Design a service that decides, for a given coordinate, which delivery zones (and their owning stores) cover it, resolving overlaps by priority, at the read scale of a marketplace checkout and browse flow.

Operating context. Merchants draw polygonal serviceable areas around their stores. At checkout and while browsing, the product asks: for this address, which stores can deliver here, and is store X serviceable? Zones overlap (competing stores, tiered coverage), so a lookup must resolve them deterministically. Merchants edit zones frequently and expect changes to take effect quickly, but a lookup must never block on a config write. Reads dominate heavily.

Out of scope. The delivery-fee and pricing rules beyond identifying the covering zones, the merchant onboarding UI, dispatch and driver assignment, and store inventory/availability. Assume those consume your coverage result.

What to produce. A high-level architecture covering: how zone polygons are stored and spatially indexed so a lookup tests only nearby candidates; the point-in-polygon evaluation at scale, including complex polygons; deterministic overlap resolution by priority or smallest-area; caching coverage results by cell and invalidating on edits; zone versioning and how an edit propagates within a bounded staleness window; regional partitioning; and failure behavior when the config store is degraded. Sketch the components and the flow of one coverage lookup; we will probe specifics in checkpoints.