Location search and search at scale

One illustrative launch night in Bengaluru and the location bugs that break it, then the fixes: why a B-tree on (lat, lng) reads a 600 km strip; cells, coverings and refine; geohash and its edge problem (cell plus 8 neighbours); quadtrees, S2's numbered Hilbert cells and H3's equidistant hexagons; R-trees in PostGIS and BKD points in Elasticsearch; radius queries end to end and a safe ring count; k-nearest with a stopping bound; moving partners as soft state in memory (75,000 writes a second, only about 1,700 cell moves); sharding by load and hot cells; and the search cluster's topology, shard sizing and alias reindexing.