Database Sharding

This is a solid sharding implementation guide that covers hash, range, and directory strategies with working TypeScript routers you can actually use. It's strongest on the gotchas: wrong shard key selection causing hotspots, cross-shard transactions without two-phase commit, and the modulo rebalancing trap that breaks all your keys when you add a fifth shard. The shard key criteria checklist is genuinely helpful (high cardinality, immutable, in 80% of queries). If you're hitting billions of records or need multi-tenant isolation, this walks you through the decision tree and gives you production patterns from companies like Instagram and Discord. The error catalog is worth reading even if you're not implementing sharding yet.