People really underestimate the value of Occasional Consistency. Occasionally Consistent databases, like MongoDB, are great for approximation algorithms, sublinear time algorithms, and similar applications.
The issue isn't that MongoDB is eventually consistent, it's that the documentation claims that in some cases it's strictly consistent[1] while Kyle found that:
"MongoDB, even at the strongest consistency levels, allows reads to see old values of documents or even values that never should have been written."
I didn't say Eventual Consistency. I said Occasional Consistency. MongoDB is has hard Occasional Consistency. Indeed, it is the most occasionally consistent database I know of. I once wrote a few million records into Mongo. It was consistent before the write, but never again after.
Great for sub-linear time algorithms! At that point, all my algorithms ran at less than O(n) on the size of the data I had written in.
From a business perspective, Occasional Consistency is also a very nice property if you are storing audit data for certain types of organizations. It gives complete plausible deniability about rule compliance.
