It depends on how many unique URLs you have. If you have like a few hundreds or even thousand I would just use a simple hash table with counters to see what's a

It depends on how many unique URLs you have. If you have like a few hundreds or even thousand I would just use a simple hash table with counters to see what's a hot sale right now (I guess that's what you want in your example). If you have several millions than maybe I would put it into Redis or something similar where I can utilize ready-to-use HyperLogLog cardinality estimation.