API Versioning

Scope: API versioning strategies, breaking changes, deprecation workflow, migration patterns Lines: ~200 Last Updated: 2025-10-18

When to Use This Skill

Activate this skill when:

• Planning API changes and deciding on versioning approach
• Managing multiple API versions in production
• Deprecating old API versions
• Migrating clients to new API versions
• Designing backward compatibility strategies
• Implementing breaking changes safely
• Establishing sunset policies for legacy APIs

Core Concepts

What is API Versioning?

Problem: APIs evolve over time. Changes can break existing clients.

Solution: Maintain multiple API versions simultaneously, allowing clients to migrate gradually.

Benefits:

• Backward compatibility for existing clients
• Safe deployment of breaking changes
• Gradual client migration
• Clear contract between API and clients

Cost:

• Maintenance burden (multiple codebases)
• Testing complexity
• Documentation overhead

---

Versioning Strategies

1. URL Path Versioning

Format: /v1/users, /v2/users

Example:

GET /v1/users/123
GET /v2/users/123

Implementation (FastAPI):

from fastapi import FastAPI

app = FastAPI()

# Version 1
@app.get("/v1/users/{user_id}")
async def get_user_v1(user_id: int):
    return {"id": user_id, "name": "Alice"}

# Version 2 (with email field)
@app.get("/v2/users/{user_id}")
async def get_user_v2(user_id: int):
    return {
        "id": user_id,
        "name": "Alice",
        "email": "alice@example.com"
    }

Pros:

• Simple and explicit
• Easy to route and cache
• Visible in logs and monitoring
• Works with all HTTP clients

Cons:

• URL changes break bookmarks
• Version in URL feels unnatural
• No granular versioning per endpoint

Best for: Public APIs, REST APIs, simple versioning schemes

---

2. Header Versioning

Format: Accept: application/vnd.api.v1+json

Example:

GET /users/123
Accept: application/vnd.api.v2+json

Implementation (FastAPI):

from fastapi import FastAPI, Header, HTTPException

app = FastAPI()

@app.get("/users/{user_id}")
async def get_user(
    user_id: int,
    accept: str = Header(default="application/vnd.api.v1+json")
):
    if "v2" in accept:
        return {
            "id": user_id,
            "name": "Alice",
            "email": "alice@example.com"
        }
    elif "v1" in accept:
        return {"id": user_id, "name": "Alice"}
    else:
        raise HTTPException(400, "Unsupported API version")

Pros:

• Clean URLs (no version pollution)
• RESTful (resource unchanged, representation varies)
• Granular control per request

Cons:

• Less discoverable (hidden in headers)
• Harder to test (need header support)
• Caching complexity

Best for: Internal APIs, strict REST adherence, content negotiation

---

3. Query Parameter Versioning

Format: /users?api_version=2

Example:

GET /users/123?api_version=2

Implementation (FastAPI):

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/users/{user_id}")
async def get_user(
    user_id: int,
    api_version: int = Query(default=1)
):
    if api_version == 2:
        return {
            "id": user_id,
            "name": "Alice",
            "email": "alice@example.com"
        }
    else:
        return {"id": user_id, "name": "Alice"}

Pros:

• Simple to implement
• Easy to test (just add parameter)
• Works with all clients

Cons:

• Version in query string feels unnatural
• Pollutes URL space
• Can conflict with other parameters

Best for: Simple internal APIs, quick prototypes

---

4. Custom Header Versioning

Format: X-API-Version: 2

Example:

GET /users/123
X-API-Version: 2

Implementation (FastAPI):

from fastapi import FastAPI, Header

app = FastAPI()

@app.get("/users/{user_id}")
async def get_user(
    user_id: int,
    x_api_version: int = Header(default=1)
):
    if x_api_version == 2:
        return {
            "id": user_id,
            "name": "Alice",
            "email": "alice@example.com"
        }
    else:
        return {"id": user_id, "name": "Alice"}

Pros:

• Clean URLs
• Explicit versioning
• Easy to add middleware

Cons:

• Custom headers not standard
• Requires client header support
• Less discoverable

Best for: Internal APIs, microservices, controlled environments

---

Breaking vs Non-Breaking Changes

Non-Breaking Changes (Safe)

Can deploy without versioning:

✅ Adding optional fields to request:

// Before
{"name": "Alice"}

// After (optional email)
{"name": "Alice", "email": "alice@example.com"}

✅ Adding new fields to response:

// Before
{"id": 1, "name": "Alice"}

// After (added email)
{"id": 1, "name": "Alice", "email": "alice@example.com"}

✅ Adding new endpoints:

POST /v1/users        (existing)
POST /v1/users/bulk   (new, safe)

✅ Adding new optional query parameters ✅ Expanding enum values (if clients ignore unknown) ✅ Relaxing validation (accepting more input)

---

Breaking Changes (Require Versioning)

Must introduce new version:

❌ Removing fields from response:

// Before
{"id": 1, "name": "Alice", "email": "alice@example.com"}

// After (removed email) - BREAKS CLIENTS
{"id": 1, "name": "Alice"}

❌ Renaming fields:

// Before
{"user_id": 1}

// After - BREAKS CLIENTS
{"id": 1}

❌ Changing field types:

// Before
{"created_at": "2025-01-15"}

// After - BREAKS CLIENTS
{"created_at": 1736899200}

❌ Making optional field required ❌ Removing endpoints ❌ Changing URL structure ❌ Stricter validation (rejecting previously valid input) ❌ Changing authentication scheme

---

Breaking Change Checklist

Before introducing breaking change:

[ ] Identified all affected clients
[ ] Planned new version number/identifier
[ ] Implemented both old and new versions
[ ] Created migration guide for clients
[ ] Set deprecation timeline (e.g., 6 months)
[ ] Added deprecation warnings to old version
[ ] Updated documentation
[ ] Communicated timeline to stakeholders
[ ] Monitored usage of old version
[ ] Planned sunset date for old version

---

Deprecation Workflow

Phase 1: Announce (Month 0)

Actions:

1. Document breaking changes
2. Announce deprecation timeline
3. Add deprecation headers to old version

Example (FastAPI):

from fastapi import Response

@app.get("/v1/users/{user_id}")
async def get_user_v1(user_id: int, response: Response):
    # Add deprecation warning
    response.headers["Deprecation"] = "true"
    response.headers["Sunset"] = "2025-07-01"
    response.headers["Link"] = '</v2/users>; rel="successor-version"'

    return {"id": user_id, "name": "Alice"}

Communication:

Subject: API v1 Deprecation Notice

We're deprecating /v1/users in favor of /v2/users.

Timeline:
- Today: v2 available, v1 still supported
- Month 3: Deprecation warnings added to v1
- Month 6: v1 sunset (will return 410 Gone)

Migration guide: https://docs.api.com/v1-to-v2

Questions? Contact api-support@example.com

---

Phase 2: Warn (Month 3)

Actions:

1. Log clients still using old version
2. Send targeted emails to heavy users
3. Add warning in response body

Example:

@app.get("/v1/users/{user_id}")
async def get_user_v1(user_id: int, response: Response):
    response.headers["X-Deprecation-Warning"] = "v1 will sunset on 2025-07-01"

    return {
        "id": user_id,
        "name": "Alice",
        "_deprecation": {
            "message": "This endpoint will be removed on 2025-07-01",
            "migration_guide": "https://docs.api.com/v1-to-v2"
        }
    }

---

Phase 3: Sunset (Month 6)

Actions:

1. Return 410 Gone for old version
2. Redirect to migration guide
3. Monitor for stragglers

Example:

@app.get("/v1/users/{user_id}")
async def get_user_v1(user_id: int):
    raise HTTPException(
        status_code=410,
        detail={
            "error": "This API version has been sunset",
            "sunset_date": "2025-07-01",
            "migration_guide": "https://docs.api.com/v1-to-v2",
            "new_endpoint": "/v2/users/{user_id}"
        }
    )

---

Deprecation Timeline Template

Month 0: Announcement
  - Release v2 alongside v1
  - Announce deprecation timeline
  - Update documentation

Month 1-2: Migration Period
  - Monitor v1 usage
  - Provide migration support
  - Send reminder emails

Month 3: Warning Phase
  - Add deprecation headers
  - Log clients using v1
  - Contact heavy users directly

Month 4-5: Final Warning
  - Increase warning visibility
  - Offer migration assistance
  - Set hard sunset date

Month 6: Sunset
  - Return 410 Gone for v1
  - Redirect to migration guide
  - Monitor for issues

Adjust timeline based on:

• API criticality (longer for critical APIs)
• Number of clients (longer for many clients)
• Migration complexity (longer for complex changes)

Recommended timelines:

• Internal APIs: 3 months
• Public APIs: 6-12 months
• Critical APIs: 12-24 months

---

Version Migration Patterns

Pattern 1: Parallel Versions

Strategy: Run old and new versions side-by-side.

# v1/users.py
@router.get("/v1/users/{user_id}")
async def get_user_v1(user_id: int):
    return {"id": user_id, "name": "Alice"}

# v2/users.py
@router.get("/v2/users/{user_id}")
async def get_user_v2(user_id: int):
    return {
        "id": user_id,
        "name": "Alice",
        "email": "alice@example.com"
    }

Pros: Clean separation, easy rollback Cons: Code duplication, maintenance burden

---

Pattern 2: Adapter Pattern

Strategy: Single core implementation, adapters for each version.

# core/users.py
def get_user_data(user_id: int):
    return {
        "id": user_id,
        "name": "Alice",
        "email": "alice@example.com"
    }

# api/v1.py
@app.get("/v1/users/{user_id}")
async def get_user_v1(user_id: int):
    data = get_user_data(user_id)
    # Adapter: remove email for v1
    return {"id": data["id"], "name": data["name"]}

# api/v2.py
@app.get("/v2/users/{user_id}")
async def get_user_v2(user_id: int):
    return get_user_data(user_id)  # Full data

Pros: Single source of truth, less duplication Cons: Adapter complexity increases over time

---

Pattern 3: Feature Flags

Strategy: Use flags to toggle new behavior.

from functools import wraps

def version_aware(func):
    @wraps(func)
    async def wrapper(user_id: int, version: int = 1):
        data = await func(user_id)

        if version == 1:
            # Remove new fields for v1
            return {k: v for k, v in data.items() if k in ["id", "name"]}
        else:
            return data

    return wrapper

@version_aware
async def get_user(user_id: int):
    return {
        "id": user_id,
        "name": "Alice",
        "email": "alice@example.com"
    }

Pros: Single endpoint, flexible Cons: Complexity in logic, harder to test

---

Backward Compatibility Techniques

Technique 1: Additive Changes Only

Rule: Only add, never remove or change.

// Version 1
{"id": 1, "name": "Alice"}

// Version 1.1 (backward compatible)
{"id": 1, "name": "Alice", "email": "alice@example.com"}

// Version 1.2 (still backward compatible)
{"id": 1, "name": "Alice", "email": "alice@example.com", "phone": "555-1234"}

Clients ignore unknown fields → No breaking changes.

---

Technique 2: Default Values

Strategy: Provide defaults for new required fields.

from pydantic import BaseModel, Field

class User(BaseModel):
    id: int
    name: str
    email: str = Field(default="noreply@example.com")  # Default for old clients

Old requests (no email) → Use default New requests (with email) → Use provided value

---

Technique 3: Field Aliases

Strategy: Support both old and new field names.

from pydantic import BaseModel, Field

class User(BaseModel):
    user_id: int = Field(alias="id")  # Accept both "id" and "user_id"
    name: str

Accepts:

{"id": 1, "name": "Alice"}         // Old clients
{"user_id": 1, "name": "Alice"}    // New clients

---

Versioning Strategy Comparison

Recommendation: Use URL Path Versioning for simplicity and discoverability unless strict REST compliance required.

---

Related Skills

• fastapi-routing.md - Organizing API routes and versioning
• api-design-patterns.md - RESTful API design principles
• api-documentation.md - Documenting versioned APIs (OpenAPI/Swagger)
• database-migrations.md - Versioning database schemas alongside APIs
• feature-flags.md - Using feature flags for gradual rollouts

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Last Updated: 2025-10-18 Format Version: 1.0 (Atomic)