API Versioning Strategies

Design APIs that evolve gracefully without breaking clients.

When to Use

• Launching a public API
• Planning breaking changes
• Supporting multiple client versions
• Implementing deprecation policies
• Designing API evolution strategy

Strategy Comparison

URL Path Versioning (Recommended)

FastAPI Structure

backend/app/
├── api/
│   ├── v1/
│   │   ├── __init__.py
│   │   ├── routes/
│   │   │   ├── users.py
│   │   │   └── analyses.py
│   │   └── router.py
│   ├── v2/
│   │   ├── __init__.py
│   │   ├── routes/
│   │   │   ├── users.py      # Updated schemas
│   │   │   └── analyses.py
│   │   └── router.py
│   └── router.py             # Combines all versions

Router Setup

# backend/app/api/router.py
from fastapi import APIRouter
from app.api.v1.router import router as v1_router
from app.api.v2.router import router as v2_router

api_router = APIRouter()
api_router.include_router(v1_router, prefix="/v1")
api_router.include_router(v2_router, prefix="/v2")

# main.py
app.include_router(api_router, prefix="/api")

Version-Specific Schemas

# v1/schemas/user.py
class UserResponseV1(BaseModel):
    id: str
    name: str  # Single name field

# v2/schemas/user.py
class UserResponseV2(BaseModel):
    id: str
    first_name: str  # Split into first/last
    last_name: str
    full_name: str   # Computed for convenience

Shared Business Logic

# services/user_service.py (version-agnostic)
class UserService:
    async def get_user(self, user_id: str) -> User:
        return await self.repo.get_by_id(user_id)

# v1/routes/users.py
@router.get("/{user_id}", response_model=UserResponseV1)
async def get_user_v1(user_id: str, service: UserService = Depends()):
    user = await service.get_user(user_id)
    return UserResponseV1(id=user.id, name=user.full_name)

# v2/routes/users.py
@router.get("/{user_id}", response_model=UserResponseV2)
async def get_user_v2(user_id: str, service: UserService = Depends()):
    user = await service.get_user(user_id)
    return UserResponseV2(
        id=user.id,
        first_name=user.first_name,
        last_name=user.last_name,
        full_name=f"{user.first_name} {user.last_name}",
    )

Header-Based Versioning

from fastapi import Header, HTTPException

async def get_api_version(
    x_api_version: str = Header(default="1", alias="X-API-Version")
) -> int:
    try:
        version = int(x_api_version)
        if version not in [1, 2]:
            raise ValueError()
        return version
    except ValueError:
        raise HTTPException(400, "Invalid API version")

@router.get("/users/{user_id}")
async def get_user(
    user_id: str,
    version: int = Depends(get_api_version),
    service: UserService = Depends(),
):
    user = await service.get_user(user_id)

    if version == 1:
        return UserResponseV1(id=user.id, name=user.full_name)
    else:
        return UserResponseV2(
            id=user.id,
            first_name=user.first_name,
            last_name=user.last_name,
        )

Content Negotiation

from fastapi import Request

MEDIA_TYPES = {
    "application/vnd.skillforge.v1+json": 1,
    "application/vnd.skillforge.v2+json": 2,
    "application/json": 2,  # Default to latest
}

async def get_version_from_accept(request: Request) -> int:
    accept = request.headers.get("Accept", "application/json")
    return MEDIA_TYPES.get(accept, 2)

@router.get("/users/{user_id}")
async def get_user(
    user_id: str,
    version: int = Depends(get_version_from_accept),
):
    ...

Deprecation Headers

from fastapi import Response
from datetime import date

def add_deprecation_headers(
    response: Response,
    deprecated_date: date,
    sunset_date: date,
    link: str,
):
    response.headers["Deprecation"] = deprecated_date.isoformat()
    response.headers["Sunset"] = sunset_date.isoformat()
    response.headers["Link"] = f'<{link}>; rel="successor-version"'

# Usage in v1 endpoints
@router.get("/users/{user_id}")
async def get_user_v1(user_id: str, response: Response):
    add_deprecation_headers(
        response,
        deprecated_date=date(2025, 1, 1),
        sunset_date=date(2025, 7, 1),
        link="https://api.example.com/v2/users",
    )
    return await service.get_user(user_id)

Version Lifecycle

┌─────────────────────────────────────────────────────────────────┐
│                     VERSION LIFECYCLE                           │
├─────────────────────────────────────────────────────────────────┤
│                                                                 │
│  ┌─────────┐   ┌─────────┐   ┌──────────┐   ┌─────────────┐   │
│  │  ALPHA  │ → │  BETA   │ → │  STABLE  │ → │ DEPRECATED  │   │
│  │ (dev)   │   │ (test)  │   │ (prod)   │   │ (sunset)    │   │
│  └─────────┘   └─────────┘   └──────────┘   └─────────────┘   │
│                                                                 │
│  v3-alpha      v3-beta        v2 (current)   v1 (6 months)     │
│                                                                 │
├─────────────────────────────────────────────────────────────────┤
│  POLICY:                                                        │
│  • Deprecation notice: 3 months before sunset                   │
│  • Sunset period: 6 months after deprecation                    │
│  • Support: Latest stable + 1 previous version                  │
└─────────────────────────────────────────────────────────────────┘

Breaking vs Non-Breaking Changes

Non-Breaking (No Version Bump)

# Adding optional fields
class UserResponse(BaseModel):
    id: str
    name: str
    avatar_url: str | None = None  # New optional field

# Adding new endpoints
@router.get("/users/{user_id}/preferences")  # New endpoint

# Adding optional query params
@router.get("/users")
async def list_users(
    limit: int = 100,
    cursor: str | None = None,  # New pagination
):

Breaking (Requires Version Bump)

# Removing fields
# Renaming fields
# Changing field types
# Changing URL structure
# Changing authentication
# Removing endpoints
# Changing error formats

OpenAPI Per Version

from fastapi import FastAPI
from fastapi.openapi.utils import get_openapi

def custom_openapi_v1():
    return get_openapi(
        title="SkillForge API",
        version="1.0.0",
        routes=v1_router.routes,
    )

def custom_openapi_v2():
    return get_openapi(
        title="SkillForge API",
        version="2.0.0",
        routes=v2_router.routes,
    )

app.mount("/docs/v1", create_docs_app(custom_openapi_v1))
app.mount("/docs/v2", create_docs_app(custom_openapi_v2))

Anti-Patterns (FORBIDDEN)

# NEVER version internal implementation
class UserServiceV1:  # Services should be version-agnostic
    ...

# NEVER break contracts without versioning
class UserResponse(BaseModel):
    # Changed from `name` to `full_name` without version bump!
    full_name: str

# NEVER sunset without notice
# Just removing v1 routes one day

# NEVER support too many versions (max 2-3)
/api/v1/...  # Ancient
/api/v2/...
/api/v3/...
/api/v4/...  # Too many!

Key Decisions

Related Skills

• api-design-framework - REST API patterns
• error-handling-rfc9457 - Consistent errors across versions
• observability-monitoring - Version usage metrics

Capability Details

url-versioning

Keywords: url version, path version, /v1/, /v2/ Solves:

• How to version REST APIs?
• URL-based API versioning

header-versioning

Keywords: header version, X-API-Version, custom header Solves:

• Clean URL versioning
• Header-based API version

deprecation

Keywords: deprecation, sunset, version lifecycle, backward compatible Solves:

• How to deprecate API versions?
• Version sunset policy

breaking-changes

Keywords: breaking change, non-breaking, backward compatible Solves:

• What requires a version bump?
• Breaking vs non-breaking changes