Links & short takes

The reference implementation everyone quietly copies. Worth reading not just for the mechanics — Idempotency-Key header, results stored and replayed, keys pruned after 24 hours — but for how much correctness they push onto a single well-designed primitive. If you’re building anything that charges a card, read this before you write a line. I riffed on why it matters here.

A concrete, production account of the ideas I keep banging on about: append-only, immutable, double-entry, built on Spanner for horizontal scale. It’s the rare ledger write-up that takes both correctness and scale seriously instead of trading one for the other. Good companion to my post on why double-entry wins — this is what it looks like when a real company commits to it.

Not a fintech book, but the one I recommend most to people building financial systems. The chapters on replication, consistency, and the real meaning of delivery guarantees are the groundwork for everything I write about here — you can’t reason honestly about at-least-once and idempotency without them. Read it slowly; it rewards a second pass.