$schema should point to meta-schema incorporating our changes to JSON Schema

[jpmckinney]

Originally reported via email by @devgateway, quoted below:

The $schema section should define the meta-schema of the current schema. This is used by validation tools that comply with the standard, to correctly identify entities that are part of the schema. Further read on the standard page:

http://json-schema.org/latest/json-schema-core.html#rfc.section.7

and another third party page linked from json-schema.org, they say it is a good read:

https://spacetelescope.github.io/understanding-json-schema/reference/schema.html

The issue is OCDS defines draft-04 as the reference meta-schema: "$schema": "http://json-schema.org/draft-04/schema#"

Because extra properties are defined in the OCDS schema files, the line above should read something like this:

"$schema": "http://standard.open-contracting.org/schema/1__1/meta-schema.json"

which should be an extension of draft-04, adding the deprecated keyword, and others that were added on top of draft-04.

This meta-schema extension should allow:

• validating the schema itself against the meta-schema (which is self-validating). For example if someone introduces now a typo in the definition of deprecated for a new deprecated field, say deprecatedVersoin instead of deprecatedVersion, this will be spotted easily.

• make validation tools know what rules to apply for validation, right now they correctly ignore all these extra keywords because $schema says it only has entities defined from draft-04 (which in the case of OCDS release files is false).

So for me, because I use one of these validating libraries in jOCDS, I need to hack the OCDS schema and either remove $schema altogether (which is a problem in itself), or write my own $schema and define your extra keywords there...

[jpmckinney]

It may be possible to simply re-use this (reformatting that info as JSON): https://github.com/open-contracting/standard-maintenance-scripts/blob/master/tests/test_json.py#L42-L57

[kindly]

@jpmckinney thanks for the link that was helpful.

In pull request #611 in the schema directory there is an adaptation of the draft-04 schema which is more strict (uses additionalProperties: false) and covers our use. It is also configured to run in the tests so travis will pick up issues. There is still some work todo on adding title and description tags to the fields we have added which @timgdavies said he would look at. Also, $ref and format properties had to be added to make the additionalProperties: false work.

I nonetheless have concerns about using this meta-schema in the the $schema tag.
Having a non-standard $schema tags means that there is not any machine way to detect what version of schema should be used when validating without looking at external docs to saying we use draft-04. The $schema tag has more meaning than just the meta-schema you are validating against it also refers to the actual draft document and the rules for each property.

Also it could be considered a breaking change and "could" break some libraries as they would no longer have a link to an official draft schema to validate against.

As our schemas are also valid against draft-4 then I think we should keep it in our schemas. draft-4 meta-schema itself does not validate itself (if you add additionalProperties: false due to the format and $ref) so this must be expected usage.

[jpmckinney]

Hmm, but isn't the JSON document we are extending (http://json-schema.org/draft-04/schema#) the 'official draft schema to validate against'? Our meta-schema, itself, would have a $schema of http://json-schema.org/draft-04/schema#, but its id would be different.

Are libraries just looking at the string 'http://json-schema.org/draft-04/schema#' and deciding on a bunch of rules to apply, independent of the contents of the JSON document? I know that JSON Schema has non-schema rules about how 'format' is validated, and about what words like 'maxItems' mean, but I don't think those rules and semantics have changed across versions of JSON Schema. In other words, libraries shouldn't need to have different code paths for different versions. I contributed to a Ruby JSON Schema validator, and it didn't have special logic that followed from which schema version was being used.

http://json-schema.org/latest/json-schema-core.html#rfc.section.6.4 says:

Implementations MAY define additional keywords to JSON Schema. Save for explicit agreement, schema authors SHALL NOT expect these additional keywords to be supported by peer implementations. Implementations SHOULD ignore keywords they do not support.

Authors of extensions to JSON Schema are encouraged to write their own meta-schemas, which extend the existing meta-schemas using "allOf". This extended meta-schema SHOULD be referenced using the "$schema" keyword, to allow tools to follow the correct behaviour.

[kindly]

From my understanding there are non-schema rules across drafts that do change and that we can not be assured of them not in future.
For example the usage of required changed from being a boolean in properties to a list between draft 3 and 4 and the python library we use ended up special casing how properties worked in them:
https://github.com/Julian/jsonschema/blob/master/jsonschema/_validators.py#L243
https://github.com/Julian/jsonschema/blob/master/jsonschema/_validators.py#L294

However, you are correct that libraries could look at the meta-schemas $schema to determine the official version to look for but I am not sure they would do that.
I also have reservations about using allOf though as an extension mechanism as it produces very opaque error messages (at least in the python library).

On reflection, I do not mind if we do this, but we should at least wait for minor bump in OCDS version number if we do. There is quite a bit of work around adding the meta-schema.json to the official schema locations that has to be worked out and making sure the references in the id/&schema tags are in sync.

[jpmckinney]

I mainly just want to kick the tires on this, so that we don't do things one way, and then decide at a later date that we should have done it the way the JSON Schema draft recommends all along.

Could you describe the specific work that would have to be done in a minor bump, while it's still fresh?

[kindly]

In a minor version bump we should do the following:

Governance

• As this is a change to our JSON schema and it arguably a semantic change I think we should go through a advisory discussion like all the changes that happened in 1.1
• We have to decide if we are adding this only to the new versions of the schema or we also do this to a new patch releases of 1.0 or to all existing schemas.
• Have to decide if we have a new metaschema per patch release (what we do for all other schemas) or that we maintain different meta-schemas versions as they should change less often.

Documentation

• We should add reference to this schema somewhere in the docs and add descriptions of each extra field, with translations etc.

Technical Changes

• Add a test to make sure all schemas point to the correct metaschema (should be fairly straight forward)
• The process of copying over the final schemas to the official locations is a manual step (purposefully so) and this procedure may need to be changed to add the meta-schema.json to those locations. This may be a lot more work if we do anything historically.
• Need to decide if just having our own full copy of the schema (as in the pull request) or extend draft-4 in some way. For our case using anyOf (i.e anyOf our meta-schema or draft-04) will not work because draft-04 does not have additionalProperties: False, which means any extra property would still pass validation.