[Traster]

It's important to think about this in an a la carte design, not one fixed solution for all use cases.

Step 1: You give people the ability to put in soft limits - "Warn me when I hit 500",

Step 2: You also give the ability to put in hard limits "Pull the plug at 10k" (caveat to both these things - you guarantee this at an eventual consistency level, like "Well you hit 500 but by the time our stats updated you were at 600",

Step 3: You introduce rate limits - "We're expecting 500 in a month, warn us if we hit 50 in a day or 10 in an hour".

Step 4: You introduce predictive warnings "Our statistics show you'll hit your monthly limit on the 23rd of the month"

Step 5: You put in predictive limits to allow scaling - "The last 3 months we've seen the following use trend, warn us if we exceed double that trend, cut off if we see 50x that trend"

You might set some of these limits or none of these limits depending how predictable your use case is.