Merge pull request #1577 from json-schema-org/gregsdennis/validation-not-required

keywords only exhibit the behaviors they're defined with

Author: gregsdennis

specs/jsonschema-core.md

@@ -394,6 +394,18 @@ Additional schema keywords MAY be defined by any entity. Save for explicit
 agreement, schema authors SHALL NOT expect these additional keywords to be
 supported by implementations that do not explicitly document such support.
 
+Extension keywords MUST NOT directly modify the operation of keywords defined by
+this document or the companion JSON Schema Validation specificiation, and SHOULD
+NOT directly modify the operation of keywords defined by other extension
+documents.[^11]
+
+[^11]: JSON Schema currently does not have a namespacing mechanism, which would
+allow multiple extensions to define the same keyword differently while also
+giving the schema author the ability to declare which definition is intended.
+Such a feature is planned for future releases. See the
+[Vocabularies / Extensions project](https://github.com/orgs/json-schema-org/projects/28/views/2)
+in GitHub for more information.
+
 Implementations MAY provide the ability to register or load handlers for
 keywords that they do not support directly. The exact mechanism for registering
 and implementing such handlers is implementation-dependent.
@@ -415,20 +427,40 @@ keywords MUST NOT begin with this prefix.
 Implementations MUST refuse to evaluate schemas which contain keywords which
 they do not know how to process or explicitly choose not to process.
 
-## Keyword Behaviors
+## Keyword Behaviors {#keyword-behaviors}
 
-JSON Schema keywords fall into several general behavior categories. Assertions
-validate that an instance satisfies constraints, producing a boolean result.
-Annotations attach information that applications may use in any way they see
-fit. Applicators apply subschemas to parts of the instance and combine their
-results.
+JSON Schema keywords may exhibit one or more behaviors. This specification
+defines three such behaviors[^7]:
+
+- Assertions validate that an instance satisfies constraints, producing a
+  boolean result: `true` if the constraints are satisfied; `false` otherwise.
+- Annotations attach information to instance locations that applications may use
+  in any way they see fit.
+- Applicators apply subschemas to parts of the instance and combine their
+  results.
+
+[^7]: This specification also defines several operational directive keywords,
+such as `$id` and `$schema`, which do not exhibit these behaviors. Instead,
+these keywords provide metadata that instruct implementations on how to
+interpret and process the schema.
 
-Extension keywords SHOULD stay within these categories, keeping in mind that
+Extension keywords SHOULD be defined using these behaviors, keeping in mind that
 annotations in particular are extremely flexible. Complex behavior is usually
 better delegated to applications on the basis of annotation data than
 implemented directly as schema keywords. However, extension keywords MAY define
 other behaviors for specialized purposes.
 
+Implementations SHOULD NOT add unspecified behaviors to keywords.
+
+For the purposes of this document, an instance "validating against a keyword"
+means that the keyword produces an assertion result of `true` if the instance
+satisfies the given constraint; otherwise an assertion result of `false` is
+produced.
+
+<!-- The next two paragraphs and the following section seem to have more to
+do with schema evaluation than keywords specifically. I'd like to move them out
+of the "keyword behaviors" h2, but will do so separately. [TODO: GD] -->
+
 Evaluating an instance against a schema involves processing all of the keywords
 in the schema against the appropriate locations within the instance. Typically,
 applicator keywords are processed until a schema object with no applicators (and
@@ -581,11 +613,19 @@ the keyword's value. Alternatively, an applicator may refer to a schema
 elsewhere in the same schema document, or in a different one. The mechanism for
 identifying such referenced schemas is defined by the keyword.
 
-Applicator keywords also define how subschema or referenced schema boolean
-[assertion](#assertions) results are modified and/or combined to produce the
-boolean result of the applicator. Applicators may apply any boolean logic
-operation to the assertion results of subschemas, but MUST NOT introduce new
-assertion conditions of their own.
+Applicator keywords also behave as assertions, using the assertion results of
+each subschema or referenced schema of the keyword. These boolean results are
+modified (e.g. the `not` keyword negates its subschema's assertion) and/or
+combined (e.g. `allOf` takes the conjunction of its subschemas' assertions) to
+produce the boolean result of the applicator. Applicators may apply any boolean
+logic operation to the assertion results of subschemas, but SHOULD NOT introduce
+new assertion conditions of their own.[^2]
+
+[^2]: It is recommended that keywords have a single resposibility. An example of
+this in action is the separation between `properties`, which verifies object
+property values have the right data _if_ they exist, and `required`, which
+verifies that object properties exist. Separating these concerns allows for more
+reusable components.
 
 [Annotation](#annotations) results from subschemas are preserved in accordance
 with {{collect}} so that applications can decide how to interpret multiple
@@ -1491,8 +1531,8 @@ operators can contact the owner of a potentially misbehaving script.
 
 ## Keywords for Applying Subschemas
 
-This section defines a set of keywords that enable schema combinations and
-composition.
+This section defines a set of applicator keywords that enable schema
+combinations and composition.
 
 ### Keywords for Applying Subschemas in Place {#in-place}
 
@@ -1752,8 +1792,8 @@ The value of this keyword MUST be a non-negative integer.
 This keyword modifies the behavior of `contains` within the same schema object,
 as described below in the section for that keyword.
 
-Validation MUST always succeed against this keyword. The value of this keyword
-is used as its annotation result.
+This keyword produces no assertion result. The value of this keyword is used as
+its annotation result.
 
 ##### `minContains`
 
@@ -1762,8 +1802,8 @@ The value of this keyword MUST be a non-negative integer.
 This keyword modifies the behavior of `contains` within the same schema object,
 as described below in the section for that keyword.
 
-Validation MUST always succeed against this keyword. The value of this keyword
-is used as its annotation result.
+This keyword has no assertion behavior. The value of this keyword is used as
+its annotation result.
 
 Per {{default-behaviors}}, omitted keywords MUST NOT produce annotation results.
 However, as described in {{contains}}, the absence of this keyword's annotation

specs/jsonschema-validation.md

@@ -62,6 +62,21 @@ information. The {{format}} keyword is intended primarily as an annotation, but
 can optionally be used as an assertion. The {{content}} keywords are annotations
 for working with documents embedded as JSON strings.
 
+## Keyword Behaviors
+
+The keywords defined by this document exhibit one or more behaviors as defined by
+the [JSON Schema Core Specification](./jsonschema-core.md#keyword-behaviors).
+
+Keywords which are not defined to exhibit a particular behavior MUST NOT affect
+that aspect of the evaluation outcome. In particular, the keywords defined in
+{{annotations}} produce no assertion result and therefore are not considered
+during validation.
+
+For the purposes of this document, an instance "validating against a keyword"
+means that the keyword produces an assertion result of `true` if the instance
+satisfies the given constraint; otherwise an assertion result of `false` is
+produced.
+
 ## Interoperability Considerations
 
 ### Validation of String Instances
@@ -652,7 +667,7 @@ structures: first the header, and then the payload. Since the JWT media type
 ensures that the JWT can be represented in a JSON string, there is no need for
 further encoding or decoding.
 
-## Keywords for Basic Meta-Data Annotations
+## Keywords for Basic Meta-Data Annotations {#annotations}
 
 These general-purpose annotation keywords provide commonly used information for
 documentation and user interface display purposes. They are not intended to form