Well-designed APIs: contracts before code

Start from the consumer, not from the database

An API that mirrors tables forces every client to rebuild the business rule on its own. An API that exposes meaningful domain operations reduces coupling and prevents the same logic from being reimplemented in five different places.

A simple exercise helps: write the call and response example you'd like to read in the documentation first. If the example is confusing, the design isn't ready yet.

• Stable, predictable names, consistent in language across the whole surface.
• Errors with a code, a readable message and a correlation identifier.
• Pagination, sorting and filters defined from the first version.

Errors and idempotency are part of the product

Networks fail, clients retry, timeouts happen. Write operations need an idempotency key so a retry doesn't produce two charges or two orders. That detail prevents most integration incidents.

Error responses deserve the same care as success responses: predictable, documented and free of leaked internal information.

Versioning means planning for change

Adding a field is safe; removing one or changing its meaning is not. When a breaking change is unavoidable, publish a new version, keep the previous one for an announced period, and offer a migration path. A written deprecation policy is worth more than any verbal promise.

Living documentation and explicit limits

A specification generated from the code, executable examples and a test environment drastically reduce integration cost. Usage limits, quotas and authentication rules should be on the first page, not in a footnote.