perl>

IMPLEMENTING A NEW PROVIDER ^

A provider implements a certain interface that is documented in Apache2::Translation::_base.

SEE ALSO ^

Apache2::Translation::DB
Apache2::Translation::BDB
Apache2::Translation::File
Apache2::Translation::Admin
Apache2::Translation::_base
Apache2::Translation::Config
mod_perl: http://perl.apache.org

TODO / WHISHLIST ^

AUTHOR ^

Torsten Foertsch, <torsten.foertsch@gmx.net>

SPONSORING ^

Sincere thanks to Arvato Direct Services (http://www.arvato.com/) for sponsoring the initial version of this module.

COPYRIGHT AND LICENSE ^

Copyright (C) 2005-2008 by Torsten Foertsch

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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