DBI(3)

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://www.symbolstone.org/technology/perl/DBI/
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 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 several times a database, the http requests will be handled very likely by different servers. Every server needs to do its own connect. It would be nice, if all servers could share the database handles. 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 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 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. It will not be used, if AutoCommit will be turned off, after the connect has been done.

This module plugs in a menu item for Apache::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 server, the one which happens to serve the current request. Other servers might have other database connections. The Apache::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 PerlRequire directive. See eg/startup.pl for an example.

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 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
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.

SEE ALSO
the Apache manpage, the mod_perl manpage, the DBI manpage

AUTHORS

COPYRIGHT
The Apache::DBI module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.