Architecture
Designing APIs Contract-First
By Vivek Jalondhara · 1 min read
Published Mar 2026 · Updated
Agree on the contract before anyone writes a line of implementation. Frontend and backend then move in parallel.
Contract-first design is the cheapest way to unblock a team. When the request/response shapes, status codes, and error schemas are agreed up front, frontend and backend can build against the same source of truth simultaneously.
I document the contract, generate types from it, and let both sides consume those types. Drift becomes a compile error instead of a 2 a.m. incident. Error schemas deserve as much care as the happy path — consistent error shapes make clients dramatically simpler.
The contract is also where you encode rate limits, pagination, and versioning conventions. Decide those once, centrally, and every endpoint inherits good behavior.
Model a booking conflict as an expected response
If two people select the same slot, one may need to choose again. A documented conflict response helps the interface recover without treating the outcome as an unexplained crash.
Illustrative example · JSON · Simplified for explanation
{
"code": "SLOT_UNAVAILABLE",
"message": "Choose another available time.",
"resourceId": "example-room",
"retryable": false
}- Reserve slot
- Server availability check
- HTTP 409 conflict
- Refresh available slots
Conceptual flow for the example above.
Tradeoff to keep in mind
A response contract describes behavior but cannot enforce concurrency safety. That must be guaranteed by the reservation operation itself.