Introduce Core Source Schema Directives

spec/Section 2 -- Source Schema.md

@@ -2,61 +2,75 @@
 
 ## Directives
 
-### @entityResolver
+### @lookup
+
+The `@lookup` directive is applied to object fields to enable the resolution of entities from a _source schema_ by a stable key. This directive marks a field as capable of performing a lookup operation.
 
 ```graphql
-directive @entityResolver on FIELD_DEFINITION
+directive @lookup on FIELD_DEFINITION
 ```
 
-Entity resolvers are fields on the query root type of a subgraph that can
-resolve an entity by a stable key. The stable key is defined by the arguments of
-the field.
+Lookup fields allow the composite schema to resolve entities from a source schema by a stable key. The stable key is defined by the arguments of the field. Only fields that are annotated with the `@lookup` directive will be recognized as lookup field.
 
 ```graphql example
 extend type Query {
   version: Int # NOT an entity resolver.
-  personById(id: ID!): Person @entityResolver
+  personById(id: ID!): Person @lookup
 }
 
-extend type Person {
-  id: ID! # matches the argument of personById
+extend type Person @key(fields "id") {
+  id: ID!
 }
 ```
 
-The arguments of an entity resolver field must match fields of the returning
-type.
+The arguments of a lookup field must correspond to the fields specified by a `@key` directive annotated on the type returned by the lookup field.
 
 ```graphql example
 extend type Query {
-  node(id: ID!): Node @entityResolver
+  node(id: ID!): Node @lookup
 }
 
-interface Node {
+interface Node @key(fields "id")  {
   id: ID!
 }
 ```
 
-When an entity resolver returns an interface all implementing types are inferred
-as entities.
+Lookup fields returning an interface or union type can be used as lookup field for all possible object types.
 
 ```graphql example
 extend type Query {
-  entityById(id: ID!, categoryId: Int): Entity @entityResolver
+  entityById(id: ID!, categoryId: Int): Entity @lookup
 }
 
 union Entity = Cat | Dog
 
-extend type Dog {
+extend type Dog @key(fields "id") {
   id: ID!
   categoryId: Int
 }
 
-extend type Cat {
+extend type Cat @key(fields "id") {
   id: ID!
   categoryId: Int
 }
 ```
 
+Lookup fields must be accessible from the Query type. If not directly on the Query type, they should be accessible via fields that do not require arguments, starting from the Query type.
+
+```graphql example
+extend type Query {
+  lookups: Lookups!
+}
+
+type Lookups {
+  personById(id: ID!): Person @lookup
+}
+
+extend type Person @key(fields "id") {
+  id: ID!
+}
+```
+
 ### @is
 
 ```graphql