Apache::DBI::Cache(3) User Contributed Perl DocumentationApache::DBI::Cache(3)NAMEApache::DBI::Cache - a DBI connection cache
SYNOPSIS
use Apache::DBI::Cache debug=>3, bdb_env=>'/tmp/tmp',
plugin=>'Apache::DBI::Cache::mysql',
plugin=>['DBM', sub {...}, sub {...}],
...;
DESCRIPTION
This module is an alternative to the famous Apache::DBI module. As
Apache::DBI it provides persistent DBI connections.
It can be used with mod_perl1, mod_perl2 and even standalone.
WHY ANOTHER MODULE FOR THE SAME?
Apache::DBI has a number of limitations. Firstly, it is not possible to
get multiple connections with the same parameters. A common scenario
for example is to use one connection to perform transactions and
another to perform simple lookups in the same database. With
Apache::DBI it is very likely to get the same connection if you mean to
use different.
With Apache::DBI all connections are reset at end of a request.
Apache::DBI does not regard database specific functions to cache
handles more aggressively. For example a mysql DSN can look like
dbi:mysql:test:localhost:3306
or
dbi:mysql:host=localhost;db=test
Both point to the same database but for Apache::DBI they are different.
Apache::DBI::Cache recognizes these two by means of a mysql plugin.
The plugin even recognizes connections to different databases on the
same mysql server as the same connection and issues a "USE database"
command before returning the actual handle to the user. Hence, with
Apache::DBI::Cache many the overall number of connections to a DB
server can be dramatically reduced.
HOW DOES IT WORK?
To decide whether to use Apache::DBI::Cache or not it is essential to
know how it works. As with Apache::DBI Apache::DBI::Cache uses a hook
provided by the DBI module to intercept "DBI->connect()" calls. Also
do Apache::DBI::Cache maintain a cache of active handles.
When a new connection is requested and the cache is empty a new
connection is established and returned to the user. At this point it is
not cached at all. Here Apache::DBI::Cache differs from Apache::DBI.
Later either "disconnect" is called on the handle or it simply goes out
of scope and the garbage collector calls a "DESTROY" method if
provided. Both events are intercepted by Apache::DBI::Cache. Only then
the handle is put in the cache.
This means a handle is never really disconnected. "$dbh->{Active}" will
always return true no matter how often "disconnect" is called.
Further, you can prevent a handle from getting reused by simply not
forgetting it.
USAGE
Different to Apache::DBI Apache::DBI::Cache must be "use"ed not
"require"ed. That means it's "import" function must be called.
When used with mod_perl (versions 1.x or 2.x) this is best done in a
"startup.pl" or in a "<Perl>" section in the "httpd.conf". See
mod_perl documentation for more information.
Thereafter, "DBI->connect" is called as usual. No special treatment is
needed.
When Apache::Status or Apache2::Status is used Apache::DBI::Cache
provides an extra menu item to show statistics on handles. The loading
order of the Apache::Status and Apache::DBI::Cache is irrelevant.
FUNCTIONS
import - "use" parameter
Parameters to the "use" statement are given in a "key => value"
fashion.
use Apache::DBI::Cache debug=>3, logger=>sub {...},
plugin=>['driver', sub {}, sub {}],
plugin=>'Apache::DBI::Cache::mysql',
use_bdb=>1,
bdb_env=>'/tmp/mybdbenv',
bdb_memcache=>20*1024,
...;
· plugin
loads a plugin, see also "PLUGINS" below. The plugin can be
specified as a 3-element array or by name. In the second case the
"import" simply "use"s the module. This option can be given
multiple times.
· use_bdb, bdb_env and bdb_memcache
Apache::DBI::Cache can use BerleleyDB as a shared memory
implementation to maintain statistics for a group of processes
instead of a single process.
"use_bdb" specify whether to use BerkeleyDB or not. If ommitted
Apache::DBI::Cache will try to load and use BerkeleyDB. If that
fails it silently provides per process statistics. If "use_bdb"
is true Apache::DBI::Cache dies if it cannot use BerkeleyDB. If
"use_bdb" is false per process statistics are maintained and
BerkeleyDB is not used.
"bdb_env" specifies a path to a directory where BerkeleyDB can
put it's temporary files. If omitted "/tmp/Apache::DBI::Cache" is
used. The parent directory of this directory must exists and be
writeable.
"bdb_memcache" specifies the size of the shared memory segment
that is allocated by BerkeleyDB. Depending on the number of
handles in your configuration a few kilobytes are enough. If
omitted 20 kB are used.
"bdb_env" and "bdb_memcache" can also be specified by the
"APACHE_DBI_CACHE_ENVPATH" and "APACHE_DBI_CACHE_CACHESIZE"
environment variables.
· debug
set a debug level. Under mod_perl this is almost irrelevant, see
"logger" below.
· logger
here a logger function can be specified. It is called with the
message verbosity level as the first parameter. The remaining
parameters are concatenated to build the actual message.
Currently there are 2 verbosity levels used 1 and 2. 0 is
reserved for real errors. 1 mentions that the module has been
initialized. 2 rattle off normal processing messages.
Apache::DBI::Cache provides 2 logger functions. One is controlled
by the "debug" level setting (see above). A message is printed to
STDERR if it's level is equal or greater the current debug level.
The other logger is used when running under mod_perl. It is
mainly controlled by the Apache "LogLevel" setting. Messages at
level 0 are printed as "$log->error", level 1 as "$log->info" and
level 2 as "$log->debug". For level 2 messages additionally the
current debug level is checked to be greater or equal 2.
· delimiter
Here the internal key delimiter can be changed. It defaults to
"\1". Changing it is necessary only when your DSN, username or
password contain it or to provide more readable debugging
messages.
statistics
returns a reference to the statistics hash. If BerkeleyDB is used
it is tied to BerkeleyDB::Btree.
statistics_as_html
returns a reference to an array of HTML fragments. If mod_perl and
Apache::Status or Apache::Status2 is used the output of this
function is shown under http://HOST/STATUS/URI?DBI_conn.
plugin( 'name', \&mangle, \&setup )
installs a new plugin, see "PLUGINS" below. If a plugin for the
specified database type was already installed it is returned as a
2-element list:
($old_mangle, $old_setup)=
plugin( 'name', \&new_mangle, \&new_setup );
If called with an name only the current plugin is returned:
($old_mangle, $old_setup)=plugin( 'name' );
To delete a plugin call
($old_mangle, $old_setup)=plugin( 'name', undef, undef );
connect_on_init
call this function multiple times with parameters you would pass to
"DBI->connect" before calling "Apache::DBI::Cache::init", i.e. in
your "startup.pl". Then "init" will establish all these
connections.
init
This function is called once per child process to initialize
Apache::DBI::Cache. If mod_perl is used this is done automatically
in a PerlChildInitHandler
finish
This function must be called before a process is going to
terminate. Under mod_perl it is automatically called in a
PerlChildExitHandler.
As of version 0.08 calling this function is optional.
undef_at_request_cleanup( \$dbh1, \$dbh2, ... )
When an application uses global variables to store handles they
probably won't be reused because a global variable is ..., well
global. This can be fixed by explicitly undefining these handles at
request cleanup or by using this function. It simply collects all
handle references passed in between 2 calls to
"Apache::DBI::Cache::request_cleanup". When
"Apache::DBI::Cache::request_cleanup" is called all these handles
are undefined. The first call to this function during a request
cycle installs "Apache::DBI::Cache::request_cleanup" as
PerlCleanupHandler.
With mod_perl2 this requires the PerlOption "GlobalRequest" to be
set:
PerlOption +GlobalRequest
in your httpd.conf.
request_cleanup
This is the PerlCleanupHandler. If Apache::DBI::Cache is used
standalone the application can call it from time to time.
EXPORT
Nothing.
DBI SUBCLASSING
For a module like Apache::DBI::Cache it is complicated to cope with DBI
subclasses. There are 2 problems to solve. First, make sure that our
"disconnect" and "DESTROY" methods are called instead of the original.
Apache::DBI::Cache solves this problem by inserting its own methods
into the foreign class.
Hence, if a subclass provides "disconnect" and "DESTROY" methods they
will never be called. This is ugly but works in most cases.
To insert our methods into the subclass we need to know its name. This
is the second problem. To create a subclassed DBI handle one calls
either
DBI->connect( $dsn, $user, $passwd, {RootClass=>SUBCLASS} );
or
SUBCLASS->connect( $dsn, $user, $passwd, {} );
The first case is simple since the attribute hash is passed to our
connect method. Unfortunately, the second case is not simple since
"SUBCLASS" is known only by "DBI::connect". This is solved by searching
the current call stack for the DBI::connect call. Then we use its first
parameter.
That works for me but is even uglier. If you encounter problems don't
hesitate to mail me.
Class::DBI and Ima::DBI
To make "Class::DBI" or "Ima::DBI" work with "Apache::DBI::Cache" see
Apache::DBI::Cache::ImaDBI.
PLUGINS
Plugins are used to modify the caching for certain database types. They
can change the caching key, issue database commands just before a
handle is returned to the user or prevent handle caching entirely for a
database type.
There can only be one plugin per database type at a time.
A plugin registers itself by calling "Apache::DBI::Cache::plugin"
passing 3 parameters. The first parameter is simply the name of the
database type. It matches the DBI driver name. Thus, a MySQL plugin
passes the string "mysql" since the corresponding DBI driver is named
"DBD::mysql". Whereas a PostgreSQL plugin passes either "Pg" or "PgPP"
depending on the driver actually used.
The 2nd and 3rd parameters are CODE references that are called just
before a connection is chosen from the cache or newly established and
after the connection is made just before it is returned to the caller.
The first function can mangle the connection parameters the second
perform additional setup steps. Further, I will call them mangle and
setup.
Thus, a plugin is registered this way:
Apache::DBI::Cache::plugin('Name', \&mangle, \&setup);
Normally, it is implemented as a separate module according to the
following template, see Apache::DBI::Cache::mysql for example:
package Apache::DBI::Cache::DRIVER;
use strict;
BEGIN {
die "Please load Apache::DBI::Cache before"
unless defined &Apache::DBI::Cache::plugin;
...
Apache::DBI::Cache::plugin
(
'DRIVER',
sub {},
sub {}
);
}
1;
Calling Conventions
· mangle
($dsn, $user, $passwd, $attr, $ctx, $nocache)=
mangle($dsn, $user, $passwd, $attr);
mangle is called with almost the same parameters as the original call
to "DBI->connect". The "dbi:DRIVER:" prefix is stripped from the DSN.
It is expected to return similar values plus an arbitrary context
that is later passed to setup and an optional "nocache" flag.
If $nocache is true or mangle returns an empty list a new connection
is made and the handle is directly passed to the caller without
further processing. Also setup will not be called. Such a handle will
not be cached on "disconnect" or "DESTROY".
mangle can change all parameters. The MySQL plugin for example
deletes the actual database name from the DSN, reformats it according
to a standard format and adds the standard port if it is omitted. The
database is put in the context.
· setup
$rc=setup($dbh, $dsn, $user, $passwd, $attr, $ctx);
The setup function performs additional setup steps on $dbh. The MySQL
plugin for example issues a "USE database" command using the database
from the context.
A connection is considered dead if setup returns false.
TODO
· periodically ping all cached handles
· correct statistics when a process is finishes without calling
"finish()"
· redirect BerkeleyDB errors to logger
SEE ALSO
Apache::DBI::Cache::mysql
Apache::DBI
AUTHOR
Torsten Foertsch, <torsten.foertsch@gmx.net>
With suggestions from
Andreas Nolte < andreas dot nolte at bertelsmann dot de >
Dietmar Hanisch < dietmar dot hanisch at bertelsmann dot de > and
Ewald Hinrichs < ewald dot hinrichs at bertelsmann dot de >
SPONSORING
Sincere thanks to Arvato Direct Services (http://www.arvato.com/) for
sponsoring this module and providing a test platform with several
thousand DBI connections.
COPYRIGHT AND LICENSE
Copyright (C) 2005-2006 by Torsten Foertsch
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.14.1 2006-05-22 Apache::DBI::Cache(3)
