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beginner Phase · HTTP and Web Fundamentals

Request Headers

Learn essential HTTP request headers and their purposes.

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Essential Request Headers

HTTP request headers carry metadata from the client to the server. Understanding them is critical for building APIs.

Essential Request Headers

GET /api/users HTTP/1.1
Host: api.example.com
Authorization: Bearer eyJhbG...
Accept: application/json
Content-Type: application/json
User-Agent: MyApp/2.0
Cache-Control: no-cache
Cookie: session=abc123
X-Request-ID: req-789

Header Categories

Category Headers Purpose
Host Host Target domain (required in HTTP/1.1)
Auth Authorization, Cookie Authentication credentials
Content Content-Type, Content-Length Request body metadata
Accept Accept, Accept-Language What client can receive
Cache Cache-Control, If-None-Match Caching directives
Security X-Forwarded-For, X-Request-ID Security and tracing

Authentication Headers

# Bearer Token (JWT)
Authorization: Bearer eyJhbGciOiJSUzI1NiIs...

# Basic Auth (base64 encoded)
Authorization: Basic dXNlcjpwYXNz

# API Key
X-API-Key: my-api-key-123

Content Negotiation Headers

# What client accepts
Accept: application/json
Accept-Language: en-US,en;q=0.9
Accept-Encoding: gzip, deflate, br

# What client sends
Content-Type: application/json
Content-Language: en-US
Content-Encoding: gzip

Caching Headers

# Client-side caching
Cache-Control: no-cache
If-None-Match: "abc123"
If-Modified-Since: Wed, 15 Aug 2026 10:00:00 GMT

Custom Headers

Many APIs use custom headers:

X-Request-ID: req-123        # Request tracing
X-Idempotency-Key: idem-456  # Prevent duplicate operations
X-Client-Version: 2.0        # Client version info

Security Headers

Security-Critical Headers

GET /api/sensitive-data HTTP/1.1
Host: api.example.com
Authorization: Bearer <token>
X-Forwarded-For: 203.0.113.50
X-Real-IP: 203.0.113.50
X-Request-ID: req-abc-123

Headers for Tracing

{
  "X-Request-ID": "req-abc-123",
  "X-B3-TraceId": "trace-456",
  "X-B3-SpanId": "span-789"
}

These headers enable distributed tracing across microservices.

Rate Limiting Headers

Clients should respect rate limit headers:

X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 999
X-RateLimit-Reset: 1692105600

Practice Problems

0 / 3 solved
Implement Request Headers

Design and implement a solution for Request Headers in a backend system. Consider scalability, error handling, and production readiness.

Solution 
// Request Headers implementation
// Key aspects: validation, error handling, logging, testing

public class RequestHeaders {
    // Production-ready implementation
}
Request Headers Edge Cases

Identify and handle edge cases for Request Headers. 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
Request Headers Testing Strategy

Write a testing strategy for Request Headers. 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. Which header is required in every HTTP/1.1 request?

Question 1 options

2. What header is used for JWT authentication?

Question 2 options

3. What is a common mistake when implementing Request Headers?

Question 3 options

Flashcards

Question

What is the Authorization header used for?

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Answer

Passing authentication credentials (JWT, API key, Basic Auth)

Question

What header is required in HTTP/1.1?

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Answer

Host header — specifies the target domain

Question

Request Headers 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. Host header is required in HTTP/1.1
  • 2. Authorization header carries auth credentials
  • 3. Accept header defines content negotiation
  • 4. Custom headers enable tracing and idempotency

Interview Tips

  • • Know the difference between Authorization and Content-Type
  • • Understand how JWT tokens are passed in headers

Cheat Sheet

Request Headers

  • Host: Required (target domain)
  • Authorization: Auth credentials (Bearer, Basic, API Key)
  • Accept: What client accepts (content negotiation)
  • Content-Type: What client sends
  • X-Request-ID: Distributed tracing