OpenAPI and JSON Schema Divergence
Update 2020-02-03: OpenAPI v3.1 will be using JSON Schema Draft
so tooling vendors should get to work on upgrading support for JSON Schema
2019-09 and the other OpenAPI v3.1 changes.
This article is going to explain OpenAPI and JSON Schema divergence, which I’ve
been calling the subset/superset/sideset problem. It’ll finish up explaining how
we’re going to solve it.
Whenever talking about API descriptions it is impossible to avoid mentioning
OpenAPI and JSON Schema. They’re the two main solutions for any sort of API that
doesn’t have a type system forcibly jammed into it by default.
Often you’ll need OpenAPI for one thing, and JSON Schema for another. OpenAPI
has amazing API documentation
fancy SDK generators, and handles loads of API-specific functionality that JSON
Schema doesn’t even go near. It also has a focus on keeping this static, for
strictly typed languages, where properties should be 1 type and 1 type only.
JSON Schema focuses on very flexible data modeling with the same sort of
validation vocabulary as OpenAPI, but for more flexible data sets. Whilst it
doesn’t focus just on APIs, by using more advanced vocabularies like JSON
it can model a fully RESTful API and its hypermedia controls (HATEOAS). JSON
Schema can offer server-defined client-side
and a bunch of other fantastic stuff that OpenAPI doesn’t really aim to do.
Over the last year I’ve been chasing the perfect
and one of my main requirements when evaluating the common API specs was JSON
Schema support. The situation overall was pretty bleak, not just in supporting
JSON Schema, but a lot of tooling was just... not great.
Eight months after that article things are better, and the API design-first
workflow has been
around me, to a point where I’m really happy about most stuff!
OpenAPI is often described as an extension of JSON Schema, but both specs have
changed over time and grown independently. OpenAPI v2 based based on JSON Schema
draft 4 with a long list of deviations, but OpenAPI v3 shrank that list, upping
their support to draft v4 and making the list of discrepancies shorter. Despite
OpenAPI v3 closing the gap, the issue of JSON Schema divergence has not been
resolved fully, and with newer drafts of JSON Schema coming out, the divergence
is actually getting worse over time. Currently OpenAPI is still on draft 5, and
JSON Schema is on draft 7.
A list of caveats to the JSON Schema support in OpenAPI v3.0.
I’ve been punting this issue for a while in my articles and recommendations at
work. The hope was that by the time folks at work had upgraded to v3, there
might be a v3.1 out solving the situation, but that has not come to pass. Now I
find myself suggesting folks find some way to convert one to the other, or try
to write JSON Schema that is compatible with OpenAPI.
Carefully writing JSON Schema for your data model kiiiinda works
The latter can be done, but eventually you’ll get bit by something. At work
we’ve been writing JSON Schema files, using them for contract testing and a
bunch of other stuff, then rendering them as part of our OpenAPI Docs with
ReDoc. ReDoc will let you use
type: [string, null], but now we've got
Speccy linting our packages, it's reporting
that as invalid OpenAPI...
$ speccy lint docs/openapi.ymlSpecification schema is invalid. #/paths/~1foo/post/requestBody/content/application~1json/properties/user_uuid expected Array [ 'string', 'null' ] to be a string expected Array [ 'string', 'null' ] to have type string expected 'object' to be 'string'
If I change that to valid OpenAPI and use
type: string with
instead, validators like Speccy will be happy, but my JSON Schema contract tests
will break as they no longer know that
null is an acceptable value for that
This error was the final straw for me. At work I have been recommending everyone
enable Speccy on CircleCI to (amongst other things) make sure we’re writing
valid OpenAPI, and I am failing to write valid OpenAPI in an API I manage. I’m
also a little tired of explaining this awkward difference to people who would
like to use some JSON Schema-based tools.
After grumping at Darrel Miller (a contributor to OpenAPI) and others on the
APIs You Won’t Hate slack, some good ideas
started to pop up. Darrel is going to try and draft up an extension to OpenAPI
that could theoretically end up in a future version — like 3.1 or 4.0:
x-oas-draft-alternate-schema: schemaType: json-schema schemaRef: ./real-json-schema.json
This would allow support for various JSON Schema drafts, and any other data
model you can think of; including protobuf. The decisions of which data model
formats to support would be in the hands of tool vendors. Of course this would
increase work for these vendors, and decrease portability for a while as you can
only use tools that support your alternate schema, but ultimately solve a lot
There’s some stuff to flesh out, and obvious limitations around which schemas
$ref which other schemas, but there is definitely a solution here. It’ll
take some time to get it done, and in that time we all need a solution.
One approach would be trying to use OpenAPI for all the things, write validators
and RSpec tools that do this, but we’d have to be 100% OpenAPI for everything,
and we’d never get to play with client validation, Hyper-Schema, etc.
Another approach would be converting our OpenAPI models to JSON Schema, but that
seems a bit lossy. OpenAPI v3 is based on JSON Schema draft v5, and at time of
writing JSON Schema is up to draft v8... This also adds a build step that gets in
the way. If you are using JSON Schema for your contract testing, with something
you would need to edit your OpenAPI model, run the conversion, then run the
tests. Or crowbar a conversion into your test suite, meaning the tool handling
the conversion needs to be written in that specific language... or pipe a shell
command to the CLI... AGH RUN AWAY.
No, I think making JSON Schema (latest possible draft) the one and only source
of truth for the data model, then "downgrading" to a flavour of JSON Schema that
OpenAPI likes, is going to be the way to go.
Anyway, gonna keep at this until I come up with some solutions.
Part Two: Problem Solved!