if i remember my lore correctly it's not a distributed system. It was originally either a mainframe or like AS400 application. I want to say it was related to Saber somehow... maybe they leased it. When i interviewed there years ago they were porting it to some kind of Java stack or maybe they had already ported it and were building it out further. On a tangent, I have to admit, their interview was the best one i've ever had. Very competent people, well rounded process, it was actually more fun than stressful but super challenging too.
/ this was a lonng time ago, like 10+ years, it could be a very very different animal now
I don't think Southwest was ever on Sabre. Their scheduling used to be in-house, now on Amadeus. Computerized operations started with Braniff's software and have ... ah, grown since then. Name almost any technology and it's probably running somewhere in their stack.
Especially when you outsource it to the lowest bidding WITCH shop you can find. Incredible that the US is still pumping out MBAs that see IT as a cost center. If you are of a sufficient size and depend on software to function, you are a software company and need to allocate your resources accordingly.
