27 September 2015

REST APIs don’t need a versioning strategy – they need a change strategy

Change in an API is inevitable as your knowledge and experience of a system improves. Managing the impact of this change can be quite a challenge when it threatens to break existing client integrations.

Developers often try to decide on a versioning strategy as soon as they start work on an API. This is understandable but it’s not always the smartest way of looking at the problem of managing change. Brandon Byers summed this up by borrowing Jamie Zawinski’s dig at regular expressions:

Some people, when confronted with a problem, think “I know, I’ll use versioning.” Now they have 2.1.0 problems.

How can you version resources in REST?

REST doesn’t provide for any specific versioning but the more commonly used approaches fall into three camps: putting it on the URI, using a custom request header or a adding it to the HTTP Accept header.

Using the URI is the most straightforward approach though it does upset REST advocates who insist that a URI should refer to a unique resource. You are also guaranteed to break client integrations when a version is updated no matter how heavily you invest in creative routing and client communication.

A custom header allows you to preserve your URIs between versions though it is effectively a duplicate of the content negotiation behaviour implemented by the existing Accept header. A client can use this header to send a list of supported versions while the server responds with the version used in the Content-Type header.

Content negotiation may let you to preserve a clean set of URLs but you still have to deal with the complexity of serving different versions of content somewhere. This burden tends to be moved up the stack to your API controllers which become responsible for figuring out which version of a resource to send. The end result tends to be a more complex API as clients have to know which headers to specify before requesting a resource.

The version number isn’t the problem

Given the contentious nature of REST, you’ll always be wrong in somebody’s eyes no matter what approach you take. The point is that version numbering itself is a red herring.

The real challenge here is in managing a code base that can serve up multiple versions of resources. If you keep all versions in the same code base then older versions become vulnerable to unexpected changes. If you separate the code bases then the operational and support overhead escalates. In both cases, code bloat and increased complexity are an inevitable consequence.

A strict approach to versioning does give you much-needed certainty over the contract but it does tend to undermine a system’s capacity to change. Versioning can become a barrier to improvement as any requirements that lead to version changes are resisted. I have seen APIs with strict versioning policies stuck on the same version for years due to legitimate concerns over the amount of work and risk involved in change.

What’s the alternative to versioning?

A coherent version strategy should address how you will manage change in your API whilst providing a stable contract to clients. This doesn’t have to include issuing new versions in response to changes.

One approach is to build in the possibility of change by making provision for backwards compatibility in API changes. This approach does carry significant risk as you cannot be sure that a change will not break existing clients even with exhaustive regression testing.

You can even take backwards compatibility a step further by adding features such as optional parameters and wildcard properties that anticipate future changes. This kind of “forwards compatibility” tends to produce a coarse contract that places a considerable burden of validation onto the client. The end result is often a messy set of switches and codes required for each call.

Bertrand Meyer’s Open\Closed principle suggests that software entities should be “open for extension, but closed for modification”. When applied to APIs the implication is that you can augment your resources but not change them.

This approach could offer the certainty of stricter versioning without the regression risks involved in backwards compatibility. Augmentation is not without its problems though as it can give rise to bloated contracts. Without careful discipline an API can become littered with duplicate methods or resources that provide several slightly different ways of achieving the same thing.

Can you share the responsibility?

You could do more to share the burden of change between API and client. Postel’s law, often referred to as the Robustness principle, states that you should be “liberal in what you accept and conservative in what you send”. In terms of APIs this implies a certain tolerance in consuming services.

For example, strict serialization techniques can be unnecessarily intolerant of change. A more tolerant reader should only be concerned with data that it needs and ignore every other part of the response. This means that the majority of changes are unlikely to break the integration.

Another approach could be for the consumer to declare the data they are interested in as part of a request. This consumer-driven contract pattern does not specify the form that these consumer assertions should take, but an implementation could allow an API to detect when a request is out of date.

Unfortunately, these approaches can only be applied to relatively closed communities of services. Public-facing APIs rarely have the luxury of being able to dictate the style of client integration. The only enforceable contract you have between service and client is made up of the data and protocol.

This is why careful discipline is at the heart of any sensible change strategy. A good API doesn’t come into being by accident. It has to be curated. Whatever approach you take to managing change you will need consistent and active governance over the evolving contract.

Filed under API design, Integration, Rants, REST, Web services.