A book that turns over hourly
Listings appear and vanish continuously, so a snapshot taken once is a snapshot of something that no longer exists. Velocity — how fast inventory moves at a price — is the signal brokers actually pay for, and it only exists if you never miss a turn.
What we doEvent-level change detection drives the refresh: events whose book is moving get swept in minutes, quiet events fall back to hours. Listing appearance and disappearance are recorded as events rather than inferred from diffs.