Top 30 Most Common Api Interview Questions You Should Prepare For
Which API interview questions are most important to prepare?
Short answer: Focus on fundamentals (HTTP, REST, JSON), testing and tooling (Postman, automation), security/auth, REST-specific patterns (idempotency, versioning), and behavioral stories showing problem-solving.
Why this matters: Interviewers probe both conceptual understanding and practical skills. Knowing how an endpoint is designed is useful — but being able to show how you tested, automated, or secured it is what separates candidates. Prioritize clear definitions, one-line examples, and 2–3 short anecdotes that show results. Takeaway: Master the basics, then map them to real examples you can describe clearly in 60–90 seconds.
What are the top 30 API interview questions (with concise answers) you should know?
Short answer: Learn the top 30 across five categories—fundamentals, testing/tools, security, REST specifics, and behavioral/process—each with a one-line answer and a short example.
Below are 30 common questions grouped by theme, with succinct answers you can memorize and expand during interviews.
What is an API? — An interface for systems to communicate, exposing endpoints and contracts (e.g., JSON over HTTP).
What is REST? — An architectural style using stateless resources, standard HTTP verbs, and resource URIs.
Explain HTTP methods (GET, POST, PUT, DELETE, PATCH). — GET reads, POST creates, PUT replaces, PATCH updates partially, DELETE removes.
What is JSON and why used? — Lightweight, language-agnostic data format that’s easy to parse and transmit.
What are status codes (200, 201, 4xx, 5xx)? — 2xx success, 4xx client errors, 5xx server errors; 201 for created.
What is idempotency? — Repeating the same request produces the same result (important for safe retries).
What are API contracts and schemas? — Definitions (OpenAPI/JSON Schema) that describe endpoints, payloads, and validation rules.
Fundamentals (7)
How do you test an API using Postman? — Create requests, set environments, write tests/assertions, and run collections.
What’s API automation? — Using code (e.g., REST-assured, pytest) or tools to run repeatable API tests in CI.
How do you validate JSON responses? — Use schema validation, field assertions, and status checks.
What are common API test types? — Functional, integration, contract, performance, security, and negative tests.
Which tools for API testing? — Postman, curl, REST-assured, Newman, JMeter, k6.
How do you mock dependencies in API tests? — Use stubs, mock servers, or contract testing (e.g., WireMock).
How to handle flaky API tests? — Add proper waits, retries, isolate environments, and avoid brittle assertions.
API Testing & Tools (7)
How do you secure a REST API? — TLS, authentication, authorization, input validation, rate limiting, logging.
Difference between API keys, OAuth, and JWT? — API keys are simple tokens; OAuth is delegated auth; JWT is a signed token for claims.
What is CORS and why does it matter? — Browser mechanism controlling cross-origin requests; servers add headers to allow origins.
How to prevent common API vulnerabilities? — Sanitize inputs, use prepared statements, enforce auth, and follow least privilege.
What is rate limiting? — Throttling requests per client to protect resources and prevent abuse.
How to log and monitor APIs securely? — Redact sensitive fields, use centralized logging, and monitor anomalies.
Security & Authentication (6)
How to version an API? — URI versioning (/v1), header versioning, or content negotiation with clear deprecation plans.
What are cache-control headers? — Direct browser/CDN caching behavior (e.g., Cache-Control, ETag).
PUT vs PATCH? — PUT replaces resource; PATCH applies partial modifications.
What is HATEOAS? — Hypermedia links in responses guiding clients through available actions.
When to use query params vs. request body? — Query for filtering/pagination; body for resource creation/updates.
REST-Specific Concepts (5)
How did you design or improve an API? — Describe problem, trade-offs, chosen design, and measured outcome.
How did you handle a production API failure? — Explain detection, rollback/patch, root cause, and prevention steps.
How do you prioritize API test cases? — Risk-based: auth, critical flows, data integrity, and common failure modes first.
How do you collaborate with frontend/backends? — Use contracts, mock servers, API docs, and regular syncs.
How do you measure API performance? — Latency percentiles, throughput, error rates, and resource utilization.
Behavioral & Process Questions (5)
Sources & further reading: See practical interview and testing guides from GeeksforGeeks, Browserless, and Simplilearn. Takeaway: Memorize concise answers, but prepare 1–2 short stories showing impact.
How do I answer API testing and tooling questions (Postman, automation) in an interview?
Short answer: Demonstrate both practical steps (how you run tests) and decision-making (why you chose a tool or pattern), with a short demo-like explanation or a mini code snippet described verbally.
Start with a one-line summary: “I use Postman for exploratory and REST-assured/pytest for automation in CI.”
Walk through a concrete example: creating a Postman collection, using environment variables for endpoints/keys, asserting JSON fields, running Newman in CI with reports.
Show technical depth: mention schema validation, contract tests, mocking, and how you handle flaky tests (retries, timeouts).
Cite tools and resources: Postman collections and Newman for CI, REST-assured or requests+pytest for code-based automation, and testRigor or Testleaf for common approaches.
Tips to structure your reply:
Example answer snippet: “In CI I run a smoke collection (Newman) that validates auth, one core happy-path, schema checks, and a negative test; if it fails, the pipeline blocks and triggers a Slack alert.” Takeaway: Be ready to describe one end-to-end flow and the reasons behind your tooling choices.
How should you explain API security, authentication, and CORS in interviews?
Short answer: Explain objectives first (confidentiality, integrity, availability), then map specific controls (TLS, auth types, rate limits) to those objectives, and give examples of trade-offs.
TLS (HTTPS) is non-negotiable for confidentiality and integrity.
Authentication vs authorization: auth proves identity (OAuth2 flows, API keys), authorization enforces permissions (RBAC, scopes).
JWT vs OAuth2: JWTs carry claims and can be stateless; OAuth2 handles delegated access and token lifecycle.
CORS: Describe why browsers block cross-origin requests and how Access-Control-Allow-* headers enable safe cross-origin calls.
Practical hardening: input validation, escaping, SQL injection prevention, parameterized queries, logging, and monitoring.
Rate limiting, quotas, and throttling to prevent abuse, combined with exponential backoff on clients.
Key points to cover:
Reference reading: practical security Q&A from Browserless and security-focused testing guides on GeeksforGeeks. Takeaway: Frame security answers around risk mitigation and show a short, real-world example where you applied one control.
What REST API concepts interviewers expect you to explain (idempotency, versioning, headers)?
Short answer: Expect questions about idempotency, caching, versioning, headers/ETags, and how those decisions affect client behavior and backward compatibility.
Idempotency: PUT and some POST variants should be idempotent when clients retry; recommend idempotency keys for payment endpoints.
Caching/ETag: Use ETag and conditional requests to avoid re-sending full payloads; set Cache-Control for CDNs.
Versioning: Explain URI vs header versioning and describe a deprecation strategy (announce, support overlap, migrate).
Headers and content negotiation: Accept, Content-Type, and Vary headers for different representations (JSON, XML).
Error handling: Use consistent error shapes, structured error codes, and helpful messages to aid debugging.
Explain with examples:
Sources: In-depth REST patterns and examples can be found at Simplilearn and Merge.dev. Takeaway: Combine the concept with a short design decision and an operational implication.
How do I present my experience and behavioral answers for API roles?
Short answer: Use a concise STAR or CAR framework: Situation, Task, Action, Result — quantify impact and highlight trade-offs and learning.
Situation: One sentence context (project scale, team size, business goal).
Task: Concrete responsibility (design API, improve performance, add auth).
Action: Technical steps you led (schema changes, caching, load testing, CI pipelines).
Result: Metrics or outcomes (reduced latency by X%, cut errors by Y%, faster feature delivery).
Practical structure:
Example: “On a payments API, I designed idempotent endpoints and added idempotency keys — this reduced duplicate transactions by 95% and improved customer trust.”
Also be ready to discuss teamwork, cross-functional collaboration, and how you validated changes (contract testing, staging deployments). For practical prompts and model answers, see automation role examples at Testleaf and testRigor. Takeaway: Quantify outcomes and show ownership of technical and process-level improvements.
What study plan and resources should I use to prepare for API interviews?
Short answer: Build a 4–6 week plan combining theory, hands-on practice, security, and mock interviews; use curated resources and code-based exercises.
Week 1 — Fundamentals: HTTP, REST, JSON, status codes, and common patterns. Use articles and reference docs.
Week 2 — Testing & Tools: Hands-on with Postman, Newman, REST-assured, and small automation scripts. Follow tutorials and run collections.
Week 3 — Security & Auth: Study OAuth2, JWTs, TLS, CORS, and rate limiting; do threat-modeling exercises.
Week 4 — REST patterns & Performance: Caching, ETags, versioning, idempotency, load testing with JMeter or k6.
Week 5 — Mock interviews: Do timed answers, whiteboard API design, and behavioral STAR practice.
Week 6 — Polish & review: Revisit weak spots and rehearse 10–15 core Qs with sample answers.
Suggested weekly plan:
Top resources: Practical Q&A and tutorials on GeeksforGeeks, deep REST guides at Browserless, and structured lists from Simplilearn and Merge.dev. Also explore community collections like the GitHub API design questions repo and targeted practice on InterviewBit. Takeaway: Mix reading, hands-on tasks, and mock interviews; quality practice beats quantity.
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What Are the Most Common Questions About This Topic
Q: Can I use Postman in interviews?
A: Yes - demo using a Postman collection, explain environments, tests, and show assertions for responses.
Q: How to prepare for API security questions?
A: Focus on auth (OAuth, JWT), TLS, input validation, rate limiting, CORS, and common vulnerabilities with examples.
Q: What's the difference between PUT and PATCH?
A: PUT replaces a full resource; PATCH applies partial updates. Mention idempotency and use-cases in examples.
Q: How to prepare API automation scripts?
A: Start with test cases, parameterize inputs, validate schemas, use CI, retries/fixtures, mock external services.
Q: How long to prepare for API interviews?
A: Plan 4-6 weeks: fundamentals week, testing/tools week, security week, REST patterns week, mock interviews and review.
(Note: each answer is a concise prep action that you can rehearse in practice sessions.)
Conclusion
Recap: Top interviews test both API theory and practical skills—HTTP basics, testing tools, security, REST patterns, and behavioral storytelling. Prepare by combining concise answers with 1–2 real examples that show impact and measurable results. Structured practice (frameworked answers, mock interviews, and hands-on tests) builds confidence and clarity.
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