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What Is an API

Understand what APIs are and why they matter in backend development.

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What Is an API

An API (Application Programming Interface) is a contract that defines how two software systems communicate. It specifies the requests, responses, and data formats.

Types of APIs

Type Protocol Use Case
REST HTTP/HTTPS Web services, most common
GraphQL HTTP Flexible queries, single endpoint
gRPC HTTP/2 High-performance, inter-service
WebSocket WS/WSS Real-time, bidirectional
SOAP HTTP/XML Legacy enterprise systems

API in Backend Development

Client (Mobile App)
    |
    |  GET /api/products/123
    v
API Gateway / Load Balancer
    |
    v
Backend Service
    |
    +--> Database
    |
    +--> Cache
    |
    v
Response: { "id": 123, "name": "Laptop" }

Why APIs Matter

  1. Separation of concerns — Frontend and backend evolve independently
  2. Reusability — Multiple clients consume the same API
  3. Scalability — Services can be scaled independently
  4. Testability — APIs can be tested in isolation
  5. Integration — Third-party systems can connect

API vs Library vs Framework

Concept What It Is Runs On
Library Code you call Your code
Framework Code that calls you Your code
API Contract over network Remote system

API Best Practices

Design Principles

  • Use nouns, not verbs
  • Plural resource names
  • Consistent naming conventions
  • Proper HTTP status codes

Versioning

  • URI versioning (/v1/resource)
  • Header versioning
  • Deprecation policy

Documentation

  • OpenAPI/Swagger specs
  • Request/Response examples
  • Error code documentation
  • Rate limit documentation

Key Points

  • Understanding What Is an API is essential for production systems
  • Always consider scalability and maintainability
  • Test thoroughly before deploying to production
  • Monitor performance and set up alerting

Common Patterns

  1. Validation: Always validate input at the boundary
  2. Error Handling: Use structured error responses
  3. Logging: Log key events for debugging
  4. Testing: Unit, integration, and load tests
  5. Documentation: Keep docs updated with code changes

Practice Problems

0/3solved
Implement What Is an API

Design and implement a solution for What Is an API in a backend system. Consider scalability, error handling, and production readiness.

Solution
// What Is an API implementation
// Key aspects: validation, error handling, logging, testing

public class WhatIsanAPI {
    // Production-ready implementation
}
What Is an API Edge Cases

Identify and handle edge cases for What Is an API. What happens under high load, with invalid input, or during failures?

Solution
// Edge case handling:
// 1. Null/empty input -> validation
// 2. High load -> rate limiting, queuing
// 3. Failures -> retries, circuit breaker
// 4. Concurrent access -> locks, idempotency
What Is an API Testing Strategy

Write a testing strategy for What Is an API. Include unit tests, integration tests, and performance tests.

Solution
// Test plan:
// - Unit: 80% coverage target
// - Integration: API contracts
// - Performance: latency, throughput
// - Chaos: failure injection

Quiz

1. What is an API?

Question 1 options

2. Which API type is most common for web services?

Question 2 options

3. What is the primary purpose of What Is an API?

Question 3 options

4. What is a common mistake when implementing What Is an API?

Question 4 options

Flashcards

Question

What is an API?

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Answer

A contract defining how two software systems communicate

Question

Most common API type for web services?

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Answer

REST (Representational State Transfer)

Question

What is What Is an API?

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Answer

What Is an API is a key concept in backend development.

Question

When to use What Is an API?

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Answer

Use What Is an API when building production systems that require reliability, scalability, and maintainability.

Question

What Is an API best practices

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Answer

Follow SOLID principles, write clean code, test thoroughly, document decisions, and monitor in production.

Revision Notes

Key Takeaways

  • 1.API = contract for software communication
  • 2.REST is the most common web API type
  • 3.APIs enable separation of concerns and reusability

Interview Tips

  • •Explain what an API is in simple terms
  • •Know the difference between REST, GraphQL, and gRPC

Cheat Sheet

API Basics

  • API: Contract for software communication
  • Types: REST, GraphQL, gRPC, WebSocket, SOAP
  • REST: Most common, uses HTTP methods
  • Benefits: Decoupling, reusability, scalability