API Versioning Patterns

Evolve your API confidently. Version correctly, deprecate gracefully, migrate safely — without breaking existing consumers.

Versioning Strategies

Pick one strategy and apply it consistently across your entire API surface.

URL Path Versioning

The most common strategy. Version lives in the URL, making it immediately visible.

from fastapi import FastAPI, APIRouter

v1 = APIRouter(prefix="/api/v1")
v2 = APIRouter(prefix="/api/v2")

@v1.get("/users")
async def list_users_v1():
    return {"users": [...]}

@v2.get("/users")
async def list_users_v2():
    return {"data": {"users": [...]}, "meta": {...}}

app = FastAPI()
app.include_router(v1)
app.include_router(v2)

Rules:

• Always prefix: /api/v1/... not /v1/api/...
• Major version only: /api/v1/, never /api/v1.2/ or /api/v1.2.3/
• Every endpoint must be versioned — no mixing versioned and unversioned paths

Header Versioning

Version specified via request headers, keeping URLs clean.

function versionRouter(req, res, next) {
  const version = req.headers['accept-version'] || 'v2'; // default to latest
  req.apiVersion = version;
  next();
}

app.get('/api/users', versionRouter, (req, res) => {
  if (req.apiVersion === 'v1') return res.json({ users: [...] });
  if (req.apiVersion === 'v2') return res.json({ data: { users: [...] }, meta: {} });
  return res.status(400).json({ error: `Unsupported version: ${req.apiVersion}` });
});

Always define fallback behavior when no version header is sent — default to latest stable or return 400 Bad Request.

Semantic Versioning for APIs

Only MAJOR versions appear in URL paths. Communicate MINOR and PATCH through changelogs.

Breaking vs Non-Breaking Changes

Breaking — Require New Version

Non-Breaking — Safe Under Same Version

Deprecation Strategy

Never remove a version without warning. Follow this timeline:

Phase 1: ANNOUNCE
  • Sunset header on responses  • Changelog entry
  • Email/webhook to consumers  • Docs marked "deprecated"

Phase 2: SUNSET PERIOD
  • v1 still works but warns     • Monitor v1 traffic
  • Contact remaining consumers  • Provide migration support

Phase 3: REMOVAL
  • v1 returns 410 Gone
  • Response body includes migration guide URL
  • Redirect docs to v2

Minimum deprecation periods: Public API: 12 months · Partner API: 6 months · Internal API: 1–3 months

Sunset HTTP Header (RFC 8594)

Include on every response from a deprecated version:

HTTP/1.1 200 OK
Sunset: Sat, 01 Mar 2025 00:00:00 GMT
Deprecation: true
Link: <https://api.example.com/docs/migrate-v1-v2>; rel="sunset"
X-API-Warn: "v1 is deprecated. Migrate to v2 by 2025-03-01."

Retired Version Response

When past sunset, return 410 Gone:

{
  "error": "VersionRetired",
  "message": "API v1 was retired on 2025-03-01.",
  "migration_guide": "https://api.example.com/docs/migrate-v1-v2",
  "current_version": "v2"
}

Migration Patterns

Adapter Pattern

Shared business logic, version-specific serialization:

class UserService:
    async def get_user(self, user_id: str) -> User:
        return await self.repo.find(user_id)

def to_v1(user: User) -> dict:
    return {"id": user.id, "name": user.full_name, "email": user.email}

def to_v2(user: User) -> dict:
    return {
        "id": user.id,
        "name": {"first": user.first_name, "last": user.last_name},
        "emails": [{"address": e, "primary": i == 0} for i, e in enumerate(user.emails)],
        "created_at": user.created_at.isoformat(),
    }

Facade Pattern

Single entry point delegates to the correct versioned handler:

async def get_user(user_id: str, version: int):
    user = await user_service.get_user(user_id)
    serializers = {1: to_v1, 2: to_v2}
    serialize = serializers.get(version)
    if not serialize:
        raise UnsupportedVersionError(version)
    return serialize(user)

Versioned Controllers

Separate controller files per version, shared service layer:

api/
  v1/
    users.py      # v1 request/response shapes
    orders.py
  v2/
    users.py      # v2 request/response shapes
    orders.py
  services/
    user_service.py   # version-agnostic business logic
    order_service.py

API Gateway Routing

Route versions at infrastructure layer:

routes:
  - match: /api/v1/*
    upstream: api-v1-service:8080
  - match: /api/v2/*
    upstream: api-v2-service:8080

Multi-Version Support

Architecture:

Request → API Gateway → Version Router → v1 Handler → Shared Service Layer → DB
                                        → v2 Handler ↗

Principles:

1. Business logic is version-agnostic. Services, repositories, and domain models are shared.
2. Serialization is version-specific. Each version has its own request validators and response serializers.
3. Transformations are explicit. A v1_to_v2 transformer documents every field mapping.
4. Tests cover all active versions. Every supported version has its own integration test suite.

Maximum concurrent versions: 2–3 active (current + 1–2 deprecated). More than 3 creates unsustainable maintenance burden.

Client Communication

Changelog

Publish a changelog for every release, tagged by version and change type:

## v2.3.0 — 2025-02-01
### Added
- `avatar_url` field on User response
- `GET /api/v2/users/{id}/activity` endpoint
### Deprecated
- `name` field on User — use `first_name` and `last_name` (removal in v3)

Migration Guides

For every major version bump, provide:

• Field-by-field mapping table (v1 → v2)
• Before/after request and response examples
• Code snippets for common languages/SDKs
• Timeline with key dates (announcement, sunset, removal)

SDK Versioning

Align SDK major versions with API major versions:

api-client@1.x  →  /api/v1
api-client@2.x  →  /api/v2

Ship the new SDK before announcing API deprecation.

Anti-Patterns

NEVER Do

1. NEVER remove a field, endpoint, or change a type without bumping the major version
2. NEVER sunset a public API version with less than 6 months notice
3. NEVER mix versioning strategies in the same API (URL path for some, headers for others)
4. NEVER use minor or patch versions in URL paths (/api/v1.2/ is wrong — use /api/v1/)
5. NEVER version individual endpoints independently — version the entire API surface as a unit
6. NEVER deploy a breaking change under an existing version number, even if "nobody uses that field"
7. NEVER skip documenting differences between versions — every breaking change needs a migration guide entry