[bialecki]

I'm sure other people are wondering the same thing, so a quick take.

The problem is not if you're counting one thing (or even 100). The problem is when you want analytics and you want it to scale to 1,000s or 1,000,000s of counters. That may seem ridiculous (who could possibly need that many counters?). But it happens quickly when you say, "How many DAUs do we have? How many from country X? How many using device Y? How many from country X and using device Y?"

Also, to address an idea you mentioned around bitmaps. Bitmaps are great until you have lots of counters and lots of users/things to count. Then the problem is they get very sparse. Imagine user #100,000 does something. You need to allocation 97k of space (lots of zeros behind that 100,000th bit) just to count that one thing. Are bitmaps a good idea? Sure, in a lot of cases they are. The problem is they just break down at some point and that's when these other tricks are really nice.