partial-promise-proxy

Combines a partially defined object with a function that returns a promise of the rest of the partial object. Any properties defined in the initial partial object will be available immediately, while other properties will wait for the resolution of the promise. (credit goes to @brysgo for the idea)

Install

yarn add partial-promise-proxy

Or using npm

npm install --save partial-promise-proxy

Usage

function partialPromiseProxy(partialObject, partialFunc)

Arguments

• partialObject: a javascript object that consists of the properties that should be available immediately
• partialFunc: a function that returns a promise for the remaining properties of the object

Why would you ever want to do this?

When building out a GraphQL API that wraps a set of legacy services, its not uncommon to end up with a scenario that looks like this:

const Address = new GraphQLObjectType({
  name: "address",
  fields: {
    country: { type: GraphQLString },
    city: { type: GraphQLString },
    streetName: { type: GraphQLString },
    streetNumber: { type: GraphQLString }
  }
});

const Customer = new GraphQLObjectType({
  name: "customer",
  fields: {
    id: { type: new GraphQLNonNull(GraphQLID) },
    firstName: { type: GraphQLString },
    lastName: { type: GraphQLString },
    address: { 
      type: Address,
      resolve: (customer, _, context) => {
        const { legacyAddressService } = context;
        const { id } = customer;
        return legacyAddressService.getAddress(id);
      } 
    }
  },
});

const customerSchema = new GraphQLSchema({
  query: new GraphQLObjectType({
    name: 'query',
    fields: {
      GetCustomer: {
        type: Customer,
        resolve: (_, args, context) => {
          const { legacyCustomerService } = context;
          const { id } = args;
          return legacyCustomerService.getCustomer(id);
        }
      }
    }
  })
});

In this instance, there is a Customer type with a nested Address type, where the resolution of both rely on different back-end systems. When resolving a query against Customer and Address, GraphQL will execute the call to the legacyCustomerService, but wait on the response before calling the legacyAddressService. Both legacy services only require the customer id argument passed in to getCustomer on the root query, so why can't we call both without waiting?

Because GraphQL can handle resolvers that return either the value or a promise of the value, we can create a partial proxy to the requested type where some fields resolve to their values, while some resolve to a promise which GraphQL will wait for.

Changing the code above to use a partial-promise-proxy allows both legacy service calls to execute without waiting:

import { partialPromiseProxy } from "partial-promise-proxy";

const Address = new GraphQLObjectType({
  name: "address",
  fields: {
    country: { type: GraphQLString },
    city: { type: GraphQLString },
    streetName: { type: GraphQLString },
    streetNumber: { type: GraphQLString }
  }
});

const Customer = new GraphQLObjectType({
  name: "customer",
  fields: {
    id: { type: new GraphQLNonNull(GraphQLID) },
    firstName: { type: GraphQLString },
    lastName: { type: GraphQLString },
    address: { 
      type: Address,
      resolve: (customer, _, context) => {
        const { legacyAddressService } = context;
        const { id } = customer;
        return legacyAddressService.getAddress(id);
      } 
    }
  },
});

const customerSchema = new GraphQLSchema({
  query: new GraphQLObjectType({
    name: 'query',
    fields: {
      GetCustomer: {
        type: Customer,
        resolve: (_, args, context) => {
          const { legacyCustomerService } = context;
          const { id } = args;
          return partialPromiseProxy({ id }, () => legacyCustomerService.getCustomer(id)); // <-- wrap the promise in a partial proxy
        }
      }
    }
  })
});

Passing in { id } to the partialPromiseProxy creates a proxy object where id will be available immediately, while any other properties that are accessed will return a promise that GraphQL will wait to resolve.