For work, we're building a system which allows people to specify an object model in Perl code and have the database get automatically built for it. Yes, I know this has been done before, but we do some nifty tricks to push business logic into the database layer. Before those of you who know about the "object-relational impedance mismatch" choke, I have news for you: the impedance mismatch is a myth. Seriously, it's a complete myth. There's an object-dbms impedance mismatch, but that's a rant for another day. Suffice it to say that if one understands and properly applies relational theory, the apparent impedance mismatch is significantly lower than folks think.
Returning to LiveJournal, you may have noticed that it holds up very, very well to severe server loads. Many sites get taken down by Slashdot stories like the one about my identify theft, but LiveJournal held up quite well. Part of the reason for that is they created software called memcached. This is a nice tool which makes it easy to cache data and take the load off of the database. Unlike many open source projects, this one turned out to be phenomenally easy to compile and use. Two thumbs up!
Since we're also using Catalyst to map actions to commands for our Web UI, we're using Catalyst::Plugin::Session::Store::Memcached to manage our sessions. Once again, it turned out to be very easy to use and install. This makes Ovid a very happy programmer.