Advanced GraphQL Schema Stitching Techniques

GraphQL has gained tremendous popularity in recent years due to its flexibility and efficiency in handling complex data fetching requirements. As applications scale and their data requirements grow, it becomes essential to use advanced techniques for managing schemas. One such powerful technique is schema stitching, which enables developers to combine multiple GraphQL schemas into a single, unified schema. In this blog post, we'll delve deep into advanced GraphQL schema stitching techniques, complete with code examples and clear explanations suitable for beginners and experienced developers alike.

What is Schema Stitching?

Schema stitching is the process of merging multiple GraphQL schemas into a single schema. This technique is especially useful when working with microservices or third-party APIs that expose their data through GraphQL. By stitching together different schemas, developers can create a unified API that simplifies client-side queries and improves overall application architecture.

Benefits of Schema Stitching

Some of the primary benefits of schema stitching include:

1. Simplified data fetching: Instead of making multiple queries to different services, clients can fetch all required data through a single query to the unified schema.
2. Improved maintainability: Each service can focus on its domain-specific functionality and evolve independently, reducing the risk of breaking changes.
3. Easier integration: Combining schemas from different sources (e.g., third-party APIs) becomes much more manageable, as the stitched schema can handle the complexities of communicating with multiple services.

Getting Started with Schema Stitching

Before diving into advanced techniques, let's first set up a simple example of schema stitching. We'll be using the graphql-tools package, which provides utilities for working with GraphQL schemas. To begin, install the package:

npm install graphql-tools

Now, let's create two simple GraphQL schemas that we'll later stitch together. The first schema represents a User service:

const { makeExecutableSchema } = require('graphql-tools');

const userType = `
  type User {
    id: ID!
    name: String
  }

  type Query {
    user(id: ID!): User
  }
`;

const userResolvers = {
  Query: {
    user: (_, { id }) => {
      // Fetch the user by ID from the data store (e.g., a database)
    },
  },
};

const userSchema = makeExecutableSchema({ typeDefs: userType, resolvers: userResolvers });

The second schema represents a Post service:

const postType = `
  type Post {
    id: ID!
    title: String
    authorId: ID!
  }

  type Query {
    post(id: ID!): Post
  }
`;

const postResolvers = {
  Query: {
    post: (_, { id }) => {
      // Fetch the post by ID from the data store (e.g., a database)
    },
  },
};

const postSchema = makeExecutableSchema({ typeDefs: postType, resolvers: postResolvers });

With these two schemas defined, let's stitch them together using the mergeSchemas function from graphql-tools:

const { mergeSchemas } = require('graphql-tools');

const stitchedSchema = mergeSchemas({
  schemas: [userSchema, postSchema],
});

At this point, we have a basic stitched schema that includes both User and Post types. However, there's no relationship between the two. To create a more useful schema, we'll need to delve into some advanced techniques.

Advanced Schema Stitching Techniques

Adding Relationships Between Types

To create relationships between types from different schemas, we can use the mergeInfo argument provided by graphql-tools. This object contains useful methods for delegating parts of a query to different schemas.

Inour example, let's assume that each Post is written by a User. We'll add a User field to the Post type and use the mergeInfo object to resolve this relationship:

First, update the Post type definition to include the User field:

const postType = `
  type Post {
    id: ID!
    title: String
    authorId: ID!
    author: User
  }

  type Query {
    post(id: ID!): Post
  }
`;

Next, modify the Post resolvers to use the mergeInfo object for resolving the User field:

const postResolvers = {
  Post: {
    author: {
      fragment: '... on Post { authorId }',
      resolve(parent, args, context, info, { mergeInfo }) {
        return mergeInfo.delegateToSchema({
          schema: userSchema,
          operation: 'query',
          fieldName: 'user',
          args: {
            id: parent.authorId,
          },
          context,
          info,
        });
      },
    },
  },
  Query: {
    post: (_, { id }) => {
      // Fetch the post by ID from the data store (e.g., a database)
    },
  },
};

Now, when querying for a Post, the associated User object will also be fetched, allowing clients to request both pieces of data in a single query:

query {
  post(id: "1") {
    title
    author {
      name
    }
  }
}

Handling Conflicting Type Names

In some cases, you may encounter conflicting type names when stitching together multiple schemas. To resolve these conflicts, you can use the transform option of the mergeSchemas function to rename types.

For example, let's say both our User and Post schemas define a Timestamp scalar:

const userType = `
  scalar Timestamp

  type User {
    id: ID!
    name: String
    createdAt: Timestamp
  }

  type Query {
    user(id: ID!): User
  }
`;

const postType = `
  scalar Timestamp

  type Post {
    id: ID!
    title: String
    authorId: ID!
    author: User
    createdAt: Timestamp
  }

  type Query {
    post(id: ID!): Post
  }
`;

To avoid conflicts, we can rename the Timestamp scalar in the Post schema:

const { RenameTypes } = require('graphql-tools');

const stitchedSchema = mergeSchemas({
  schemas: [userSchema, postSchema],
  transforms: [
    new RenameTypes((name) => (name === 'Timestamp' ? 'PostTimestamp' : name)),
  ],
});

Now, the Timestamp scalar in the Post schema will be renamed to PostTimestamp, and the conflict is resolved.

Linking Multiple Schemas with Type Extensions

Another advanced technique for schema stitching is using type extensions. Type extensions allow you to extend a type with additional fields without modifying the original schema.

For example, if you want to add a posts field to the User type to fetch all posts written by a user, you can create a type extension like this:

const userPostsTypeExtension = `
  extend type User {
    posts: [Post]
  }
`;

Then, add a resolver for the new posts field using the mergeInfo object:

const userPostsResolvers = {
  User: {
    posts: {
      fragment: '... on User { id }',
      resolve(parent, args, context, info, { mergeInfo }) {
        return mergeInfo.delegateToSchema({
          schema: postSchema,
          operation: 'query',
          fieldName: 'postsByAuthor',
          args: {
            authorId: parent.id,
          },
          context,
          info,
        });
      },
    },
  },
};

const userPostsSchema = makeExecutableSchema({
  typeDefs: [userPostsTypeExtension],
  resolvers: userPostsResolvers,
});

Note that we assumed there's a postsByAuthor query in the Post schema. Update the Post schema accordingly:

const postType = `
  scalar Timestamp

  type Post {
    id: ID!
    title: String
    authorId: ID!
    author: User
    createdAt: Timestamp
  }

  type Query {
    post(id: ID!): Post
    postsByAuthor(authorId: ID!): [Post]
  }
`;

const postResolvers = {
  // ...
  Query: {
    post: (_, { id }) => {
      // Fetch the post by ID from the data store (e.g., a database)
    },
    postsByAuthor: (_, { authorId }) => {
      // Fetch all posts by the specified author from the data store
    },
  },
};

Finally, merge the new schema extension with the existing schemas:

const stitchedSchema = mergeSchemas({
  schemas: [userSchema, postSchema, userPostsSchema],
});

Now, when querying a User, you can also fetch all their posts:

query {
  user(id: "1") {
    name
    posts {
      title
    }
  }
}

FAQ

Q: Can I stitch together schemas from different GraphQL servers?

A: Yes, you can stitch together schemas from different GraphQL servers using the wrapSchema function from the graphql-tools package. This function creates a proxy schema that forwards queries to the remote server. You can then merge the proxy schema with your local schemas using the mergeSchemas function.

Q: Can I use schema stitching with GraphQL subscriptions?

A: Yes, schema stitching supports GraphQL subscriptions. To stitch together schemas with subscriptions, ensure that your resolvers include subscription fields and merge the subscription fields using the mergeInfo object, similar to how you would for queries and mutations.

Q: How does schema stitching affect performance?

A: Schema stitching can introduce some overhead, as queries might be delegated across multiple schemas or services. However, this performance impact is generally negligible compared to the benefits of a well-structured application architecture. To optimize performance, consider using tools like DataLoader for batching and caching requests.

Q: Can I use schema stitching with Apollo Server?

A: Yes, you can use schema stitching with Apollo Server. Once you've merged your schemas using graphql-tools, simply provide the resulting stitched schema to the ApolloServer constructor.