Apache::DBI(3pm) User Contributed Perl Documentation Apache::DBI(3pm)
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 looks if the environment variable GATEWAY_INTERFACE starts
with 'CGI-Perl' and if the module Apache::DBI has been loaded. In this
case every connect request will be forwarded to the Apache::DBI module.
This looks 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 han-
dle. 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 estab-
lished 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 con-
nections persistent on a per process basis. The problem is, if a user
accesses several times a database, the http requests will be handled
very likely by different servers. Every server needs to do its own con-
nect. It would be nice, if all servers could share the database han-
dles. Currently this is not possible, because of the distinct name-
space of every process. Also it is not possible to create a database
handle upon startup of the httpd and then inheriting this handle to
every subsequent server. This will cause clashes when the handle is
used by two processes at the same time.
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 time of inactivity. The module tries to validate the
database handle using the ping-method of the DBI-module. This method
returns true as default. If the database handle is not valid and the
driver has no implementation for the ping method, you will get an error
when accessing the database. As a work-around you can try to replace
the ping method by any database command, which is cheap and safe or you
can deactivate the usage of the ping method (see CONFIGURATION below).
Here is generalized ping method, which can be added to the driver mod-
ule:
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 roll-
back whenever the script exits. In the case of persistent database con-
nections, the database handle will not be destroyed and hence no auto-
matic rollback occurs. At a first glance it seems even to be possible,
to handle a transaction over multiple requests. But this should be
avoided, because different requests are handled by different servers
and a server does not know the state of a specific transaction which
has been started by another server. In general it is good practice to
perform an explicit commit or rollback at the end of every script. 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 consid-
ered incomplete because of the limitations explained above. It shows
the current database connections for one specific server, the one which
happens to serve the current request. Other servers 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 via the PerlRe-
quire 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, $auth, \%attr)
This can be used as a simple way to have apache servers establish con-
nections on process startup.
Apache::DBI->setPingTimeOut($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-acti-
vate 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::Sta-
tus/Apache2::Status BEFORE Apache::DBI ! For an example of the configu-
ration 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
Note that this module needs mod_perl-1.08 or higher, apache_1.3.0 or
higher and that mod_perl needs to be configured with the appropriate
call-back hooks:
PERL_CHILD_INIT=1 PERL_STACKED_HANDLERS=1.
MOD_PERL 2.0
Apache::DBI version 0.96 and should work under mod_perl 2.0 RC5 and
later. See the Changes file for more information. Beware that it has
only been tested very lightly.
SEE ALSO
Apache, mod_perl, DBI
AUTHORS
* Philip M. Gollucci <pgollucci@p6m7g8.com> is currently packaging new
releases. Ask Bjoern Hansen <;ask@develooper.com> package 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.
perl v5.8.8 2006-10-24 Apache::DBI(3pm)
