[rshrin]

Querying each instance can lead to availability and latency challenges. Moreover, HLL is not suited for tasks like increments/decrements, TTLs on counts, and clearing of counts. Count-Min Sketch could work if we're willing to forgo certain requirements like TTLs, but relying solely on in-memory data structures on every instance isn't ideal (think about instance crashes, restarts, new code deployments etc.) Instead, using data stores that support HLL or Count-Min Sketch, like Redis, would offer better reliability. That said, we prefer to build on top of data stores we already operate. Also, the "Best-Effort" counter is literally 5 lines of code for EvCache. The bulk of the post focuses on "Eventually Consistent Accurate" counters, along with a way to track which systems sent the increments, which probabilistic counting is not ideal for.

[rshrin]

Just to add, we are also trying to support idempotency wherever possible to enable safe hedging and retries. This is mentioned a bunch in the article on Accurate counters. So take that into consideration.