@wundergraph/composition

WunderGraph Composition

The WunderGraph composition library facilitates the federation of multiple subgraph schemas into a single federated GraphQL schema.

Prerequisites

• Node.js 16 LTS or higher

Federating subgraphs

The federateSubgraphs function is responsible for producing a valid federated graph. Each subgraph will be normalized and validated before federation. This normalization process does not affect the upstream schema. The final federated graph will also be validated. The function must be provided with an array of at least one Subgraph object. An example federation of two simple subgraphs:

import { federateSubgraphs, FederationResult, Subgraph } from '@wundergraph/composition';
import { parse } from 'graphql';

const result: FederationResult = federateSubgraphs([subgraphA, subgraphB]);

const subgraphA: Subgraph = {
  name: 'subgraph-a',
  url: 'http://localhost:4001',
  definitions: parse(`
    type User @key(fields: "id") {
      id: ID!
      name: String!
    }
  `),
};

const subgraphB: Subgraph = {
  name: 'subgraph-b',
  url: 'http://localhost:4002',
  definitions: parse(`
    type Query {
      users: [User!]!
    }
      
    type User @key(fields: "id") {
      id: ID!
      interests: [String!]!
    }
  `),
};

FederationResult

The federateSubgraphs function returns FederationResult, which is a union of FederationResultSuccess and FederationResultFailure. Both types in the union always define the following mutual properties:

property	Description	type
success	assertion of composition success	boolean
warnings	array of composition warnings (if any)	Array

FederationResultSuccess

If federation was successful, the return type is FederationResultSuccess.

property	Description	type
federatedGraphAST	an AST object representation of the federated graph sdl	graphql.DocumentNode
federatedGraphSchema	a schema object representation of the federated graph sdl	graphql.GraphQLSchema
success	assertion that composition was successful	true
warnings	array of composition warnings (if any)	Array

FederationResultFailure

If federation was unsuccessful, the return type is FederationResultFailure.

property	Description	type
errors	array of composition errors	Array
success	assertion that composition was unsuccessful	false
warnings	array of composition warnings (if any)	Array

Debugging

If normalization of any subgraph fails, or the federated graph itself is invalid, the AST and schema will not be produced (undefined properties). In these cases, the errors array will be defined and populated. An example of a simple debugging framework might be:

import { federateSubgraphs, FederationResult, Subgraph } from '@wundergraph.composition';
import { print, printSchema } from 'graphql';

const result: FederationResult = federateSubgraphs([subgraphA, subgraphB]);

if (result.success) {
  // Both options to print the federated graph as a string are included for documentational purposes only
  console.log(print(result.federatedGraphAST)); // log the federated graph AST as a string
  console.log(printSchema(result.federatedGraphSchema)); // log the federated graph schema as a string
} else {
  for (const err of result.errors) {
    console.log(err.message);
  }
}
for (const warning of result.warnings) {
  console.log(warning);
}

// subgraph definitions would be below [removed for brevity]

Errors

Errors can happen in three main stages:

1. While validating the subgraph metadata, e.g., validating that each Subgraph object has a unique name.
2. During the normalization process, which prepares the subgraph for federation. (if this stage fails, federation will not be attempted)
3. During the federation process itself.

All errors will be appended to the FederationResultFailure.errors array.

Subgraph object

The Subgraph object is the core of the WunderGraph composition library. The definitions property must be provided as a graphQL.DocumentNode type. This is easily achieved by passing string representation of the subgraph SDL to the graphql.js parse function. An example is shown below:

import { Subgraph } from '@wundergraph/composition'
import { parse } from 'graphql';

const subgraphA: Subgraph = {
  name: 'subgraph-a',
  url: 'http://localhost:4001',
  definitions: parse(`
    type Query {
      user: User!
    }

    type User {
      name: String!
    }
  `),
};

Subgraph Properties

property	Description	type
name	unique name of the subgraph	string
url	unique endpoint for the subgraph	string
definitions	an AST representation of the subgraph SDL	graphql.DocumentNode

Contributing

When adding or changing error, please ensure GraphQL types begin with a capital letter for clarity:

• Enum
• Input Object
• Interface
• Object
• Scalar
• Union