Pay no attention to that callback behind the curtain!

So you’ve got some perl code that connects to a particular database via a particular DBI driver. You want it to connect to a different database or driver. But you can’t change that part of the code. What can you do?

I ran into this problem recently. A large application is using an old version of DBIx::HA which doesn’t support DBD::Gofer. DBIx::HA can’t be upgraded (long story, don’t ask) but I wanted to use DBD::Gofer to provide client-side caching via Cache::FastMmap. (I’ll save more details of that, and the 40% reduction in database requests it gave, for another post.)

I needed a way for DBIx::HA to think that it was connecting to a particular driver and database, but for it to actually connect to another. Using $ENV{DBI_AUTOPROXY} wasn’t an option because that has global effect whereas I needed fine control over which connections were affected. It’s also fairly blunt instrument in other ways.

It seemed like I was stuck. Then I remembered the DBI callback mechanism – it would provide an elegant solution to this. I added it to DBI 1.49 back in November 2005 and enhanced it further in 1.55. I’d never documented it though. I think I was never quite sure it had sufficient functionality to be really useful. Now I’m sure it has.

The DBI callback mechanism lets you intercept, and optionally replace, any method call on a DBI handle. At the extreme, it lets you become a puppet master, deceiving the application in any way you want.

Here’s how the code looked (with a few irrelevant details changed):

    # The following section of code uses the DBI Callback mechanism to
    # intercept connect() calls to DBD::Sybase and, where appropriate, 
    # reroute them to DBD::Gofer.
    our $in_callback;

    # get Gofer $drh and make it pretend to be named Sybase
    # to keep DBIx::HA 0.62 happy
    my $gofer_drh  = DBI->install_driver("Gofer");
    $gofer_drh->{Name} = "Sybase";

    # get the Sybase drh and install a callback to intercept connect()s
    my $sybase_drh = DBI->install_driver("Sybase");
    $sybase_drh->{Callbacks} = {
        connect => sub {
            # protect against recursion when gofer itself makes a connection
            return if $in_callback; local $in_callback = 1;

            my $drh = shift;
            my ($dsn, $u, $p, $attr) = @_;
            warn "connect via callback $drh $dsn\n" if $DEBUG;

            # we're only interested in connections to particular databases
            return unless $dsn =~ /some pattern/;

            # rewrite the DSN to connect to the same DSN via Gofer
            # using the null transport so we can use Gofer caching
            $dsn = "transport=null;dsn=dbi:Sybase(ReadOnly=1):$dsn";

            my $dbh = $gofer_drh->connect($dsn, $u, $p, $attr);

            if (not $dbh) { # gofer connection failed for some reason
                warn "connect via gofer failed: $DBI::errstr\n"
                    unless our $connect_via_gofer_err++; # warn once
                return; # DBI will now call original connect method
            }

            undef $_;    # tell DBI not to call original connect method
            return $dbh; # tell DBI to return this $dbh instead
        },
    };

So the application, via DBIx::HA, executed

  $dbh = DBI->connect("dbi:Sybase:foo",...)

but what it got back was a DBD::Gofer dbh, as if the application has executed

  $dbh = DBI->connect("dbi:Gofer:transport=null;dsn=dbi:Sybase(ReadOnly=1):foo",...).

I guess I should document the callback mechanism now. Meanwhile the closest thing to documentation is the test file.

I’ve always enjoyed this kind of “plumbing”. If you come up with any interesting uses of DBI callbacks, do let me know.

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Perl Myths

Update: several more recent versions of my Perl Myths talk are available. These have significant updates. Slides can be found on slideshare.net and screencasts can be found on my blip.tv channel.

I’ve uploaded my Perl Myths presentation to slideshare.net and google video:

“Perl has it’s share of myths. This presentation debunks a few popular ones with hard facts. Surprise yourself with the realities.”

While I agree with Andy Lester that Good Perl code is the best form of evangelism, I wanted to put together a presentation that others could refer to when they encounter misinformation about Perl. I cover these myths that I’ve heard recently:

  • Perl is dead
  • Perl is hard to read / test / maintain
  • Perl 6 is killing Perl 5

and pull in a wealth of upto date information, some of it quite surprising even to those familiar with Perl and its community. There are two versions, plus a video. I recommend the one with notes (which have useful extra detail and context for the slides) which is best viewed as a PDF. There’s also one without notes which I’ve embedded here:

I videoed an extended version of this presentation at IWTC in Dublin in February. The first 40 minutes or so correspond with the slides above. In the remaining 30 minutes or so I talk about Parrot and Perl 6. I’ve embedded the video below, but wordpress forces me to use a small size so you’ll probably prefer to view it at video.google.com: