Part 2: Engineering for money

Part 1 explained how value moves on a blockchain. This part explains why moving it reliably is hard, and how the engineering world solves each difficulty. None of these ideas is specific to crypto: they are how banks, payment companies and databases have always kept money correct. On a blockchain they matter even more, because nothing can be undone.

#	Lesson	The problem it solves
10	When networks fail	A request timed out. Did the payment happen or not?
11	Idempotency	How do you retry safely, when retrying might pay twice?
12	Persistence and crash recovery	The process died halfway through a payment. Now what?
13	Concurrency: locks, leases and fencing	Ten workers share the work. How do they not collide?
14	Trust: one server's word is not proof	A server says the payment failed. Is it telling the truth?
15	Secrets and key custody	Where do private keys live, and how do secrets leak?

Each lesson ends by showing how crypto-aio applies the idea. After lesson 15, the Developer tour puts them all together, inside the library.