Every few weeks someone posts the same checklist: as a backend engineer, learn System Design, APIs, Databases, Distributed Systems, Caching, Security, DevOps, Performance, Cloud, Monitoring… and the list is correct but can be confusing without specifying when any of it matters.
Lets take one endpoint, POST /transfers, which moves money from one account to another. I picked the scariest endpoint on purpose: when an order endpoint has a bug you ship a duplicate t-shirt; when a transfer endpoint has a bug you move someone's rent twice. Each thing that breaks teaches one concept, in the order you'd actually hit it.
Code is Spring Boot. Lessons aren't (These are applicable in any backend framework of your choice)
Stage 0: It works on my machine
Here's version one. A controller, a repository, done.
