GraphQL mesh gateway with local federation

Disclaimer: This is 100% glue around the the following excellent packages:

• @graphql-mesh
• @theguild/federation-composition

... But for this specific workflow it sets things up in a simple and clean way and removes un-needed boilerplate.

The package exposes only 2 functions:

1. createSupergraph - Takes external services and the local schema to produce the supergraph definition
2. createMeshInstance - Takes the supergraph definition from createSupergraph and local schema to build a executable gateway configuration that can be used with graphql-yoga

Workflow

graph LR;
    subgraph Instance
        gateway(Graphql\ngateway):::local--context-->local(Local executable schema):::local
        db{{Supergraph\ndefinition}}:::data<-.load.->gateway
    end
    gateway--headers--->federated1(External service);
    gateway--headers--->federated2(External service);

	classDef local fill:#00b54f

Define external services

import type { SubgraphService } from "mesh-local-federation";

const subgraphs: SubgraphService[] = [
	{
		subgraphName: "users",
		endpoint: "https://federated.users.service.endpoint/graphql",
	},
	{
		subgraphName: "orders",
		endpoint: "https://federated.orders.service.endpoint/graphql",
	},
	// ...
];

Create local executable schema

import { buildSubgraphSchema } from "@graphql-tools/federation";

export const localSchema = buildSubgraphSchema({
	typeDefs: /* GraphQL */ `
		type Query {
			hello: String
		}
	`,
	resolvers: {
		Query: {
			hello: (obj, args, context, info) => {
				// Full yoga server context is passed to the local subgraph
				// See server creation in the last paragraph
				return "world";
			},
		},
	},
});

Build supergraph definition

import { createSupergraph } from "mesh-local-federation";

const supergraphSDL = await createSupergraph({
	subgraphs,
	localSchema,
	onRemoteRequestHeaders: ({ endpoint }) => {
		return {
			Authorization: `Bearer ${await getToken(endpoint)}`,
		};
	},
});

Result of this step can be cached and the resulting schema definition can be used for the next steps to speed up the precess significantly.

materialize-ts-function is a simple way to do this during build process.

Create server

import { createServer } from "node:http";
import { createMeshInstance } from "mesh-local-federation";
import { createYoga } from "graphql-yoga";

const config = await createMeshInstance({
	supergraphSDL,
	localSchema: harness.localSchema,
	onRemoteRequestHeaders: ({ endpoint }) => {
		return {
			Authorization: `Bearer ${await getToken(endpoint)}`,
		};
	},
});

const yoga = createYoga({
	...config,
	context: ({ request }) => {
		// context will be available in onRemoteRequestHeaders
		// and will be passed to local graph
	},
});

const server = createServer(yoga);

server.listen(4000, () => {
	console.info("Server is running on http://localhost:4000/graphql");
});

Thats it ...

... happy coding :)