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beginner Phase · HTTP and Web Fundamentals

PATCH

Deep dive into the PATCH method for partial updates.

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The PATCH Method

PATCH applies partial modifications to a resource. Unlike PUT, you only send the fields you want to change.

PATCH Characteristics

PATCH /api/users/123 HTTP/1.1
Host: api.example.com
Content-Type: application/json

{
  "email": "alice.updated@example.com"
}

→ Not safe: Modifies server state
→ Not idempotent (in general)
→ Not cacheable by default
→ Has a request body (partial resource)

PATCH vs PUT

Aspect PATCH PUT
Purpose Partial update Full replacement
Body Only changed fields Complete resource
Idempotent Generally no Yes
Network efficiency More efficient Less efficient

PATCH Examples

# Update only email
PATCH /api/users/123
Content-Type: application/json
{"email": "new@example.com"}

# Update multiple fields
PATCH /api/orders/456
Content-Type: application/json
{"status": "SHIPPED", "trackingNumber": "1Z999AA10123456784"}

# JSON Patch (RFC 6902)
PATCH /api/users/123
Content-Type: application/json-patch+json
[
  {"op": "replace", "path": "/email", "value": "new@example.com"},
  {"op": "remove", "path": "/phone"}
]

Making PATCH Idempotent

While PATCH isn't inherently idempotent, you can make it idempotent by:

  • Storing the full state and re-applying the same changes
  • Using a unique request ID to deduplicate
// Idempotent PATCH (replaces with same result)
PATCH /api/users/123
{"email": "new@example.com"}
// Server stores: "email update from old to new"
// Same request again → no-op (already applied)

PATCH Design Best Practices

  1. Return the updated resource after PATCH
  2. Support JSON Merge Patch (RFC 7396) for simple updates
  3. Consider JSON Patch (RFC 6902) for complex operations
  4. Validate partial updates — Ensure required fields aren't removed

HTTP Best Practices

Methods

  • GET: Read (safe, idempotent)
  • POST: Create
  • PUT: Replace (idempotent)
  • PATCH: Partial update
  • DELETE: Remove (idempotent)

Headers

  • Content-Type: Body format
  • Cache-Control: Caching rules
  • Authorization: Authentication
  • Accept: Desired response format

Status Codes

  • 2xx: Success
  • 3xx: Redirection
  • 4xx: Client error
  • 5xx: Server error

Key Points

  • Understanding PATCH 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 / 3 solved
Implement PATCH

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

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

public class PATCH {
    // Production-ready implementation
}
PATCH Edge Cases

Identify and handle edge cases for PATCH. 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
PATCH Testing Strategy

Write a testing strategy for PATCH. 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 PATCH used for?

Question 1 options

2. Is PATCH idempotent by default?

Question 2 options

3. What is a common mistake when implementing PATCH?

Question 3 options

Flashcards

Question

What does PATCH do?

Click to reveal answer

Answer

Partially updates a resource — sends only changed fields

Question

PATCH vs PUT?

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Answer

PATCH = partial update; PUT = full replacement

Question

PATCH 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. PATCH applies partial updates to a resource
  • 2. PATCH is not idempotent by default
  • 3. Only send changed fields in the request body
  • 4. JSON Patch (RFC 6902) and JSON Merge Patch (RFC 7396) are standards

Interview Tips

  • • Know when to use PATCH vs PUT
  • • Understand JSON Patch format

Cheat Sheet

PATCH

  • Purpose: Partial update (only changed fields)
  • Properties: Not safe, Not idempotent
  • Standards: JSON Patch (RFC 6902), JSON Merge Patch (RFC 7396)
  • vs PUT: PATCH=partial, PUT=full replacement