Each test is 5 questions with varying difficulty.
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RESTful API design remains the foundational architectural style for modern web services in 2026. Despite the rise of gRPC and GraphQL, REST's reliance on standard HTTP semantics makes it the primary choice for public-facing APIs and microservices communication. Interviewers focus on RESTful design to assess a candidate's ability to model complex domain logic into intuitive, scalable, and predictable resource-based endpoints. At the junior level, candidates are expected to understand standard HTTP methods and status codes. Senior engineers are tested on advanced architectural concerns: designing for idempotency in distributed systems, managing complex resource relationships, implementing versioning without breaking clients, and ensuring API discoverability via HATEOAS. Mastery of these concepts demonstrates a deep understanding of network protocols, system consistency, and developer experience (DX).
RESTful API design is the interface between business logic and the external world. A well-designed API reduces integration friction, minimizes client-side bugs, and ensures long-term system maintainability. In 2026, as systems become increasingly distributed, the ability to design APIs that handle concurrent updates gracefully—using idempotency keys and proper status codes—is critical for preventing data corruption. For example, a poorly designed payment endpoint without idempotency can lead to duplicate charges, directly impacting revenue. Interviewers use this topic to gauge whether a candidate thinks about the 'consumer' of the code. A strong answer reflects an understanding of the trade-offs between strict REST compliance and practical performance needs, such as when to use a POST request for a non-idempotent search query versus a GET request. This topic is high-signal because it reveals how a candidate handles the tension between theoretical purity and real-world system constraints.
The RESTful execution model relies on the stateless request-response cycle where the server acts as a resource provider. The client initiates a request, which is routed through an API Gateway or Load Balancer, hitting the application logic which interacts with the persistence layer before returning a representation.
[Client Request]
↓
[API Gateway / Load Balancer]
↓
[Resource Controller (Routing)]
↓
[Service Layer (Logic/Validation)]
↓ ↓
[Cache Layer] [Persistence Layer]
↓ ↓
[Response Serialization]
↓
[HTTP Response (2xx/4xx/5xx)]Client sends a unique header (e.g., X-Idempotency-Key) which the server stores to prevent duplicate processing.
Trade-offs: Increases storage overhead on the server; requires cleanup strategies.
Using query parameters like ?expand=author to fetch related resources in a single request.
Trade-offs: Reduces network round trips but increases server-side join complexity.
Using custom headers (e.g., X-API-Version: 2) to route requests to specific handlers.
Trade-offs: Cleaner URLs but harder to cache and debug via browser.
| Reliability | Use idempotency keys for all state-changing operations and implement circuit breakers for downstream service calls. |
| Scalability | Stateless design allows horizontal scaling; use CDNs for caching GET responses. |
| Performance | Use field filtering, pagination, and compression (Gzip/Brotli) to reduce payload size. |
| Cost | Minimize egress costs by reducing payload size and using efficient serialization like Protobuf where applicable. |
| Security | Enforce mTLS, validate JWTs, and sanitize all inputs at the API Gateway level. |
| Monitoring | Track latency (p99), error rates (4xx vs 5xx), and throughput per endpoint. |
PUT is used for full resource replacement, where the client sends the entire representation of the resource. PATCH is used for partial updates, where the client sends only the fields that need to be changed. PUT is idempotent, while PATCH may or may not be, depending on the implementation.
REST is stateless because each request from a client to a server must contain all the information necessary for the server to understand and complete the request. The server does not store any client context between requests, which simplifies scaling and improves reliability.
HATEOAS stands for Hypermedia as the Engine of Application State. It means the API returns links that guide the client on what actions are available next. It is often skipped because it adds complexity to both the server and client, and many developers prefer simpler JSON responses.
Common strategies include URI versioning (e.g., /v1/users), header-based versioning (e.g., X-API-Version: 1), or media type versioning (e.g., Accept: application/vnd.myapi.v1+json). URI versioning is the most common and easiest to cache, while header versioning is cleaner but harder to debug.
No. REST is an architectural style that uses HTTP as its primary transport protocol. While RESTful APIs heavily rely on HTTP methods, status codes, and headers, you could technically implement REST over other protocols, though it is rarely done.
An idempotent request is one that can be made multiple times without changing the result beyond the initial application. For example, a DELETE request is idempotent because deleting a resource once or ten times results in the same state (the resource is gone).
Use GET for retrieving data where the request is safe and idempotent. Use POST for actions that change server state, are not idempotent, or involve sensitive data that should not appear in URL query parameters.
Use eager loading (e.g., JOINs in SQL) to fetch related resources in a single query, or implement field filtering/expansion to allow clients to request only the necessary related data, reducing the number of round trips to the database.
The 204 No Content status code indicates that the request was successfully processed, but there is no body content to return. This is commonly used for successful DELETE requests or updates where the client already has the necessary information.
REST is inherently request-response, while WebSockets are full-duplex, persistent connections. You can use them together by using REST for initial setup or resource management and WebSockets for real-time updates, but they serve different communication patterns.
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