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If you set up a DBI connection handle like this, the connection will not be made until the handle is first used. The connection persists after that, and it will try to reconnect if ping fails. No connection is made to the database server if the handle goes unused.

This demonstrates Tie::Constrained as a factoring tool for real code. The alternative is to find the code branches which use the database and hard-code the test and connection in each.

DBD::Mock is used as a stand-in for the real connection because it has an off switch ("mock_can_connect").

(Added) Thanks, jZed++, for suggesting $DBI::errstr as the message when things come apart.

use DBI;
use Tie::Constrained;

our $dbh;
tie $dbh, 'Tie::Constrained' => {
    test => sub {
        $_[0]->isa('DBI::db')
            and $_[0]->ping;
    },
    value =>  DBI->connect('DBI:Mock:','','', \%attr),
    fail => sub {
        # warn  "Connecting $dsn . . .";
        $_[1] =
            DBI->connect($dsn, $user, $passwd, \%attr)
                or die $DBI::errstr;
    }
};

$dbh->{'mock_can_connect'} = 0;
$Tie::Constrained::STRICT = 1;
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In reply to Lazy DBI Connection Handles by Zaxo

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