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REST API Design: Reasoning Through an Interview ​

By InterviewPrep.World · Published October 19, 2026

Practice REST API design with resource modeling, request validation, pagination, authorization, idempotency, and actionable error responses. This guide focuses on explaining decisions clearly and testing your understanding with a concrete scenario.

Model resources around the business operation ​

Start with a domain such as orders rather than choosing endpoints by habit. Identify the resources, their identifiers, and the transitions users are allowed to request. An order collection can support creation and listing, while an individual order supports retrieval. Cancellation may need an explicit operation when it has meaningful rules and side effects. Explain your choice using the business contract. A tidy URL cannot compensate for unclear behavior about who may cancel an order, when cancellation is allowed, and whether inventory or payment must also change.

Define request and response contracts ​

For each operation, describe required fields, validation rules, response shape, and error semantics. Distinguish an invalid request from missing authentication, insufficient permission, a missing resource, and a state conflict. Return an error identifier that clients can handle without parsing a human sentence. Avoid exposing stack traces or internal infrastructure details. When listing records, define ordering and pagination together so clients know whether new records can move between pages. Cursor pagination can help some changing datasets, but the cursor must preserve a meaningful and stable ordering relationship.

Make retries safe where they matter ​

A network timeout does not tell a caller whether the server completed a request. For an operation such as creating a payment, blindly retrying can create duplicate effects. An idempotency key lets the server recognize repeated attempts within a defined scope and lifetime. Explain how the original result is stored and how conflicting payloads using the same key are treated. Do not equate an HTTP method name with a complete safety guarantee. The implementation, dependencies, and transaction boundaries determine whether repeated requests actually have the promised behavior.

Review security and evolution together ​

Authenticate the caller, then authorize access to the specific resource rather than trusting an identifier supplied by the client. Limit accepted fields to prevent unintended updates. Add rate limits and request size limits appropriate to the service. For evolving contracts, prefer compatible additions when possible and document changes that require coordinated client work. A useful interview example walks through one successful request and several failure paths. That demonstrates a service contract clients can depend on, rather than a diagram containing endpoints with no defined operational behavior.

Practice and review ​

Open the backend interview questions practice section and choose a question connected to the ideas above. Before reading its answer, write down your assumptions and attempt an explanation in your own words. Use a small example that another person can follow without needing your project history. Identify the point where your explanation depends on a fact you have not verified, and turn that uncertainty into a focused follow-up task.

Next, change one constraint in your example and explain which part of the solution must change. Ask a practice partner to challenge your reasoning, or record a short walkthrough and listen for missing steps. Keep a brief error log with the original misunderstanding, the correction, and a new example that demonstrates the correction. Revisit that example after a few days. This checks whether you can apply the idea independently rather than recognize a familiar answer.

Continue preparing ​