Write an API versioning strategy document

API Versioning Strategy

Produce a complete API versioning strategy document that gives a service team durable, consistent rules for evolving their API without breaking consumers. This document covers the versioning scheme selection (with rationale), lifecycle policy from introduction through sunset, a precise breaking-change classification, and all the communication artifacts a team needs when deprecating a version. Engineers should be able to hand this document to a new team member or external consumer and have them understand exactly what to expect.

Required Inputs

Ask for these if not already provided:

• API type — REST, GraphQL, or gRPC (each has different versioning mechanics)
• Current versioning approach — URL path (/v1/), request header, query parameter, or none; if none, document starts fresh
• Number of existing versions and active consumer count — needed to size the lifecycle policy and migration scope
• Deprecation timeline constraints — any hard deadlines (contract SLAs, compliance windows, annual release cycles)
• Consumer type — internal teams only, external partners, public API, or mix (affects communication channel choices)

If any input is missing, ask before producing the document. For GraphQL, note that the versioning approach differs substantially (schema evolution over versioning) and tailor the scheme section accordingly.

Output Format

---

API Versioning Strategy: [Service Name]

Owner: [Team Name] API Type: [REST / GraphQL / gRPC] Document Version: 1.0 Last Reviewed: [Date] Next Review: [Date + 6 months]

---

1. Versioning Scheme

Selected Approach: [URL Path / Request Header / Query Parameter]

Scheme	Example	Pros	Cons	Verdict
URL Path	/v2/orders	Visible in logs and bookmarks; trivial to route	Violates strict REST resource identity; clutters URL space	Recommended for public-facing REST APIs
Accept Header	Accept: application/vnd.[service].v2+json	Keeps URLs clean; proper content negotiation	Harder to test in browser; less visible in logs	Recommended for internal APIs with controlled clients
Query Parameter	/orders?version=2	Easy to retrofit without URL restructuring	Often missed in client code; cache-key complications	Acceptable only for read-heavy APIs already in production
GraphQL Schema Evolution	Field deprecation + @deprecated directive	No versioning needed for additive changes	Requires disciplined schema design	Recommended for GraphQL APIs

Rationale for [chosen scheme]: [One paragraph explaining why this scheme fits the API type, consumer type, and operational context provided. Reference the specific inputs — e.g., "Because this API has external partners who integrate via generated clients, URL path versioning provides the most predictable routing behavior and eliminates header negotiation complexity."]

Version Format

[Base URL]/v{MAJOR}/{resource}

Examples:
  https://api.[company].com/v1/orders
  https://api.[company].com/v2/orders/{id}/items

Version identifier: integer only (v1, v2, v3)
No minor versions in the URL — minor/patch changes are non-breaking and deployed continuously.

---

2. Version Lifecycle Policy

Lifecycle Stages

  STABLE ──────────────────────────────────────────────────►
      │
      ├─ STABLE        Active development, full SLA, new consumers allowed
      │
      ├─ DEPRECATED    Announced, timeline posted, migration docs live.
      │                New consumers blocked. Existing consumers receive warnings.
      │
      ├─ SUNSET        Requests return HTTP 410 Gone + migration pointer.
      │                30-day window before routing is removed.
      │
      └─ RETIRED       Routing removed, docs archived, no traffic accepted.

Stage	Duration	SLA Applies	New Consumers Allowed	Required Action
Stable	Until superseded	Yes — full	Yes	None
Deprecated	[12 months / adjust per constraint]	Yes — degraded acceptable	No	Migrate before sunset date
Sunset	30-day window	Best-effort only	No	Migrate immediately
Retired	Permanent	None	No	—

Minimum Stable Period: A version must remain Stable for at least [6 / 12] months before deprecation can be announced.

Maximum Simultaneous Versions: No more than [2] versions in Stable or Deprecated status at any time. Releasing v3 requires committing to a sunset date for v1 in the same announcement.

---

3. Breaking vs. Non-Breaking Change Classification

Apply this table before every API change. If a change is marked Breaking, it requires a new major version. When uncertain, default to Breaking.

Change Type	Specific Example	Classification	Rationale
Remove a response field	Delete order.legacy_id from response	Breaking	Clients reading this field will null-pointer or fail
Rename a field	user_name → username	Breaking	Clients referencing old name receive null
Change field type	"amount": "10.00" → "amount": 10.00	Breaking	Type mismatch at deserialization
Make optional field required	email required in POST body	Breaking	Existing callers omitting it receive 400
Remove an endpoint	DELETE /v1/widgets/{id} removed	Breaking	Existing callers receive 404
Change HTTP method	GET /search → POST /search	Breaking	Bookmarked or cached GET calls fail
Change authentication scheme	API key → OAuth2	Breaking	All clients must re-authenticate
Restructure error response shape	Error JSON schema changed	Breaking	Error-handling code misparses responses
Expand enum values (response)	New status: "on_hold" value returned	Breaking	Switch statements with no default fall through
Change pagination defaults	page_size default 20 → 50	Breaking	Response length changes unexpectedly
Tighten input validation	Max length 100 → 50	Breaking	Previously valid inputs now rejected
Add new optional field to response	Add order.tax_breakdown	Non-Breaking	Clients ignore unknown fields per spec
Add new optional request parameter	Add ?include_archived=true	Non-Breaking	Ignored by existing clients
Add a new endpoint	GET /v1/orders/{id}/audit	Non-Breaking	No existing client references it
Relax input validation	Min length 10 → 5	Non-Breaking	Existing valid inputs remain valid
Performance or latency improvement	Response time reduced	Non-Breaking	—
Add new enum value (request-only)	Accept new type: "express"	Non-Breaking	Existing values still accepted

---

4. Deprecation Process

Step-by-Step Deprecation Checklist

• T-0 (Decision day): Engineering lead approves deprecation. New version confirmed Stable. Sunset date set.
• T-0: Update API docs — add deprecation banner to all v[N] endpoint pages.
• T-0: Add Deprecation and Sunset response headers to all v[N] responses (see format below).
• T-0: Block new consumer onboarding for v[N] in API gateway and developer portal.
• T-0: Send initial deprecation notice to all registered consumers (see Section 5 template).
• T-0: Open tracking issue in engineering backlog linking all known consumers to their migration status.
• T minus 30 days: Send 30-day warning to all consumers still sending v[N] traffic.
• T minus 7 days: Send final warning. If consumer traffic > 100 req/day, escalate directly to their engineering lead.
• Sunset date: Switch v[N] routing to return HTTP 410 Gone with body pointing to migration guide.
• T plus 30 days: Remove routing rules. Archive documentation. Close tracking issue.

Deprecation Response Headers

HTTP/1.1 200 OK
Deprecation: true
Sunset: Sat, 01 Jan 2027 00:00:00 GMT
Link: <https://docs.[company].com/api/migration/v1-to-v2>; rel="successor-version"

Sunset Response Body

HTTP/1.1 410 Gone
Content-Type: application/json

{
  "error": "api_version_sunset",
  "message": "API v1 was sunset on 2027-01-01. Please migrate to v2.",
  "migration_guide": "https://docs.[company].com/api/migration/v1-to-v2",
  "support": "api-support@[company].com"
}

---

5. Client Communication Templates

Initial Deprecation Notice

Subject: [Action Required] [Service Name] API v[N] Deprecation — Sunset [Date]

Hi [Team / Partner Name],

We are deprecating [Service Name] API v[N], effective [Sunset Date].

What this means for you:
- v[N] continues to work normally until [Sunset Date]
- After [Sunset Date], all v[N] requests return HTTP 410 Gone
- v[N+1] is available today and fully stable

Your current usage: approximately [X] requests/day as of [Date].
Estimated migration effort: [Small: < 1 day | Medium: 1–3 days | Large: 3–10 days]

Migration resources:
  Migration guide:  [URL]
  Changelog:        [URL]
  Office hours:     [Date/Time/Link]
  Support:          [Slack channel or email]

Key dates:
  [Date]          Deprecation announced (today)
  [Date]          New consumer onboarding blocked for v[N]
  [Date]          30-day warning sent to remaining consumers
  [Sunset Date]   v[N] returns 410 Gone

Reply to this message or contact us at [channel] with questions.

[Your Name], [Team Name]

30-Day Warning

Subject: [30 Days Remaining] [Service Name] API v[N] sunsets [Date]

Hi [Team / Partner Name],

[Service Name] API v[N] sunsets in 30 days on [Date].

Your current v[N] traffic: [X] requests/day — migration is not yet complete.

If you have a technical blocker requiring an extension, contact us before
[Date minus 14 days]. Extensions require a documented blocker and a committed
migration completion date.

Migration guide: [URL] | Support: [channel]

---

6. Migration Guide Template

Publish one migration guide per version transition at docs.[company].com/api/migration/v[N]-to-v[N+1].

# Migration Guide: v[N] → v[N+1]

**Estimated effort:** [Small: < 1 day | Medium: 1–3 days | Large: 3–10 days]
**Breaking changes in this guide:** [count]

## Quick Start

Update your base URL:
  Before: https://api.[company].com/v[N]/
  After:  https://api.[company].com/v[N+1]/

## Breaking Changes

### 1. [Field Rename: user_name → username]

**Affected endpoints:** `GET /users/{id}`, `POST /users`

Before (v[N]):
{ "user_name": "alice" }

After (v[N+1]):
{ "username": "alice" }

Migration: Replace all references to `user_name` with `username` in request
builders and response parsers.

### 2. [Next breaking change — repeat structure]

## New Capabilities in v[N+1]

| Feature | Description | Docs |
|---------|-------------|------|
| [Feature name] | [Brief description] | [Link] |

## SDK Upgrade Reference

| Language | Package | v[N+1] Version | Install Command |
|----------|---------|----------------|-----------------|
| Python | `[company]-sdk` | `2.0.0` | `pip install [company]-sdk==2.0.0` |
| Node.js | `@[company]/sdk` | `2.0.0` | `npm install @[company]/[email protected]` |
| Go | `github.com/[company]/sdk-go` | `v2.0.0` | `go get github.com/[company]/sdk-go/v2` |
| Java | `com.[company]:sdk` | `2.0.0` | Update pom.xml / build.gradle |

## Migration Validation Checklist

- [ ] Base URL updated to v[N+1]
- [ ] All renamed fields updated in request serializers
- [ ] All renamed fields updated in response deserializers
- [ ] Error-handling code updated for new error shape
- [ ] Integration tests passing against v[N+1] in staging
- [ ] Load test completed against v[N+1] — latency within acceptable range
- [ ] Rollback plan documented if issues arise post-cutover

---

7. Version-Specific Documentation

• Maintain separate documentation pages for each Stable and Deprecated version.
• Deprecated version docs carry a persistent banner: "This version is deprecated. Sunset date: [Date]. [Migrate to v[N+1]]."
• OpenAPI specs, Protobuf definitions, or GraphQL schemas are tagged and archived per version in the repository under /api/v[N]/.
• A root-level CHANGELOG.md records every breaking and non-breaking change by version — not buried in commit history.

---

8. SDK Versioning Alignment

API Version	SDK Major Version	SDK GA Date	SDK EOL Date
v[1]	1.x	[Date]	[API Sunset + 90 days]
v[2]	2.x	[Date]	Active

• SDK major versions align 1:1 with API major versions.
• SDK minor versions track non-breaking API additions.
• SDK EOL dates trail API sunset dates by 90 days to give consumers extra runway.
• SDKs emit a runtime deprecation warning log line when the underlying API version is Deprecated.

---

Strategy authored by [Team Name] — questions to [Slack channel or email]

---

Anti-Patterns

• Do not classify expanding an enum (new response values) as non-breaking — clients with exhaustive switch statements will break when they receive an unexpected enum value
• Do not set a sunset date without confirming it is achievable for the largest consumer — a sunset that forces consumers to miss a legal deadline will be ignored or escalated
• Do not maintain more than two simultaneous stable/deprecated versions — each additional supported version multiplies maintenance burden and consumer confusion
• Do not use "monitor traffic" as the sole mechanism for knowing when all consumers have migrated — track named consumers against migration completion explicitly
• Do not skip the migration guide — consumers will delay migration indefinitely without a step-by-step guide that estimates effort

Quality Checks

• Versioning scheme recommendation includes explicit rationale tied to the API type and consumer type provided — not a generic recommendation
• Breaking-change table covers at minimum: field removal, field rename, type change, making optional field required, endpoint removal, enum expansion, and default value change
• Deprecation timeline durations are filled in with concrete values, not left as abstract placeholders
• All three communication artifacts are present: initial deprecation notice, 30-day warning, and migration guide template
• Sunset response headers (Deprecation, Sunset, Link) use correct RFC date format and real URL structure
• SDK versioning alignment table is present and ties SDK major versions explicitly to API major versions
• Maximum simultaneous supported versions is stated with a concrete number