# Changes and Versioning
> _The fundamental principle is that you can’t break existing clients, because you don’t know what they implement, and you don’t control them. In doing so, you need to turn a backwards-incompatible change into a compatible one._
>
> _– [Mark Nottingham](https://www.mnot.net/blog/2011/10/25/web_api_versioning_smackdown)_

Any change to an API **MUST NOT** break existing clients.

Any change to:
1. **Resource identifier** (resource name / URI) including any **query parameters** and their semantics
1. **Resource metadata** (e.g. HTTP headers)
1. **Action** the resource affords (e.g. available HTTP Methods)
1. **Relation** with other resources (e.g Links)
1. **Representation format** (e.g. HTTP request and response bodies)

**MUST** follow the [**Rules for Extending**](https://adidas-group.gitbooks.io/api-guidelines/content/core-principles/rules-for-extending.html).

## Identifier Stability (No URI Versioning)
A change **MUST NOT** affect **existing** resource identifiers (name / URI). Furthermore, a resource identifier **MUST NOT** contain a semantic version to convey a version of resource or its representation format.

> _The reason to make a real REST API is to get evolvability … a "v1" is a .... to your API customers, indicating RPC/HTTP (not REST)_
>
> _– [Roy T. Fielding](https://twitter.com/fielding/status/376835835670167552)_

#### Example
Adding a new action to existing resource with identifier `/greeting` doesn't change its identifier to `/v2/greeting` (or `/greeting-with-new-action` etc.).

## Backward-incompatible Changes
A change to _resource identifier_, _resource metadata_, _resource actions_ and _resource relations_ that can't follow the [Rules for Extending](https://adidas-group.gitbooks.io/api-guidelines/content/core-principles/rules-for-extending.html) **MUST** result into a **new resource variant**. Existing resource variant **MUST** be preserved.

A change to _representation format_ **SHOULD NOT** result into a new resource variant.

#### Example
Currently, optional URI Query Parameter `first` on an existing resource `/greeting?first=John&last=Appleseed` needs to be made required. Since this change violates the 3rd rule of extending and could break existing clients a new variant of the resource is created with different URI `/named-greeting?first=John&last=Appleseed`.

### Representation Format Changes
> A representation format is the serialization format (media type) used in request and response bodies, and typically it represents a resource or its part, possibly with additional hypermedia controls.

If a change can't follow the Rules for Extending the representation format media type **MUST** be changed. If the media type has been changed the previous media type, **MUST** be available via [Content Negotiation](https://adidas-group.gitbooks.io/api-guidelines/content/message/content-negotiation.html). 

If the media type conveys the version parameter, the version parameter **MUST** follow [Semantic versioning](http://semver.org/).

#### Example
Media type _before_ a breaking change:

```
application/vnd.example.resource+json; version=2
```

Media type _after_ a breaking change:

```
application/vnd.example.resource+json; version=3
```

## API Description Versioning
API Description in the OpenAPI specification format **MUST** have the `version` field. The `version` field **MUST** follow [Semantic versioning](http://semver.org/):

> Given a version number MAJOR.MINOR.PATCH, increment the:
> 
> - MAJOR version when you make **incompatible** API changes,
> - MINOR version when you add functionality in a **backwards-compatible** manner
> - PATCH version when you make **backwards-compatible bug fixes**

The API Description version **SHOULD** be updated accordingly to API design change.

#### Example
Following API Description

```yaml
swagger: '2.0'
info:
  version: '2.1.3'
  title: '[Demo] Inventory API'
  description: 'Inventory service API'
```

Has MAJOR version 2, MINOR version 1 and PATCH version 3.

#### Recommended Reading
- [Evolving HTTP APIs](https://www.mnot.net/blog/2012/12/04/api-evolution)