[avand]

We definitely could have done this in Postgres.

Our API is a Rails project and though we do a lot of work that's "off the rails," so to speak, we do try to follow a convention where possible. A convention that's worked well for us is to use Postgres only for database-backed models in the app. So when it came time to solve this problem, Redis seemed like a good choice.

Map/reduce isn't an option here. First, it's too slow. Second, it's asynchronous by nature, which doesn't work well when trying to load a webpage. We did consider using it in the background, however, to generate the counter-caches.

Thanks for reading!