NAME¶
Apache::DBI - Initiate a persistent database connection
SYNOPSIS¶
# Configuration in httpd.conf or startup.pl:
PerlModule Apache::DBI # this comes before all other modules using DBI
Do NOT change anything in your scripts. The usage of this module is absolutely
transparent !
DESCRIPTION¶
This module initiates a persistent database connection.
The database access uses Perl's DBI. For supported DBI drivers see:
http://dbi.perl.org/
When loading the DBI module (do not confuse this with the Apache::DBI module) it
checks if the environment variable 'MOD_PERL' has been set and if the module
Apache::DBI has been loaded. In this case every connect request will be
forwarded to the Apache::DBI module. This checks if a database handle from a
previous connect request is already stored and if this handle is still valid
using the ping method. If these two conditions are fulfilled it just returns
the database handle. The parameters defining the connection have to be exactly
the same, including the connect attributes! If there is no appropriate
database handle or if the ping method fails, a new connection is established
and the handle is stored for later re-use. There is no need to remove the
disconnect statements from your code. They won't do anything because the
Apache::DBI module overloads the disconnect method.
The Apache::DBI module still has a limitation: it keeps database connections
persistent on a per process basis. The problem is, if a user accesses a
database several times, the http requests will be handled very likely by
different processes. Every process needs to do its own connect. It would be
nice if all servers could share the database handles, but currently this is
not possible because of the distinct memory-space of each process. Also it is
not possible to create a database handle upon startup of the httpd and then
inherit this handle to every subsequent server. This will cause clashes when
the handle is used by two processes at the same time. Apache::DBI has built-in
protection against this. It will not make a connection persistent if it sees
that it is being opened during the server startup. This allows you to safely
open a connection for grabbing data needed at startup and disconnect it
normally before the end of startup.
With this limitation in mind, there are scenarios, where the usage of
Apache::DBI is depreciated. Think about a heavy loaded Web-site where every
user connects to the database with a unique userid. Every server would create
many database handles each of which spawning a new backend process. In a short
time this would kill the web server.
Another problem are timeouts: some databases disconnect the client after a
certain period of inactivity. The module tries to validate the database handle
using the "ping()" method of the DBI-module. This method returns
true by default. Most DBI drivers have a working "ping()" method,
but if the driver you're using doesn't have one and the database handle is no
longer valid, you will get an error when accessing the database. As a
work-around you can try to add your own "ping()" method using any
database command which is cheap and safe, or you can deactivate the usage of
the ping method (see CONFIGURATION below).
Here is a generalized ping method, which can be added to the driver module:
package DBD::xxx::db; # ====== DATABASE ======
use strict;
sub ping {
my ($dbh) = @_;
my $ret = 0;
eval {
local $SIG{__DIE__} = sub { return (0); };
local $SIG{__WARN__} = sub { return (0); };
# adapt the select statement to your database:
$ret = $dbh->do('select 1');
};
return ($@) ? 0 : $ret;
}
Transactions: a standard DBI script will automatically perform a rollback
whenever the script exits. In the case of persistent database connections, the
database handle will not be destroyed and hence no automatic rollback will
occur. At a first glance it even seems possible to handle a transaction over
multiple requests. But this should be avoided, because different requests are
handled by different processes and a process does not know the state of a
specific transaction which has been started by another process. In general, it
is good practice to perform an explicit commit or rollback at the end of every
request. In order to avoid inconsistencies in the database in case AutoCommit
is off and the script finishes without an explicit rollback, the Apache::DBI
module uses a PerlCleanupHandler to issue a rollback at the end of every
request. Note, that this CleanupHandler will only be used, if the initial
data_source sets AutoCommit = 0 or AutoCommit is turned off, after the connect
has been done (ie begin_work). However, because a connection may have set
other parameters, the handle is reset to its initial connection state before
it is returned for a second time.
This module plugs in a menu item for Apache::Status or Apache2::Status. The menu
lists the current database connections. It should be considered incomplete
because of the limitations explained above. It shows the current database
connections for one specific process, the one which happens to serve the
current request. Other processes might have other database connections. The
Apache::Status/Apache2::Status module has to be loaded before the Apache::DBI
module !
CONFIGURATION¶
The module should be loaded upon startup of the Apache daemon. Add the following
line to your httpd.conf or startup.pl:
PerlModule Apache::DBI
It is important, to load this module before any other modules using DBI !
A common usage is to load the module in a startup file called via the
PerlRequire directive. See eg/startup.pl and eg/startup2.pl for examples.
There are two configurations which are server-specific and which can be done
upon server startup:
Apache::DBI->connect_on_init($data_source, $username, $auth, \%attr)
This can be used as a simple way to have apache servers establish connections on
process startup.
Apache::DBI->setPingTimeOut($data_source, $timeout)
This configures the usage of the ping method, to validate a connection. Setting
the timeout to 0 will always validate the database connection using the ping
method (default). Setting the timeout < 0 will de-activate the validation
of the database handle. This can be used for drivers, which do not implement
the ping-method. Setting the timeout > 0 will ping the database only if the
last access was more than timeout seconds before.
For the menu item 'DBI connections' you need to call
Apache::Status/Apache2::Status BEFORE Apache::DBI ! For an example of the
configuration order see startup.pl.
To enable debugging the variable $Apache::DBI::DEBUG must be set. This can
either be done in startup.pl or in the user script. Setting the variable to 1,
just reports about a new connect. Setting the variable to 2 enables full debug
output.
PREREQUISITES¶
MOD_PERL 2.0¶
Apache::DBI version 0.96 and later should work under mod_perl 2.0 RC5 and later
with httpd 2.0.49 and later.
Apache::DBI versions less than 1.00 are NO longer supported. Additionally,
mod_perl versions less then 2.0.0 are NO longer supported.
PERL_CHILD_INIT=1 PERL_STACKED_HANDLERS=1
Apache::DBI v0.94 was the last version before dual mod_perl 2.x support was
begun. It is recommended that you use the latest version of Apache::DBI
because Apache::DBI versions less than 1.00 are NO longer supported.
DO YOU NEED THIS MODULE?¶
Note that this module is intended for use in porting existing DBI code to
mod_perl, or writing code that can run under both mod_perl and CGI. If you are
using a database abstraction layer such as Class::DBI or DBIx::Class that
already manages persistent connections for you, there is no need to use this
module in addition. (Another popular choice, Rose::DB::Object, can cooperate
with Apache::DBI or use your own custom connection handling.) If you are
developing new code that is strictly for use in mod_perl, you may choose to
use "DBI->connect_cached()" instead, but consider adding an
automatic rollback after each request, as described above.
SEE ALSO¶
Apache, mod_perl, DBI
AUTHORS¶
- •
- Philip M. Gollucci <pgollucci@p6m7g8.com> is
currently packaging new releases.
Ask Bjoern Hansen <ask@develooper.com> packaged a large number of
releases.
- •
- Edmund Mergl was the original author of Apache::DBI. It is
now supported and maintained by the modperl mailinglist, see the mod_perl
documentation for instructions on how to subscribe.
- •
- mod_perl by Doug MacEachern.
- •
- DBI by Tim Bunce <dbi-users-subscribe@perl.org>
COPYRIGHT¶
The Apache::DBI module is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.