Handling federation _entities queries without creating @Query

common/schema-builder/src/main/java/io/smallrye/graphql/schema/Annotations.java

@@ -590,6 +590,7 @@ private static Map<DotName, AnnotationInstance> getAnnotationsWithFilter(org.jbo
     public static final DotName ERROR_CODE = DotName.createSimple("io.smallrye.graphql.api.ErrorCode");
     public static final DotName DATAFETCHER = DotName.createSimple("io.smallrye.graphql.api.DataFetcher");
     public static final DotName SUBCRIPTION = DotName.createSimple("io.smallrye.graphql.api.Subscription");
+    public static final DotName RESOLVER = DotName.createSimple("io.smallrye.graphql.api.federation.Resolver");
     public static final DotName DIRECTIVE = DotName.createSimple("io.smallrye.graphql.api.Directive");
     public static final DotName DEFAULT_NON_NULL = DotName.createSimple("io.smallrye.graphql.api.DefaultNonNull");
     public static final DotName NULLABLE = DotName.createSimple("io.smallrye.graphql.api.Nullable");

common/schema-builder/src/main/java/io/smallrye/graphql/schema/SchemaBuilder.java

@@ -386,6 +386,9 @@ private void addOperations(Optional<Group> group, Schema schema, List<MethodInfo
                 } else {
                     schema.addSubscription(subscription);
                 }
+            } else if (annotationsForMethod.containsOneOfTheseAnnotations(Annotations.RESOLVER)) {
+                Operation resolver = operationCreator.createOperation(methodInfo, OperationType.RESOLVER, null);
+                schema.addResolver(resolver);
             }
         }
     }

common/schema-builder/src/main/java/io/smallrye/graphql/schema/creator/OperationCreator.java

@@ -304,6 +304,8 @@ private static DotName getOperationAnnotation(OperationType operationType) {
                 return Annotations.MUTATION;
             case SUBSCRIPTION:
                 return Annotations.SUBCRIPTION;
+            case RESOLVER:
+                return Annotations.RESOLVER;
             default:
                 break;
         }

common/schema-model/src/main/java/io/smallrye/graphql/schema/model/OperationType.java

@@ -8,5 +8,6 @@
 public enum OperationType {
     QUERY,
     MUTATION,
-    SUBSCRIPTION
+    SUBSCRIPTION,
+    RESOLVER,
 }

common/schema-model/src/main/java/io/smallrye/graphql/schema/model/Schema.java

@@ -21,6 +21,7 @@ public final class Schema implements Serializable {
     private Set<Operation> queries = new HashSet<>();
     private Set<Operation> mutations = new HashSet<>();
     private Set<Operation> subscriptions = new HashSet<>();
+    private Set<Operation> resolvers = new HashSet<>();
 
     private Map<Group, Set<Operation>> groupedQueries = new HashMap<>();
     private Map<Group, Set<Operation>> groupedMutations = new HashMap<>();
@@ -95,6 +96,22 @@ public boolean hasSubscriptions() {
         return !this.subscriptions.isEmpty();
     }
 
+    public Set<Operation> getResolvers() {
+        return resolvers;
+    }
+
+    public void setResolvers(Set<Operation> resolvers) {
+        this.resolvers = resolvers;
+    }
+
+    public void addResolver(Operation resolver) {
+        this.resolvers.add(resolver);
+    }
+
+    public boolean hasResolvers() {
+        return !this.resolvers.isEmpty();
+    }
+
     public Map<Group, Set<Operation>> getGroupedQueries() {
         return groupedQueries;
     }

docs/federation.md

@@ -84,3 +84,122 @@ public class Prices {
 
 It is crucial that the sequence of argument list matches with the order of result list. Currently, the name of the Argument `id` must match with the property name in the type.
 
+## Federation Reference Resolver
+
+In federation you also may want extend external type by some fields, without publishing queries into schema. You can do it using @Resolver
+
+```java
+@Extends
+@Key(fields = @FieldSet("upc"))
+public final class Product {
+    @External
+    @NonNull
+    private String upc;
+    @External
+    private Integer weight;
+    @External
+    private Integer price;
+    private Boolean inStock;
+    @Requires(fields = @FieldSet("price weight"))
+    private Integer shippingPrice;
+}
+
+@GraphQLApi
+public class Api {
+    @Query // 0 query, that will be added into schema
+    public Product findByUPC(String upc) {
+        return new Product(upc , ...etc);
+    }
+
+    @Resolver // 1 You dont receive external fields price weight here, just key
+    public Product resolveByUPC(String upc) {
+        return new Product(upc , ...etc);
+    }
+
+    @Resolver // 2 The order of variables doesn't matter
+    public Product resolveByUPCForShipping(int price, String upc, @Name("weight") int someWeight) {
+        return new Product(upc , someWeight, price, (price * someWeight) /*calculate shippingPrice */, ...etc);
+    }
+
+    @Resolver // 3 
+    public Product resolveByUPCForSource(int price, String upc) {
+        return new Product(upc, price, ...etc);
+    }
+
+    @Requires(fields = @FieldSet("price"))
+    public int anotherWeight(@Source Product product) {
+        return product.price() * 2;
+    }
+}
+```
+
+Will be generated next schema
+```
+type Product @extends @key(fields : "upc") {
+  anotherWeight: Int! @requires(fields : "price")
+  inStock: Boolean
+  price: Int @external
+  shippingPrice: Int @requires(fields : "price weight")
+  upc: String! @external
+  weight: Int @external
+}
+
+type Query {
+  _entities(representations: [_Any!]!): [_Entity]!
+  _service: _Service!
+}
+```
+
+These methods will only be available to the federation router, which send next request
+```
+// request 1
+query {
+  _entities(representations: [{ 
+    "__typename": "Product", 
+    "upc": "1" // just id key
+  }]) {
+    __typename
+    ... on Product {
+      inStock
+    }
+  }
+}
+
+// request 2
+query {  
+  _entities(representations: [{ 
+    "__typename": "Product", 
+    "upc": "1", // id key
+    "price": 100, // shippingPrice requires this field
+    "weight": 100  // shippingPrice requires this field
+  }]) {
+    __typename
+    ... on Product {
+      inStock
+      shippingPrice
+    }
+  }
+}
+
+// request 3
+query {
+  _entities(representations: [{
+    "__typename": "Product",
+    "upc": "2",
+    "price": 1299  // anotherWeight requires this field
+  }
+  ]) {
+    __typename
+    ... on Product {
+      anotherWeight
+    }
+  }
+}
+```
+
+Unfortunately, you will have to make separate methods with different `@External` parameters. 
+
+It is not currently possible to combine them into one separate type.
+
+You also can using @Query (if you want add queries into schema) or @Resolver (requests 0 and 1). 
+And if it was request `_entities` - @Resolvers methods are checked first (they have higher priority). 

server/api/src/main/java/io/smallrye/graphql/api/federation/Resolver.java

@@ -0,0 +1,15 @@
+package io.smallrye.graphql.api.federation;
+
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import io.smallrye.common.annotation.Experimental;
+
+@Target(ElementType.METHOD)
+@Retention(RUNTIME)
+@Experimental("Resolver method without creating query method")
+public @interface Resolver {
+}

server/implementation-servlet/src/main/java/io/smallrye/graphql/entry/http/IndexInitializer.java

@@ -41,6 +41,7 @@
 import io.smallrye.graphql.api.federation.Override;
 import io.smallrye.graphql.api.federation.Provides;
 import io.smallrye.graphql.api.federation.Requires;
+import io.smallrye.graphql.api.federation.Resolver;
 import io.smallrye.graphql.api.federation.Shareable;
 import io.smallrye.graphql.api.federation.Tag;
 import io.smallrye.graphql.api.federation.link.Import;
@@ -126,6 +127,7 @@ private IndexView createCustomIndex() {
             indexer.index(convertClassToInputStream(Shareable.class));
             indexer.index(convertClassToInputStream(Tag.class));
             indexer.index(convertClassToInputStream(OneOf.class));
+            indexer.index(convertClassToInputStream(Resolver.class));
         } catch (IOException ex) {
             throw new RuntimeException(ex);
         }

server/implementation/src/main/java/io/smallrye/graphql/bootstrap/Bootstrap.java

@@ -30,6 +30,7 @@
 
 import com.apollographql.federation.graphqljava.Federation;
 
+import graphql.Scalars;
 import graphql.introspection.Introspection.DirectiveLocation;
 import graphql.schema.Coercing;
 import graphql.schema.DataFetcher;
@@ -124,7 +125,7 @@ public static GraphQLSchema bootstrap(Schema schema) {
     }
 
     public static GraphQLSchema bootstrap(Schema schema, boolean skipInjectionValidation) {
-        if (schema != null && (schema.hasOperations())) {
+        if (schema != null && (schema.hasOperations() || Config.get().isFederationEnabled())) {
             Bootstrap bootstrap = new Bootstrap(schema, skipInjectionValidation);
             bootstrap.generateGraphQLSchema();
             return bootstrap.graphQLSchema;
@@ -185,7 +186,7 @@ private void generateGraphQLSchema() {
         createGraphQLObjectTypes();
         createGraphQLInputObjectTypes();
 
-        addQueries(schemaBuilder);
+        GraphQLObjectType queryRootType = addQueries(schemaBuilder);
         addMutations(schemaBuilder);
         addSubscriptions(schemaBuilder);
         schemaBuilder.withSchemaAppliedDirectives(Arrays.stream(
@@ -214,9 +215,23 @@ private void generateGraphQLSchema() {
 
         if (Config.get().isFederationEnabled()) {
             log.enableFederation();
+
+            if (!(schema.hasQueries() || schema.hasResolvers() || schema.hasMutations() || schema.hasSubscriptions())) {
+                throw new RuntimeException("You must define at least one method marked with one of the annotations - "
+                        + "@Query, @Mutation, @Subscription, @Resolver. An empty diagram has no meaning");
+            }
+
+            // hack! For schema build success if queries are empty.
+            // It will be overrides in Federation transformation
+            addDummySdlQuery(schemaBuilder, queryRootType);
+
+            // Build reference resolvers type, without adding to schema (just for federation)
+            GraphQLObjectType resolversType = buildResolvers();
+
             GraphQLSchema rawSchema = schemaBuilder.build();
             this.graphQLSchema = Federation.transform(rawSchema)
-                    .fetchEntities(new FederationDataFetcher(rawSchema.getQueryType(), rawSchema.getCodeRegistry()))
+                    .fetchEntities(
+                            new FederationDataFetcher(resolversType, rawSchema.getQueryType(), rawSchema.getCodeRegistry()))
                     .resolveEntityType(fetchEntityType())
                     .setFederation2(true)
                     .build();
@@ -225,6 +240,35 @@ private void generateGraphQLSchema() {
         }
     }
 
+    private void addDummySdlQuery(GraphQLSchema.Builder schemaBuilder, GraphQLObjectType queryRootType) {
+        GraphQLObjectType type = GraphQLObjectType.newObject()
+                .name("_Service")
+                .field(GraphQLFieldDefinition
+                        .newFieldDefinition().name("sdl")
+                        .type(new GraphQLNonNull(Scalars.GraphQLString))
+                        .build())
+                .build();
+
+        GraphQLFieldDefinition field = GraphQLFieldDefinition.newFieldDefinition()
+                .name("_service")
+                .type(GraphQLNonNull.nonNull(type))
+                .build();
+
+        GraphQLObjectType.Builder newQueryType = GraphQLObjectType.newObject(queryRootType);
+
+        newQueryType.field(field);
+        schemaBuilder.query(newQueryType.build());
+    }
+
+    private GraphQLObjectType buildResolvers() {
+        GraphQLObjectType.Builder queryBuilder = GraphQLObjectType.newObject()
+                .name("Resolver");
+        if (schema.hasResolvers()) {
+            addRootObject(queryBuilder, schema.getResolvers(), "Resolver");
+        }
+        return queryBuilder.build();
+    }
+
     private TypeResolver fetchEntityType() {
         return env -> {
             Object src = env.getObject();
@@ -326,7 +370,7 @@ private void createGraphQLDirectiveType(DirectiveType directiveType) {
         directiveTypes.add(directiveBuilder.build());
     }
 
-    private void addQueries(GraphQLSchema.Builder schemaBuilder) {
+    private GraphQLObjectType addQueries(GraphQLSchema.Builder schemaBuilder) {
         GraphQLObjectType.Builder queryBuilder = GraphQLObjectType.newObject()
                 .name(QUERY)
                 .description(QUERY_DESCRIPTION);
@@ -340,6 +384,7 @@ private void addQueries(GraphQLSchema.Builder schemaBuilder) {
 
         GraphQLObjectType query = queryBuilder.build();
         schemaBuilder.query(query);
+        return query;
     }
 
     private void addMutations(GraphQLSchema.Builder schemaBuilder) {

server/implementation/src/main/java/io/smallrye/graphql/bootstrap/FederationDataFetcher.java

@@ -38,10 +38,13 @@ public class FederationDataFetcher implements DataFetcher<CompletableFuture<List
 
     public static final String TYPENAME = "__typename";
     private final GraphQLObjectType queryType;
+    private final GraphQLObjectType resolversType;
     private final GraphQLCodeRegistry codeRegistry;
     private final HashMap<TypeAndArgumentNames, TypeFieldWrapper> cache = new HashMap<>();
 
-    public FederationDataFetcher(GraphQLObjectType queryType, GraphQLCodeRegistry codeRegistry) {
+    public FederationDataFetcher(GraphQLObjectType resolversType, GraphQLObjectType queryType,
+            GraphQLCodeRegistry codeRegistry) {
+        this.resolversType = resolversType;
         this.queryType = queryType;
         this.codeRegistry = codeRegistry;
     }
@@ -103,6 +106,11 @@ && matchesArguments(typeAndArgumentNames, definition)) {
     }
 
     private TypeFieldWrapper findBatchFieldDefinition(TypeAndArgumentNames typeAndArgumentNames) {
+        for (GraphQLFieldDefinition field : resolversType.getFields()) {
+            if (matchesReturnTypeList(field, typeAndArgumentNames.type) && matchesArguments(typeAndArgumentNames, field)) {
+                return new TypeFieldWrapper(resolversType, field);
+            }
+        }
         for (GraphQLFieldDefinition field : queryType.getFields()) {
             if (matchesReturnTypeList(field, typeAndArgumentNames.type) && matchesArguments(typeAndArgumentNames, field)) {
                 return new TypeFieldWrapper(queryType, field);
@@ -119,6 +127,11 @@ private TypeFieldWrapper findBatchFieldDefinition(TypeAndArgumentNames typeAndAr
     }
 
     private TypeFieldWrapper findFieldDefinition(TypeAndArgumentNames typeAndArgumentNames) {
+        for (GraphQLFieldDefinition field : resolversType.getFields()) {
+            if (matchesReturnType(field, typeAndArgumentNames.type) && matchesArguments(typeAndArgumentNames, field)) {
+                return new TypeFieldWrapper(resolversType, field);
+            }
+        }
         for (GraphQLFieldDefinition field : queryType.getFields()) {
             if (matchesReturnType(field, typeAndArgumentNames.type) && matchesArguments(typeAndArgumentNames, field)) {
                 return new TypeFieldWrapper(queryType, field);
@@ -131,7 +144,6 @@ private TypeFieldWrapper findFieldDefinition(TypeAndArgumentNames typeAndArgumen
                 return typeFieldWrapper;
             }
         }
-
         throw new RuntimeException(
                 "no query found for " + typeAndArgumentNames.type + " by " + typeAndArgumentNames.argumentNames);
     }