The answer
Resource does not exist (404), user not authenticated (401), user not authorized (403), input invalid (400), duplicate (409), and server error (500). For each, decide what the API should do.
How to Think Before Writing an API
What edge cases should I consider for an API?
More FAQs in How to Think Before Writing an API
Start with what the user is trying to do. Then define input, output, status codes, edge cases, database needs, side effects, permissions, and validation. Write a short LLD doc. Then code from the doc.
Because the API serves the user. If you cannot say what the user is trying to do, you do not understand the feature. Starting with the user anchors every other decision (input, output, edge cases).
Emails, counter updates, event emissions, notifications. For each, decide if it is sync (in the request) or async (in a queue). Sync side effects slow the request; async ones do not.
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