RFC: deduplicated incremental delivery

cspell.yml

@@ -4,6 +4,7 @@ ignoreRegExpList:
   - /[a-z]{2,}'s/
 words:
   # Terms of art
+  - deprioritization
   - endianness
   - interoperation
   - monospace

spec/Section 3 -- Type System.md

@@ -794,8 +794,9 @@ And will yield the subset of each object type queried:
 When querying an Object, the resulting mapping of fields are conceptually
 ordered in the same order in which they were encountered during execution,
 excluding fragments for which the type does not apply and fields or fragments
-that are skipped via `@skip` or `@include` directives. This ordering is
-correctly produced when using the {CollectFields()} algorithm.
+that are skipped via `@skip` or `@include` directives or temporarily skipped via
+`@defer`. This ordering is correctly produced when using the {CollectFields()}
+algorithm.
 
 Response serialization formats capable of representing ordered maps should
 maintain this ordering. Serialization formats which can only represent unordered
@@ -1941,6 +1942,11 @@ by a validator, executor, or client tool such as a code generator.
 
 GraphQL implementations should provide the `@skip` and `@include` directives.
 
+GraphQL implementations are not required to implement the `@defer` and `@stream`
+directives. If either or both of these directives are implemented, they must be
+implemented according to this specification. GraphQL implementations that do not
+support these directives must not make them available via introspection.
+
 GraphQL implementations that support the type system definition language must
 provide the `@deprecated` directive if representing deprecated portions of the
 schema.
@@ -2161,3 +2167,99 @@ to the relevant IETF specification.
 ```graphql example
 scalar UUID @specifiedBy(url: "https://tools.ietf.org/html/rfc4122")
 ```
+
+### @defer
+
+```graphql
+directive @defer(
+  label: String
+  if: Boolean! = true
+) on FRAGMENT_SPREAD | INLINE_FRAGMENT
+```
+
+The `@defer` directive may be provided for fragment spreads and inline fragments
+to inform the executor to delay the execution of the current fragment to
+indicate deprioritization of the current fragment. A query with `@defer`
+directive will cause the request to potentially return multiple responses, where
+non-deferred data is delivered in the initial response and data deferred is
+delivered in a subsequent response. `@include` and `@skip` take precedence over
+`@defer`.
+
+```graphql example
+query myQuery($shouldDefer: Boolean) {
+  user {
+    name
+    ...someFragment @defer(label: "someLabel", if: $shouldDefer)
+  }
+}
+fragment someFragment on User {
+  id
+  profile_picture {
+    uri
+  }
+}
+```
+
+#### @defer Arguments
+
+- `if: Boolean! = true` - When `true`, fragment _should_ be deferred (See
+  [related note](#note-088b7)). When `false`, fragment will not be deferred and
+  data will be included in the initial response. Defaults to `true` when
+  omitted.
+- `label: String` - May be used by GraphQL clients to identify the data from
+  responses and associate it with the corresponding defer directive. If
+  provided, the GraphQL service must add it to the corresponding payload.
+  `label` must be unique label across all `@defer` and `@stream` directives in a
+  document. `label` must not be provided as a variable.
+
+### @stream
+
+```graphql
+directive @stream(
+  label: String
+  if: Boolean! = true
+  initialCount: Int = 0
+) on FIELD
+```
+
+The `@stream` directive may be provided for a field of `List` type so that the
+backend can leverage technology such as asynchronous iterators to provide a
+partial list in the initial response, and additional list items in subsequent
+responses. `@include` and `@skip` take precedence over `@stream`.
+
+```graphql example
+query myQuery($shouldStream: Boolean) {
+  user {
+    friends(first: 10) {
+      nodes @stream(label: "friendsStream", initialCount: 5, if: $shouldStream)
+    }
+  }
+}
+```
+
+#### @stream Arguments
+
+- `if: Boolean! = true` - When `true`, field _should_ be streamed (See
+  [related note](#note-088b7)). When `false`, the field will not be streamed and
+  all list items will be included in the initial response. Defaults to `true`
+  when omitted.
+- `label: String` - May be used by GraphQL clients to identify the data from
+  responses and associate it with the corresponding stream directive. If
+  provided, the GraphQL service must add it to the corresponding payload.
+  `label` must be unique label across all `@defer` and `@stream` directives in a
+  document. `label` must not be provided as a variable.
+- `initialCount: Int` - The number of list items the service should return as
+  part of the initial response. If omitted, defaults to `0`. A field error will
+  be raised if the value of this argument is less than `0`.
+
+Note: The ability to defer and/or stream parts of a response can have a
+potentially significant impact on application performance. Developers generally
+need clear, predictable control over their application's performance. It is
+highly recommended that GraphQL services honor the `@defer` and `@stream`
+directives on each execution. However, the specification allows advanced use
+cases where the service can determine that it is more performant to not defer
+and/or stream. Therefore, GraphQL clients _must_ be able to process a response
+that ignores the `@defer` and/or `@stream` directives. This also applies to the
+`initialCount` argument on the `@stream` directive. Clients _must_ be able to
+process a streamed response that contains a different number of initial list
+items than what was specified in the `initialCount` argument.

spec/Section 5 -- Validation.md

@@ -422,6 +422,7 @@ FieldsInSetCanMerge(set):
   {set} including visiting fragments and inline fragments.
 - Given each pair of members {fieldA} and {fieldB} in {fieldsForName}:
   - {SameResponseShape(fieldA, fieldB)} must be true.
+  - {SameStreamDirective(fieldA, fieldB)} must be true.
   - If the parent types of {fieldA} and {fieldB} are equal or if either is not
     an Object Type:
     - {fieldA} and {fieldB} must have identical field names.
@@ -455,6 +456,16 @@ SameResponseShape(fieldA, fieldB):
   - If {SameResponseShape(subfieldA, subfieldB)} is false, return false.
 - Return true.
 
+SameStreamDirective(fieldA, fieldB):
+
+- If neither {fieldA} nor {fieldB} has a directive named `stream`.
+  - Return true.
+- If both {fieldA} and {fieldB} have a directive named `stream`.
+  - Let {streamA} be the directive named `stream` on {fieldA}.
+  - Let {streamB} be the directive named `stream` on {fieldB}.
+  - If {streamA} and {streamB} have identical sets of arguments, return true.
+- Return false.
+
 **Explanatory Text**
 
 If multiple field selections with the same response names are encountered during
@@ -463,7 +474,7 @@ unambiguous. Therefore any two field selections which might both be encountered
 for the same object are only valid if they are equivalent.
 
 During execution, the simultaneous execution of fields with the same response
-name is accomplished by {MergeSelectionSets()} and {CollectFields()}.
+name is accomplished by {CollectSubfields()}.
 
 For simple hand-written GraphQL, this rule is obviously a clear developer error,
 however nested fragments can make this difficult to detect manually.
@@ -1517,6 +1528,174 @@ query ($foo: Boolean = true, $bar: Boolean = false) {
 }
 ```
 
+### Defer And Stream Directives Are Used On Valid Root Field
+
+**Formal Specification**
+
+- For every {directive} in a document.
+- Let {directiveName} be the name of {directive}.
+- Let {mutationType} be the root Mutation type in {schema}.
+- Let {subscriptionType} be the root Subscription type in {schema}.
+- If {directiveName} is "defer" or "stream":
+  - The parent type of {directive} must not be {mutationType} or
+    {subscriptionType}.
+
+**Explanatory Text**
+
+The defer and stream directives are not allowed to be used on root fields of the
+mutation or subscription type.
+
+For example, the following document will not pass validation because `@defer`
+has been used on a root mutation field:
+
+```raw graphql counter-example
+mutation {
+  ... @defer {
+    mutationField
+  }
+}
+```
+
+### Defer And Stream Directives Are Used On Valid Operations
+
+**Formal Specification**
+
+- Let {subscriptionFragments} be the empty set.
+- For each {operation} in a document:
+  - If {operation} is a subscription operation:
+    - Let {fragments} be every fragment referenced by that {operation}
+      transitively.
+    - For each {fragment} in {fragments}:
+      - Let {fragmentName} be the name of {fragment}.
+      - Add {fragmentName} to {subscriptionFragments}.
+- For every {directive} in a document:
+  - If {directiveName} is not "defer" or "stream":
+    - Continue to the next {directive}.
+  - Let {ancestor} be the ancestor operation or fragment definition of
+    {directive}.
+  - If {ancestor} is a fragment definition:
+    - If the fragment name of {ancestor} is not present in
+      {subscriptionFragments}:
+      - Continue to the next {directive}.
+  - If {ancestor} is not a subscription operation:
+    - Continue to the next {directive}.
+  - Let {if} be the argument named "if" on {directive}.
+  - {if} must be defined.
+  - Let {argumentValue} be the value passed to {if}.
+  - {argumentValue} must be a variable, or the boolean value "false".
+
+**Explanatory Text**
+
+The defer and stream directives can not be used to defer or stream data in
+subscription operations. If these directives appear in a subscription operation
+they must be disabled using the "if" argument. This rule will not permit any
+defer or stream directives on a subscription operation that cannot be disabled
+using the "if" argument.
+
+For example, the following document will not pass validation because `@defer`
+has been used in a subscription operation with no "if" argument defined:
+
+```raw graphql counter-example
+subscription sub {
+  newMessage {
+    ... @defer {
+      body
+    }
+  }
+}
+```
+
+### Defer And Stream Directive Labels Are Unique
+
+**Formal Specification**
+
+- Let {labelValues} be an empty set.
+- For every {directive} in the document:
+  - Let {directiveName} be the name of {directive}.
+  - If {directiveName} is "defer" or "stream":
+    - For every {argument} in {directive}:
+      - Let {argumentName} be the name of {argument}.
+      - Let {argumentValue} be the value passed to {argument}.
+      - If {argumentName} is "label":
+        - {argumentValue} must not be a variable.
+        - {argumentValue} must not be present in {labelValues}.
+        - Append {argumentValue} to {labelValues}.
+
+**Explanatory Text**
+
+The `@defer` and `@stream` directives each accept an argument "label". This
+label may be used by GraphQL clients to uniquely identify response payloads. If
+a label is passed, it must not be a variable and it must be unique within all
+other `@defer` and `@stream` directives in the document.
+
+For example the following document is valid:
+
+```graphql example
+{
+  dog {
+    ...fragmentOne
+    ...fragmentTwo @defer(label: "dogDefer")
+  }
+  pets @stream(label: "petStream") {
+    name
+  }
+}
+
+fragment fragmentOne on Dog {
+  name
+}
+
+fragment fragmentTwo on Dog {
+  owner {
+    name
+  }
+}
+```
+
+For example, the following document will not pass validation because the same
+label is used in different `@defer` and `@stream` directives.:
+
+```raw graphql counter-example
+{
+  dog {
+    ...fragmentOne @defer(label: "MyLabel")
+  }
+  pets @stream(label: "MyLabel") {
+    name
+  }
+}
+
+fragment fragmentOne on Dog {
+  name
+}
+```
+
+### Stream Directives Are Used On List Fields
+
+**Formal Specification**
+
+- For every {directive} in a document.
+- Let {directiveName} be the name of {directive}.
+- If {directiveName} is "stream":
+  - Let {adjacent} be the AST node the directive affects.
+  - {adjacent} must be a List type.
+
+**Explanatory Text**
+
+GraphQL directive locations do not provide enough granularity to distinguish the
+type of fields used in a GraphQL document. Since the stream directive is only
+valid on list fields, an additional validation rule must be used to ensure it is
+used correctly.
+
+For example, the following document will only pass validation if `field` is
+defined as a List type in the associated schema.
+
+```graphql counter-example
+query {
+  field @stream(initialCount: 0)
+}
+```
+
 ## Variables
 
 ### Variable Uniqueness