In a new post to his blog Justin Carmony shares the story of how he took a platform running with MySQL as the backend (Dating DNA) and made the choice to move to Redis for storing compatibility scores for every user in the system.
We wanted not only for people to be able to visit a profile and see a score, which is easy to generate a score on demand. We wanted our users to be able to browse other profiles sorted by their score with them. This requires us to pre-generate and store these scores, and then later query them. [...] I believe we could have bent MySQL to our will and got it to work, but it would be at a high cost of server power, and that cost wouldn't scale well with our revenue stream.
He talks about some of the other options they sorted through including NoSQL databases or building something completely in-house. In the end, though, they decided that Redis was more of what they needed. He prefaces talking about their configuration with some of the limitations of the tool and then moves into their setup and statistics on its use (complete with pretty graphs). He points to predis as their Redis client of choice for PHP.