[Subscription API] Purpose of Filter schema object in OAS not clear

[HenriKorver]

In the OAS for the Subscription API the Filter object is included:

Filter:
      title: "Filter"
      description: "A filter from a selection of multiple filter types and dialects"
      type: object
      additionalProperties: true

The Filter object is used in all the filter dialects as the basis of the allOf construction, for instance in the ExactFilter:

ExactFilter:
      allOf:
        - $ref: "#/components/schemas/Filter"
        - type: object
          title: "exact filter"
          description: "This filter evaluates to 'true' if the 'value' exactly matches the value of the indicated CloudEvents context attribute"
          properties:
            exact:
              $ref: "#/components/schemas/CloudEventsAttribute"
          additionalProperties: false

It is not clear for me what the purpose is of this allOf construction with the Filter object as basis schema. It seems that the Filter object offers an extension mechanism to the standard filter dialects (All, Any, Not, Exact, Suffix, Prefix, etc.) because it contains the clause additionalProperties: true. Could somebody give me a concrete example of a JSON filter expression where this is used?

[HenriKorver]

In addition to the question above. In some filter dialects the Filter object is also used in the oneOf- besides the allOf-construction. This seems to be incorrect (see #1038)

AllFilter:
      allOf:
        - $ref: "#/components/schemas/Filter"
        - type: object
          description: "all filter"
          properties:
            all:
              description: "This filter evaluates to 'true' if all contained filters are 'true'"
              type: array
              minItems: 1
              items:
                title: "Filter entry"
                oneOf:
                - $ref: "#/components/schemas/Filter"
                - $ref: "#/components/schemas/AllFilter"
                - $ref: "#/components/schemas/AnyFilter"
                - $ref: "#/components/schemas/NotFilter"
                - $ref: "#/components/schemas/ExactFilter"
                - $ref: "#/components/schemas/PrefixFilter"
                - $ref: "#/components/schemas/SuffixFilter"
                - $ref: "#/components/schemas/SqlFilter"
          additionalProperties: false

[duglin]

I believe this was done to allow for new filter expressions to be defined. But as you noted #1038 perhaps we need to define it differently since it matches everything.

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[HenriKorver]

/remove-lifecycle stale

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This issue is stale because it has been open for 30 days with no
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[HenriKorver]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 30 days with no
activity. Mark as fresh by updating e.g., adding the comment /remove-lifecycle stale.

[HenriKorver]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 30 days with no
activity. Mark as fresh by updating e.g., adding the comment /remove-lifecycle stale.