The gateway

After you set up at least one federation-ready subgraph, you can configure a gateway to sit in front of your subgraphs. The gateway executes incoming operations across those subgraphs.

The @apollo/gateway package extends Apollo Server's functionality, enabling it to act as a gateway for an Apollo Federation architecture.

Setup

First, let's install the necessary packages:

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npm install @apollo/gateway apollo-server

The @apollo/gateway package includes the ApolloGateway class. To configure Apollo Server to act as a gateway, you pass an instance of ApolloGateway to the ApolloServer constructor, like so:

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const { ApolloServer } = require('apollo-server');
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const { ApolloGateway } = require('@apollo/gateway');
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const { readFileSync } = require('fs');
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const supergraphSdl = readFileSync('./supergraph.graphql').toString();
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// Initialize an ApolloGateway instance and pass it
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// the supergraph schema
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const gateway = new ApolloGateway({
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  supergraphSdl,
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});
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// Pass the ApolloGateway to the ApolloServer constructor
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const server = new ApolloServer({
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  gateway,
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});
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server.listen().then(({ url }) => {
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  console.log(`🚀 Server ready at ${url}`);
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});

Composing the supergraph schema

In the above example, we provide the supergraphSdl option to the ApolloGateway constructor. This is the string representation of our supergraph schema, which is composed from all of our subgraph schemas.

To learn how to compose your supergraph schema with the Rover CLI, see the Federation quickstart.

On startup, the gateway processes your supergraphSdl, which includes routing information for your subgraphs. It then begins accepting incoming requests and creates query plans for them that execute across one or more subgraphs.

Updating the supergraph schema

In the above example, we provide a static supergraph schema to the gateway. This approach requires the gateway to restart in order to update the supergraph schema. This is undesirable for many applications, so we also provide the ability to update the supergraph schema dynamically.

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const { ApolloServer } = require('apollo-server');
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const { ApolloGateway } = require('@apollo/gateway');
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const { readFile } = require('fs/promises');
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let supergraphUpdate;
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const gateway = new ApolloGateway({
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  async supergraphSdl({ update }) {
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    // `update` is a function which we'll save for later use
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    supergraphUpdate = update;
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    return {
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      supergraphSdl: await readFile('./supergraph.graphql', 'utf-8'),
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    }
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  },
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});
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// Pass the ApolloGateway to the ApolloServer constructor
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const server = new ApolloServer({
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  gateway,
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});
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server.listen().then(({ url }) => {
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  console.log(`🚀 Server ready at ${url}`);
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});

There are a few things happening here. Let's take a look at each of them individually.

Note that supergraphSdl is now an async function. This function is called exactly once, when ApolloServer initializes the gateway. It has the following responsibilities:

• It receives the update function, which we use to update the supergraph schema.
• It returns the initial supergraph schema, which the gateway uses at startup.

With the update function, we can now programatically update the supergraph. Polling, webhooks, and file watchers are all good examples of ways we can go about updating the supergraph.

The code below demonstrates a more complete example using a file watcher. In this example, assume that we're updating the supergraphSdl.graphql file with the Rover CLI.

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const { ApolloServer } = require('apollo-server');
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const { ApolloGateway } = require('@apollo/gateway');
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const { watch } = require('fs');
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const { readFile } = require('fs/promises');
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const server = new ApolloServer({
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  gateway: new ApolloGateway({
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    async supergraphSdl({ update, healthCheck }) {
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      // create a file watcher
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      const watcher = watch('./supergraph.graphql');
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      // subscribe to file changes
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      watcher.on('change', async () => {
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        // update the supergraph schema
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        try {
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          const updatedSupergraph = await readFile('./supergraph.graphql', 'utf-8');
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          // optional health check update to ensure our services are responsive
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          await healthCheck(updatedSupergraph);
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          // update the supergraph schema
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          update(updatedSupergraph);
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        } catch (e) {
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          // handle errors that occur during health check or while updating the supergraph schema
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          console.error(e);
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        }
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      });
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      return {
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        supergraphSdl: await readFile('./supergraph.graphql', 'utf-8'),
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        // cleanup is called when the gateway is stopped
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        async cleanup() {
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          watcher.close();
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        }
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      }
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    },
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  }),
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});
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server.listen().then(({ url }) => {
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  console.log(`🚀 Server ready at ${url}`);
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});

This example is a bit more complete. Let's take a look at what we've added.

In the supergraphSdl callback, we also receive a healthCheck function. This enables us to run a health check against each of the services in our future supergraph schema. This is useful for ensuring that our services are responsive and that we don't perform an update when it's unsafe.

We've also wrapped our call to update and healthCheck in a try block. If an error occurs during either of these, we want to handle this gracefully. In this example, we continue running the existing supergraph schema and log an error.

Finally, we return a cleanup function. This is a callback that's called when the gateway is stopped. This enables us to cleanly shut down any ongoing processes (such as file watching or polling) when the gateway is shut down via a call to ApolloServer.stop. The gateway expects cleanup to return a Promise and awaits it before shutting down.

Advanced usage

In a more complex application, you might want to create a class that handles the update and healthCheck functions, along with any additional state. In this case, you can instead provide an object (or class) with an initialize function. This function is called just like the supergraphSdl function discussed above. For an example of this, see the IntrospectAndCompose source code.

Composing subgraphs with IntrospectAndCompose

Alternatively, you can provide a subgraph array to the IntrospectAndCompose constructor, like so:

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const { ApolloGateway, IntrospectAndCompose } = require('@apollo/gateway');
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const gateway = new ApolloGateway({
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  supergraphSdl: new IntrospectAndCompose({
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    subgraphs: [
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      { name: 'accounts', url: 'http://localhost:4001' },
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      { name: 'products', url: 'http://localhost:4002' },
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      // ...additional subgraphs...
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    ],
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  }),
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});

Each item in the array is an object that specifies the name and url of one of your subgraphs. You can specify any string value for name, which is used primarily for query planner output, error messages, and logging.

On startup, the gateway fetches each subgraph's schema from its url and composes those schemas into a supergraph schema. It then begins accepting incoming requests and creates query plans for them that execute across one or more subgraphs.

Additional configuration options can be found in the IntrospectAndCompose API documentation.

However, IntrospectAndCompose has important limitations.

Limitations of IntrospectAndCompose

The IntrospectAndCompose option can sometimes be helpful for local development, but it is strongly discouraged for any other environment. Here are some reasons why:

• Composition might fail. With IntrospectAndCompose, your gateway performs composition dynamically on startup, which requires network communication with each subgraph. If composition fails, your gateway throws errors and experiences unplanned downtime.
  • With the static or dynamic supergraphSdl configuration, you instead provide a supergraph schema that has already been composed successfully. This prevents composition errors and enables faster startup.
• Gateway instances might differ. If you deploy multiple instances of your gateway while deploying updates to your subgraphs, your gateway instances might fetch different schemas from the same subgraph. This can result in sporadic composition failures or inconsistent supergraph schemas between instances.
  • When you deploy multiple instances with supergraphSdl, you provide the exact same static artifact to each instance, enabling more predictable behavior.

Updating the gateway

You can confirm the currently installed version of the @apollo/gateway library with the npm list command:

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npm list @apollo/gateway

To update the library, use the npm update command:

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npm update @apollo/gateway

This updates the library to the most recent version allowed by your package.json file. Learn more about dependency constraints.

To update to a particular version (including a version that exceeds your dependency constraints), use npm install instead:

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npm install @apollo/gateway@0.34.0

Customizing requests and responses

The gateway can modify the details of an incoming request before executing it across your subgraphs. For example, your subgraphs might all use the same authorization token to associate an incoming request with a particular user. The gateway can add that token to each operation it sends to your subgraphs.

Similarly, the gateway can modify the details of its response to a client, based on the result returned by each subgraph.

Customizing requests

In the following example, each incoming request to the gateway includes an Authorization header. The gateway sets the shared context for an operation by pulling the value of that header and using it to fetch the associated user's ID.

After adding the userId to the shared context object, the gateway can then add that value to a header that it includes in its requests to each subgraph.

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const { ApolloServer } = require('apollo-server');
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const { ApolloGateway, RemoteGraphQLDataSource } = require('@apollo/gateway');
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const { readFileSync } = require('fs');
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const supergraphSdl = readFileSync('./supergraph.graphql').toString();
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class AuthenticatedDataSource extends RemoteGraphQLDataSource {
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  willSendRequest({ request, context }) {
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    // Pass the user's id from the context to each subgraph
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    // as a header called `user-id`
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    request.http.headers.set('user-id', context.userId);
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  }
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}
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const gateway = new ApolloGateway({
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  supergraphSdl,
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  buildService({ name, url }) {
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    return new AuthenticatedDataSource({ url });
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  },
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});
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const server = new ApolloServer({
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  gateway,
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  context: ({ req }) => {
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    // Get the user token from the headers
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    const token = req.headers.authorization || '';
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    // Try to retrieve a user with the token
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    const userId = getUserId(token);
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    // Add the user ID to the context
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    return { userId };
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  },
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});
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server.listen().then(({ url }) => {
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  console.log(`🚀 Server ready at ${url}`);
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});

The buildService function enables us to customize the requests that are sent to our subgraphs. In this example, we return a custom RemoteGraphQLDataSource. The datasource allows us to modify the outgoing request with information from the Apollo Server context before it's sent. Here, we add the user-id header to pass an authenticated user ID to downstream services.

Customizing responses

Let's say that whenever a subgraph returns an operation result to the gateway, it includes a Server-Id header in the response. The value of the header uniquely identifies the subgraph in our graph.

When the gateway then responds to a client, we want its Server-Id header to include the identifier for every subgraph that contributed to the response. In this case, we can tell the gateway to aggregate the various server IDs into a single, comma-separated list.

The flow for processing a single operation from a client application then looks like this:

To implement this flow, we can use the didReceiveResponse callback of the RemoteGraphQLDataSource class to inspect each subgraph's result as it comes in. We can add the Server-Id to the shared context in this callback, then pull the full list from the context when sending the final response to the client.

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const { ApolloServer } = require('apollo-server');
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const { ApolloGateway, RemoteGraphQLDataSource } = require('@apollo/gateway');
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const { readFileSync } = require('fs');
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const supergraphSdl = readFileSync('./supergraph.graphql').toString();
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class DataSourceWithServerId extends RemoteGraphQLDataSource {
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  async didReceiveResponse({ response, request, context }) {
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    // Parse the Server-Id header and add it to the array on context
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    const serverId = response.http.headers.get('Server-Id');
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    if (serverId) {
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      context.serverIds.push(serverId);
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    }
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    // Return the response, even when unchanged.
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    return response;
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  }
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}
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const gateway = new ApolloGateway({
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  supergraphSdl,
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  buildService({ url }) {
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    return new DataSourceWithServerId({ url });
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  }
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});
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const server = new ApolloServer({
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  gateway,
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  context() {
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    return { serverIds: [] };
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  },
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  plugins: [
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    {
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      requestDidStart() {
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        return {
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          willSendResponse({ context, response }) {
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            // Append our final result to the outgoing response headers
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            response.http.headers.set(
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              'Server-Id',
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              context.serverIds.join(',')
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            );
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          }
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        };
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      }
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    }
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  ]
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});
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server.listen().then(({ url }) => {
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  console.log(`🚀 Server ready at ${url}`);
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});

To learn more about buildService and RemoteGraphQLDataSource, see the API docs.

Custom directive support

The @apollo/gateway library supports the use of custom directives in your subgraph schemas. This support differs depending on whether a given directive is a type system directive or an executable directive.

Type system directives

Type system directives are directives that are applied to one of these locations. These directives are not used within operations, but rather are applied to locations within the schema itself.

The @deprecated directive below is an example of a type system directive:

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directive @deprecated(
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  reason: String = "No longer supported"
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) on FIELD_DEFINITION | ENUM_VALUE
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type ExampleType {
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  newField: String
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  oldField: String @deprecated(reason: "Use `newField`.")
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}

At composition time, ApolloGateway strips all definitions and uses of type system directives from your composed schema. This has no effect on your subgraph schemas, which retain this information.

Effectively, the gateway supports type system directives by ignoring them, making them the responsibility of the subgraphs that define them.

To learn about using custom directives in your subgraph schemas, see Custom directives in subgraphs.

Executable directives

Executable directives are directives that are applied to one of these locations. These directives are defined in your schema, but they're used in operations that are sent by clients.

Here's an example of an executable directive definition:

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# Uppercase this field's value (assuming it's a string)
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directive @uppercase on FIELD

And here's an example of a query that uses that directive:

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query GetUppercaseUsernames {
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  users {
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    name @uppercase
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  }
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}

At composition time, ApolloGateway makes sure that all of your subgraphs define the exact same set of executable directives. If any service is missing a definition, or if definitions differ in their locations, arguments, or argument types, a composition error occurs.