@Nullable
    private static String toStringFromJson(@Nullable Object value) throws GraphqlWebSocketCloseException {
        if (value == null) {
            return null;
        }

        if (value instanceof String) {
            return (String) value;
        } else {
            throw new GraphqlWebSocketCloseException(4400, "Expected string value");
        }
    }
Receives an event and returns a response if one should be sent.
graphql/src/main/java/com/linecorp/armeria/server/graphql/GraphqlWSSubProtocol.java