The graph router
The entry point to your federated supergraph
After you set up at least one federation-ready subgraph, you can configure a graph router (also known as a gateway) to sit in front of your subgraphs. The router serves as the entry point to your supergraph, and it executes incoming operations across one or more of your subgraphs:
Choosing a router library
Apollo actively supports two options for your graph router:
- The Apollo Router (recommended): This is a high-performance, precompiled Rust binary.
- Go to the Apollo Router docs.
- See the federation quickstart (which uses the Apollo Router).
- Apollo Server: Apollo Server can act as your graph router via the
@apollo/gateway
extension library.
We recommend starting with the Apollo Router. It's faster to configure, it's more performant (especially with high request loads), and it rarely requires writing custom code.
If you'll use the Apollo Router, get started with the resources linked above. The remainder of this article covers using Apollo Server with @apollo/gateway
.
Node.js gateway setup
This section walks through setting up a basic graph router using Apollo Server and the @apollo/gateway
library. It currently requires Node.js version 14 or 16.
Create a new Node.js project with npm init
, then install the necessary packages:
npm install @apollo/gateway apollo-server graphql
The @apollo/gateway
package includes the ApolloGateway
class. To configure Apollo Server to act as a graph router, you pass an instance of ApolloGateway
to the ApolloServer
constructor, like so:
const { ApolloServer, gql } = require('apollo-server');const { ApolloGateway } = require('@apollo/gateway');const { readFileSync } = require('fs');const supergraphSdl = readFileSync('./supergraph.graphql').toString();// Initialize an ApolloGateway instance and pass it// the supergraph schema as a stringconst gateway = new ApolloGateway({supergraphSdl,});// Pass the ApolloGateway to the ApolloServer constructorconst server = new ApolloServer({gateway,});server.listen().then(({ url }) => {console.log(`🚀 Server ready at ${url}`);});
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, see Supported methods.
In production, we strongly recommend running the gateway in a managed mode with Apollo Studio, which enables your gateway to update its configuration without a restart. For details, see Setting up managed federation.
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.
const { ApolloServer } = require('apollo-server');const { ApolloGateway } = require('@apollo/gateway');const { readFile } = require('fs/promises');let supergraphUpdate;const gateway = new ApolloGateway({async supergraphSdl({ update }) {// `update` is a function that we'll save for later usesupergraphUpdate = update;return {supergraphSdl: await readFile('./supergraph.graphql', 'utf-8'),}},});// Pass the ApolloGateway to the ApolloServer constructorconst server = new ApolloServer({gateway,});server.listen().then(({ url }) => {console.log(`🚀 Server ready at ${url}`);});
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 schema. Polling, webhooks, and file watchers are all examples of ways we can go about doing this.
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.
const { ApolloServer } = require('apollo-server');const { ApolloGateway } = require('@apollo/gateway');const { watch } = require('fs');const { readFile } = require('fs/promises');const server = new ApolloServer({gateway: new ApolloGateway({async supergraphSdl({ update, healthCheck }) {// create a file watcherconst watcher = watch('./supergraph.graphql');// subscribe to file changeswatcher.on('change', async () => {// update the supergraph schematry {const updatedSupergraph = await readFile('./supergraph.graphql', 'utf-8');// optional health check update to ensure our services are responsiveawait healthCheck(updatedSupergraph);// update the supergraph schemaupdate(updatedSupergraph);} catch (e) {// handle errors that occur during health check or while updating the supergraph schemaconsole.error(e);}});return {supergraphSdl: await readFile('./supergraph.graphql', 'utf-8'),// cleanup is called when the gateway is stoppedasync cleanup() {watcher.close();}}},}),});server.listen().then(({ url }) => {console.log(`🚀 Server ready at ${url}`);});
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 await
s 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
⚠️ We strongly recommend against using IntrospectAndCompose
in production. For details, see Limitations of IntrospectAndCompose
.
Instead of providing a composed supergraph schema to the gateway, you can instruct the gateway to fetch all of your subgraph schemas and perform composition itself. To do so, provide an instance of the IntrospectAndCompose
class with a subgraphs
array, like so:
const { ApolloGateway, IntrospectAndCompose } = require('@apollo/gateway');const gateway = new ApolloGateway({supergraphSdl: new IntrospectAndCompose({subgraphs: [{ name: 'accounts', url: 'http://localhost:4001' },{ name: 'products', url: 'http://localhost:4002' },// ...additional subgraphs...],}),});
Each item in the subgraphs
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's 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.
- With the static or dynamic
- 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.
- When you deploy multiple instances with
Updating the gateway
Before updating your gateway's version, check the changelog for potential breaking changes.
We strongly recommend updating your gateway in local and test environments before deploying updates to staging or production.
You can confirm the currently installed version of the @apollo/gateway
library with the npm list
command:
npm list @apollo/gateway
To update the library, use the npm update
command:
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:
npm install @apollo/gateway@2.0.0
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.
The fields of the object passed to your context
function differ if you're using middleware besides Express. See the API reference for details.
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.
In this example, multiple calls to didReceiveResponse
are push
ing a value onto the shared context.serverIds
array. The order of these calls cannot be guaranteed. If you write logic that modifies the shared context
object, make sure that modifications are not destructive, and that the order of modifications doesn't matter.
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:
directive @deprecated(reason: String = "No longer supported") on FIELD_DEFINITION | ENUM_VALUEtype ExampleType {newField: StringoldField: String @deprecated(reason: "Use `newField`.")}
The gateway strips all definitions and uses of type system directives from your graph's API 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.
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.
Although the @apollo/gateway
library supports executable directives, Apollo Server itself does not. This guidance is provided primarily for architectures that use the @apollo/gateway
library in combination with subgraphs that do not use Apollo Server.
Here's an example of an executable directive definition:
# Uppercase this field's value (assuming it's a string)directive @uppercase on FIELD
And here's an example of a query that uses that directive:
query GetUppercaseUsernames {users {name @uppercase}}
It's strongly recommended that all of your subgraphs use the exact same logic for a given executable directive. Otherwise, operations might produce inconsistent or confusing results for clients.