Switch to ajv/2020 for fastify 5

[kibertoad]

Prerequisites

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🚀 Feature Proposal

I wonder if we should consider switching to ajv/dist/2020 during our next SemVer major.

According to sinclairzx81/typebox#490 (comment), default ajv is still using Draft 7 version of JSON schema.

Motivation

Support latest additions to the JSON schema standard. Considering the scope and impact of this change, I doubt we can do this outside of a SemVer major.

Example

I don't expect usage patterns for ajv or ajv-compiler to change, it will simply allow to support the features introduced in later versions of JSON Schema.

[climba03003]

Draft 7 and Draft 2020-12 is great different.
It also provides a large set of incompatibility.

[kibertoad]

@climba03003 Do you mean that existing Draft 7 schemas will stop working under Draft 2020?

[climba03003]

Do you mean that existing Draft 7 schemas will stop working under Draft 2020?

Mostly, yes. If it can works, I don't see a reason why, ajv would split into different version.

https://json-schema.org/draft/2019-09/release-notes
https://json-schema.org/draft/2020-12/release-notes

[metcoder95]

Draft-2020 is also marked as non-backwards compatible by ajv: https://ajv.js.org/json-schema.html#draft-2020-12

It would be nice but a lot of schemas uses mostly the 2019-draft, and doing this make them prone to break and cause friction with the adoption of Fastify v5.

Might be better to allow them do it if they want or still use 2019's draft

[gurgunday]

It's also important to note that, and this is what changed my mind on this, the AJV docs recommend sticking with 07 if we don't need the two new properties as all schemas will be affected by the reduced performance required to support them

fastify/ajv-compiler#105 (comment)
https://ajv.js.org/guide/schema-language.html#draft-2019-09-and-draft-2020-12

So it seems logical for me to keep the default 7 and let the user configure the setting

[Eomm]

Switching the ajv version is as easy as writing:

const factoryValidator = require('@fastify/ajv-compiler')()

const app = fastify({
  schemaController: {
    compilersFactory: {
      buildValidator: factoryValidator
    }
  }
})

Since changing the default would be a massive breaking change for the user (as it should rewrite all its schemas) - we could just ship a new @fastify/ajv-compiler major with ajv/2020 and let those users that want the new standard set the buildValidator option accordingly.

[mcollina]

I think we would need fast-json-stringify to support it first.