Google Maps — Mock Interview

You're designing Google Maps for a live system-design interview. Users pan and zoom a map of the world, search for places and addresses, and ask for driving directions with a live ETA that keeps updating as they drive.

Scale assumptions:

• ~1B users; ~100M request directions on a given day, ~5 requests each.
• Road graph: ~10⁸ intersections and ~2×10⁸ directed segments — the routable graph fits in memory (~12 GB).
• ~5M concurrent turn-by-turn sessions stream GPS probes (~1M/s) that feed live traffic.
• Peak reads: ~250k map-tile fetches/s and ~25k route requests/s.
• Targets: route < 1 s p99, tiles < 100 ms, traffic fresh within ~1–2 min.

The defining challenge is routing over a graph whose edge weights change every minute with traffic, while keeping the read-heavy tile path cheap. We'll go through requirements, estimation, architecture, the routing + ETA deep dive, and trade-offs. Think out loud and state your assumptions.