Expert ASP.NET Web API Interview Questions

Resilience protects APIs from transient failures and cascading outages:

• Use Polly for retry, circuit breaker, timeout, fallback, and bulkhead isolation policies.
• Retry only for safe operations (idempotent GET or PUT).
• Use exponential backoff and jitter to avoid retry storms.
• Wrap external API calls, DB operations, and messaging clients with policies.
• Monitor policy behavior to detect fragile dependencies.

Versioned Swagger ensures clarity when multiple API versions coexist:

• Configure multiple SwaggerDoc groups in AddSwaggerGen().
• Use ApiExplorerSettings to group endpoints by version.
• Apply versioned routes (v1, v2) or header/query-based versioning.
• Expose separate Swagger UI endpoints for each version.
• Helps clients migrate safely while maintaining backward compatibility.

Consistent querying ensures scalable and predictable API behavior:

• Use query parameters: page, pageSize, sort, filter.
• Implement logic in repositories or service layer for maintainability.
• Combine pagination with filtering and sorting before DB execution.
• Cache frequently accessed listings for performance.
• Return metadata: total records, total pages, current page, page size.

HATEOAS improves API discoverability and client navigation:

• Embed hypermedia links (self, edit, delete, related resources) in response models.
• Generate URLs using UrlHelper or route names.
• Encapsulate link-building logic in DTOs or dedicated services.
• Useful in large RESTful ecosystems where clients discover workflows dynamically.

Correlation IDs enable tracking a request across microservices:

• Generate or read a CorrelationId header in middleware.
• Pass it to downstream services and logs.
• Use logging scopes so all log entries share the same ID.
• Send correlationId in the API response for client troubleshooting.
• Critical for debugging distributed API flows.

Response compression improves performance for large payloads:

• Enable AddResponseCompression() middleware.
• Use Gzip or Brotli for optimal compression ratios.
• Whitelisted MIME types ensure only necessary content is compressed.
• Compression reduces bandwidth use and improves client response times.

Effective testing ensures correctness, stability, and performance:

• Unit test controllers using mocked services (Moq, NSubstitute).
• Use WebApplicationFactory or TestServer for integration tests.
• Use Postman/Newman or CI pipelines for end-to-end validation.
• Validate headers, status codes, responses, and edge cases.
• Load test APIs using JMeter, k6, or Locust to verify scalability.

Backward compatibility prevents breaking existing clients:

• Version APIs instead of modifying existing contracts.
• Deprecate older versions gradually with communication.
• Use DTOs to control exposed data without altering entities.
• Document breaking changes and provide migration guides.
• Avoid removing fields abruptly; mark them as deprecated first.