Schema Directives

Adding Behaviour

Schema Definition Language (SDL) allows you to define your graphql types in a declarative manner without using code. However you still need to wire in all the logic that backs those types and their fields.

Schema directives allow you to do this. You can place directives on SDL elements and then write the backing logic once and have it apply in many places.

This idea of "writing it once" is the key concept here. There is only code place where logic needs to be written and it is then applied to all the places in the SDL that have a named directive.

This is a more powerful model than wiring in 10-100s of data fetchers like you might with the conventional runtime wiring.

For example imagine you have a type like the following

type Employee
  id : ID
  name : String!
  startDate : String!
  salary : Float
}

Publishing salary information to every one who can see this employee's name might not be want you want. Rather you want some sort of access control to be in place such that if your role is that of a manager, you can see salaries, otherwise you get no data back.

Directives can help you declare this more easily. Our declaration above would become something like the following:

directive @auth(role : String!) on FIELD_DEFINITION

type Employee
  id : ID
  name : String!
  startDate : String!
  salary : Float @auth(role : "manager")
}

So we have said that only people who have the role manager are authorised to see this field. We can now use this directive on ANY field that needs manager role authorisation.

directive @auth(role : String!) on FIELD_DEFINITION

type Employee
  id : ID
  name : String!
  startDate : String!
  salary : Float @auth(role : "manager")
}

type Department {
  id : ID
  name : String
  yearlyOperatingBudget : Float @auth(role : "manager")
  monthlyMarketingBudget : Float @auth(role : "manager")
}

We now need to wire in the code that can handle any field with this @auth directive. We use graphql.schema.idl.SchemaDirectiveWiring to do this.

class AuthorisationDirective implements SchemaDirectiveWiring {

    @Override
    public GraphQLFieldDefinition onField(SchemaDirectiveWiringEnvironment<GraphQLFieldDefinition> environment) {
        String targetAuthRole = (String) environment.getDirective().getArgument("role").getArgumentValue().getValue();

        //
        // build a data fetcher that first checks authorisation roles before then calling the original data fetcher
        //
        DataFetcher originalDataFetcher = environment.getFieldDataFetcher();
        DataFetcher authDataFetcher = new DataFetcher() {
            @Override
            public Object get(DataFetchingEnvironment dataFetchingEnvironment) throws Exception {
                Map<String, Object> contextMap = dataFetchingEnvironment.getContext();
                AuthorisationCtx authContext = (AuthorisationCtx) contextMap.get("authContext");

                if (authContext.hasRole(targetAuthRole)) {
                    return originalDataFetcher.get(dataFetchingEnvironment);
                } else {
                    return null;
                }
            }
        };
        //
        // now change the field definition to have the new authorising data fetcher
        return environment.setFieldDataFetcher(authDataFetcher);
    }
}

//
// we wire this into the runtime by directive name
//
RuntimeWiring.newRuntimeWiring()
        .directive("auth", new AuthorisationDirective())
        .build();

This has modified the GraphQLFieldDefinition so that its original data fetcher will ONLY be called if the current authorisation context has the manager role. You can use any mechanism for authorisation, for example Spring Security or anything else.

You would provide this authorisation checker into the execution "context" object of the graphql input, so it can then be accessed later in the DataFetchingEnvironment.

AuthorisationCtx authCtx = AuthorisationCtx.obtain();

ExecutionInput executionInput = ExecutionInput.newExecutionInput()
        .query(query)
        .context(authCtx)
        .build();

Declaring Directives

In order to use a directive in SDL, the graphql specification requires that you MUST declare its shape before using it. Our @auth directive example above needs to be declared like so before use.

# This is a directive declaration
directive @auth(role : String!) on FIELD_DEFINITION

type Employee
  id : ID

  # and this is a usage of that declared directive
  salary : Float @auth(role : "manager")
}

The one exception to this is the @deprecated directive which is implicitly declared for you as follows:

directive @deprecated(reason: String = "No longer supported") on FIELD_DEFINITION | ENUM_VALUE

The valid SDL directive locations are as follows:

SCHEMA,
SCALAR,
OBJECT,
FIELD_DEFINITION,
ARGUMENT_DEFINITION,
INTERFACE,
UNION,
ENUM,
ENUM_VALUE,
INPUT_OBJECT,
INPUT_FIELD_DEFINITION

Directives are commonly applied to fields definitions but as you can see there are a number of places they can be applied.

Another Example - Date Formatting

Date formatting is a cross-cutting concern that we should only have to write once and apply it in many areas.

The following demonstrates an example schema directive that can apply date formatting to fields that are LocaleDate objects.

What's great in this example is that it adds an extra format argument to each field that it is applied to. So the clients can opt into what ever date formatting you provide per request.

directive @dateFormat on FIELD_DEFINITION

type Query {
  dateField : String @dateFormat
}

Then our runtime code could be :

public static class DateFormatting implements SchemaDirectiveWiring {
    @Override
    public GraphQLFieldDefinition onField(SchemaDirectiveWiringEnvironment<GraphQLFieldDefinition> environment) {
        GraphQLFieldDefinition field = environment.getElement();
        GraphQLFieldsContainer parentType = environment.getFieldsContainer();
        //
        // DataFetcherFactories.wrapDataFetcher is a helper to wrap data fetchers so that CompletionStage is handled correctly
        // along with POJOs
        //
        DataFetcher originalFetcher = environment.getCodeRegistry().getDataFetcher(parentType, field);
        DataFetcher dataFetcher = DataFetcherFactories.wrapDataFetcher(originalFetcher, ((dataFetchingEnvironment, value) -> {
            DateTimeFormatter dateTimeFormatter = buildFormatter(dataFetchingEnvironment.getArgument("format"));
            if (value instanceof LocalDateTime) {
                return dateTimeFormatter.format((LocalDateTime) value);
            }
            return value;
        }));

        //
        // This will extend the field by adding a new "format" argument to it for the date formatting
        // which allows clients to opt into that as well as wrapping the base data fetcher so it
        // performs the formatting over top of the base values.
        //
        FieldCoordinates coordinates = FieldCoordinates.coordinates(parentType, field);
        environment.getCodeRegistry().dataFetcher(coordinates, dataFetcher);

        return field.transform(builder -> builder
                .argument(GraphQLArgument
                        .newArgument()
                        .name("format")
                        .type(Scalars.GraphQLString)
                        .defaultValueProgrammatic("dd-MM-YYYY")
                )
        );
    }

    private DateTimeFormatter buildFormatter(String format) {
        String dtFormat = format != null ? format : "dd-MM-YYYY";
        return DateTimeFormatter.ofPattern(dtFormat);
    }
}

static GraphQLSchema buildSchema() {

    String sdlSpec = "directive @dateFormat on FIELD_DEFINITION\n" +
                  "type Query {\n" +
                  "    dateField : String @dateFormat \n" +
                  "}";

    TypeDefinitionRegistry registry = new SchemaParser().parse(sdlSpec);

    RuntimeWiring runtimeWiring = RuntimeWiring.newRuntimeWiring()
            .directive("dateFormat", new DateFormatting())
            .build();

    return new SchemaGenerator().makeExecutableSchema(registry, runtimeWiring);
}

public static void main(String[] args) {
    GraphQLSchema schema = buildSchema();
    GraphQL graphql = GraphQL.newGraphQL(schema).build();

    Map<String, Object> root = new HashMap<>();
    root.put("dateField", LocalDateTime.of(1969, 10, 8, 0, 0));

    String query = "" +
            "query {\n" +
            "    default : dateField \n" +
            "    usa : dateField(format : \"MM-dd-YYYY\") \n" +
            "}";

    ExecutionInput executionInput = ExecutionInput.newExecutionInput()
            .root(root)
            .query(query)
            .build();

    ExecutionResult executionResult = graphql.execute(executionInput);
    Map<String, Object> data = executionResult.getData();

    // data['default'] == '08-10-1969'
    // data['usa'] == '10-08-1969'
}

Notice the SDL definition did not have a format argument yet once the directive wiring is applied, it is added to the field definition and hence clients can begin to use it.

Please note that graphql-java does not ship with this implementation. It is merely provided here as an example of what you could add yourself.

Chaining Behaviour

The directives are applied in the order they are encountered. For example imagine directives that changed the case of a field value.

directive @uppercase on FIELD_DEFINITION
directive @lowercase on FIELD_DEFINITION
directive @mixedcase on FIELD_DEFINITION
directive @reversed on FIELD_DEFINITION

type Query {
  lowerCaseValue : String @uppercase
  upperCaseValue : String @lowercase
  mixedCaseValue : String @mixedcase

  #
  # directives are applied in order hence this will be lower, then upper, then mixed then reversed
  #
  allTogetherNow : String @lowercase @uppercase @mixedcase @reversed
}

When the above was executed each directive would be applied one on top of the other. Each directive implementation should be careful to preserve the previous data fetcher to retain behaviour (unless of course you mean to throw it away)