APIs (Application Programming Interfaces) are crucial components of modern software applications. They allow different systems to communicate with each other by exposing functionality and data. As APIs evolve, it often becomes necessary to release new major versions while maintaining backward compatibility with older versions. This allows existing users to upgrade on their own timeline while enabling new users to use the latest features.
There are several strategies for maintaining different API versions. Choosing the right approach depends on the specific needs and constraints of your project.
Use Version Numbers in the URL
One popular technique is to include the version number directly in the URL of the API. For example:
- https://api.example.com/v1/users
- https://api.example.com/v2/users
This strategy provides an explicit separation between API versions. When you release v2, it will have its own distinct endpoints. Each version can evolve independently and introduce breaking changes without impacting the other. The major downside is that the API URLs are now tightly coupled to a version number.
Benefits
- Clean separation of concerns – each version can evolve independently
- No compatibility hacks needed – breaking changes are isolated
- Explicit versioning is obvious to API consumers
Drawbacks
- URLs are not very RESTful
- Frontend clients need to be updated to use new URLs
- Switching versions requires changing application code
Custom Request Headers
Instead of putting the version in the URL, you can use a custom HTTP request header like “API-Version”. For example:
- API-Version: v1
- API-Version: v2
The server looks at this header to determine which version of the API to call. The URLs remain the same between versions. This approach decouples the API endpoints from the versioning scheme.
Benefits
- URLs stay clean and RESTful
- Switching versions does not require new URLs
- Versions are still explicit and obvious
Drawbacks
- Clients need to be updated to pass the custom header
- More logic required on the server side to handle the header
- Headers can be missed or spoofed by clients
Content Negotiation
Content negotiation takes advantage of the “Accept” HTTP header. The client specifies the version they want using the media type in the Accept header:
- Accept: application/vnd.example.v1+json
- Accept: application/vnd.example.v2+json
The server parses the header and returns the response in the requested format. The URLs remain static between versions.
Benefits
- No changes needed to API URLs
- Version information is still explicit
- Leverages existing HTTP headers
Drawbacks
- Additional logic required on server to handle media types
- Clients must be updated to send the proper Accept header
- Media types can get verbose with long version numbers
Query Parameters
Another option is to specify the API version through a query parameter, like /users?version=1. For example:
- GET /users?version=1
- GET /users?version=2
The server can check the query param to determine which version of the logic to execute. Query parameters are easy to pass from clients.
Benefits
- Very easy to pass version information in clients
- No changes needed to the URL structure
- Explicit versioning visible to API consumers
Drawbacks
- Can slightly pollute URL semantics
- Version information not in header, less RESTful
- May require changes in client code to handle param
Vendor MIME media types
An extension of standard content negotiation, you can leverage custom vendor MIME media types to represent your API versions. For example:
- Accept: application/vnd.example.v1+json
- Accept: application/vnd.example.v2+json
The key difference from regular content negotiation is that the media types are namespaced with your vendor prefix “vnd.example”. This helps avoid potential conflicts.
Benefits
- No changes needed to API URLs
- Versions are explicit and self-documenting
- Custom types avoid conflicts
Drawbacks
- Additional logic required on server side
- Clients must be updated to send custom header
- Media types can get long and verbose
Versioning at the Domain
For larger changes, you can version APIs at the domain level. For example:
- api.v1.example.com
- api.v2.example.com
This allows each version to live fully independently, at the cost of more operational overhead.
Benefits
- Full isolation and flexibility for each version
- No collisions between versions
- Can scale independently as needed
Drawbacks
- More difficult to manage infrastructure
- Costly to operate separate domains
- Doesn’t work well for small API changes
Picking an API Versioning Strategy
There are pros and cons to each of the above techniques. Here are some criteria to consider when choosing:
- Change significance – major or minor new version?
- Client impact – how will clients be affected?
- Implementation complexity – how hard is it to build?
- Visibility – is version obvious to consumer?
- Flexibility – can versions evolve independently?
For most cases, putting the version in the URL or using custom headers provide a good balance of simplicity and visibility. Here is a comparison:
| Approach | Change Significance | Client Impact | Implementation | Visibility | Flexibility |
|---|---|---|---|---|---|
| URL versioning | Major | High | Easy | Explicit | High |
| Custom headers | Major or Minor | Medium | Moderate | Explicit | High |
| Content negotiation | Minor | Medium | Moderate | Explicit | Medium |
| Query param | Minor | Low | Easy | Explicit | Medium |
| Domain | Major | High | Hard | Explicit | High |
The larger the changes in an API revision, the more isolation you typically want between versions. Minor changes and iterations can often simply use a query parameter or custom header. Major versions with breaking changes benefit from complete isolation in URLs or domains.
Dealing with Deprecation
Once you release a new API version, you’ll eventually want to deprecate and sunset older ones. Make sure to give clients ample warning time and detailed migration instructions. Here are some best practices for deprecating API versions:
- Announce deprecation well in advance – give clients 6-12 months notice.
- Clearly document the timeframe for deprecation.
- Provide specific migration instructions to upgrade clients.
- Return warnings in responses as the deprecation date approaches.
- Monitor usage to see if any clients still hit deprecated APIs.
- Eventually close down deprecated API endpoints.
Keep deprecated versions running for as long as possible during the transition period. You don’t want to break any legacy clients still using the old API. Monitor usage carefully before finally shutting down endpoints.
Graceful Deprecation Strategies
- Run both v1 and v2 side by side for a transition period.
- Redirect v1 requests to v2 and return v1 responses.
- Reject v1 requests but advise clients to upgrade to v2.
The redirection approach maintains compatibility for legacy clients even after formally deprecating v1. Only completely remove deprecated APIs once usage drops to zero.
Versioning Strategies – Recap
Choosing the right versioning technique involves tradeoffs around flexibility, visibility, and ease of implementation. Here is a quick recap of viable strategies:
- URL versioning – /v1/users, /v2/users
- Custom headers – X-API-Version: 1
- Content negotiation – Accept: application/vnd.v1+json
- Query parameters – /users?version=1
- Domain – api.v1.example.com, api.v2.example.com
Think about the size of changes, desired flexibility, and client impact as you decide which strategy is right for your API. Incremental iterations can start simple with headers or query params. Major versions often benefit from complete isolation.
Conclusion
APIs require deliberate versioning strategies to avoid issues as they evolve. Leverage approaches like URL versioning and custom headers to explicitly separate major API releases. Monitor adoption of new versions. Clearly communicate timelines for deprecation and migrations. With some careful planning, you can introduce new API versions without negatively impacting existing consumers.
