Consistency is a form of reliability

Consistency and reliability are usually considered to be in tension because of the CAP theorem. And when working on a distributed system, it’s normal to get focused on trying to improve service reliability. Sometimes, we can take shortcuts that will increase reliability at the expense of returning stale results more often.

But consistency shouldn’t be seen as opposing reliability. In fact, I think they’re very similar. If your service has 6 9s of reliability (number of successful requests / number of total requests) but only 3 9s of consistency (number of consistent requests / number of successful requests), do you really have 6 9s of reliability? After all, you’re getting some of those extra 9s just by returning a wrong answer.

With my definition of consistency (how many successful requests are consistent), it’s arguably better to prioritize adding more 9s of consistency than reliability.