A module to create an Apollo Federation v1 gateway with mercurius.
npm i fastify @mercuriusjs/federation @mercuriusjs/gatewayCreate the user service
'use strict'
const Fastify = require('fastify')
const { mercuriusFederationPlugin } = require('@mercuriusjs/federation')
const users = {
1: {
id: '1',
name: 'John',
username: '@john'
},
2: {
id: '2',
name: 'Jane',
username: '@jane'
}
}
const service = Fastify()
const schema = `
extend type Query {
me: User
}
type User @key(fields: "id") {
id: ID!
name: String
username: String
}
`
const resolvers = {
Query: {
me: () => {
return users['1']
}
},
User: {
__resolveReference: (source, args, context, info) => {
return users[source.id]
}
}
}
service.register(mercuriusFederationPlugin, {
schema,
resolvers
})
service.listen({ port: 4001 })Create the post service
'use strict'
const Fastify = require('fastify')
const { mercuriusFederationPlugin } = require('@mercuriusjs/federation')
const posts = {
p1: {
pid: 'p1',
title: 'Post 1',
content: 'Content 1',
authorId: 'u1'
},
p2: {
pid: 'p2',
title: 'Post 2',
content: 'Content 2',
authorId: 'u2'
}
}
const service = Fastify()
const schema = `
extend type Query {
topPosts(count: Int): [Post]
}
type Post @key(fields: "pid") {
pid: ID!
title: String
content: String
author: User @requires(fields: "pid title")
}
`
const resolvers = {
Query: {
topPosts: (root, { count = 2 }) => Object.values(posts).slice(0, count)
},
Post: {
__resolveReference: post => {
return posts[post.pid]
},
author: post => {
return {
__typename: 'User',
id: post.authorId
}
}
}
}
service.register(mercuriusFederationPlugin, {
schema,
resolvers
})
service.listen({ port: 4002 })Create the gateway
'use strict'
const Fastify = require('fastify')
const mercuriusGateway = require('@mercuriusjs/gateway')
const gateway = Fastify()
gateway.register(mercuriusGateway, {
gateway: {
services: [
{
name: 'user',
url: 'http://localhost:4001/graphql'
},
{
name: 'post',
url: 'http://localhost:4002/graphql'
}
]
}
})
gateway.listen({ port: 3000 })Register the gateway in fastify.
const mercuriusGateway = require('@mercurius/gateway')
const schema = ...
const resolvers = ...
const app = Fastify()
app.register(mercuriusGateway, {
gateway: [
services: [
{
name: 'user',
url: 'http://localhost:4001/graphql'
},
{
name: 'post',
url: 'http://localhost:4002/graphql'
}
]
}
})-
all the mercurius plugin options
-
gateway: Object. Run the GraphQL server in gateway mode.gateway.services:Service[]|FunctionAn array of GraphQL services that are part of the gateway or aFunctionthat returns aPromisethat resolves to an array of services. Required.service.name: A unique name for the service. Required.service.url: The URL of the service endpoint. It can also be anArrayof URLs and in which case all the requests will be load balanced throughout the URLs. Required.service.mandatory:BooleanMarks service as mandatory. If any of the mandatory services are unavailable, gateway will exit with an error. (Default:false)service.useSecureParse:BooleanMarks if the service response needs to be parsed securely using secure-json-parse. (Default:false)service.rewriteHeaders:FunctionA function that gets the original headers as a parameter and returns an object containing values that should be added to the headersservice.setResponseHeaders:FunctionA function that getsreplyas a parameter and can be used to set headers for the federated response to be sent to the client.service.initHeaders:FunctionorObjectAn object or a function that returns the headers sent to the service for the initial _service SDL query.service.connections: The number of clients to create. (Default:10)service.agent: An optional, fully configured undici agent/pool instance to use to perform network requests. If used, you must set all connections options on the instance as the request related options from theserviceconfiguration will not be applied.service.bodyTimeout: The timeout after which a request will time out, in milliseconds. (Default:30e3- 30 seconds)service.headersTimeout: The amount of time the parser will wait to receive the complete HTTP headers, in milliseconds. (Default:30e3- 30 seconds)service.keepAliveMaxTimeout: The maximum allowed keepAliveTimeout. (Default:5e3- 5 seconds)service.maxHeaderSize: The maximum length of request headers in bytes. (Default:16384- 16KiB)service.keepAlive: The amount of time pass between the keep-alive messages sent from the gateway to the service, ifundefined, no keep-alive messages will be sent. (Default:undefined)service.wsUrl: The url of the websocket endpointservice.wsConnectionParams:FunctionorObjectwsConnectionParams.connectionInitPayload:FunctionorObjectAn object or a function that returns theconnection_initpayload sent to the service.wsConnectionParams.reconnect:BooleanEnable reconnect on connection close (Default:false)wsConnectionParams.maxReconnectAttempts:NumberDefines the maximum reconnect attempts if reconnect is enabled (Default:Infinity)wsConnectionParams.connectionCallback:FunctionA function called after aconnection_ackmessage is received.wsConnectionParams.failedConnectionCallback:FunctionA function called after aconnection_errormessage is received, the first argument contains the message payload.wsConnectionParams.failedReconnectCallback:FunctionA function called if reconnect is enabled and maxReconnectAttempts is reached.wsConnectionParams.rewriteConnectionInitPayload:FunctionA function that gets the originalconnection_initpayload along with the context as a parameter and returns an object that replaces the originalconnection_initpayload before forwarding it to the federated service
service.collectors:Objectcollectors.collectHeaders:booleanAdds tocontextthecollectors.responseHeadersobject in which are stored the response headers from federated services.collectors.collectStatutsCodes:booleanAdds tocontextthecollectors.statusCodesobject in which are stored the status codes of the response from federated services.collectors.collectExtensions:booleanAdds tocontextthecollectors.extensionsobject in which are stored the extensions field of the response from federated services.
gateway.retryServicesCount:NumberSpecifies the maximum number of retries when a service fails to start on gateway initialization. (Default: 10)gateway.retryServicesInterval:NumberThe amount of time(in milliseconds) between service retry attempts in case a service fails to start on gateway initialization. (Default: 3000)gateway.pollingInterval:NumberThe amount of time (milliseconds) between polling the services for schema updates. If not specified, the gateway will not poll for schema updates. Whengateway.servicesis a function, the list of services is updated with the result of the function everygateway.pollingIntervalmilliseconds.
Hooks are registered with the fastify.graphqlGateway.addHook method
and allow you to listen to specific events in the GraphQL request/response lifecycle.
You have to register a hook before the event is triggered, otherwise the event is lost.
By using hooks you can interact directly with the GraphQL lifecycle of Mercurius gateway. There are GraphQL Request and Subscription hooks:
Notice: these hooks are only supported with async/await or returning a Promise.
The schema of the internal lifecycle of Mercurius gateway.
On the right branch of every section there is the next phase of the lifecycle, on the left branch there is the corresponding GraphQL error(s) that will be generated if the parent throws an error (note that all the errors are automatically handled by Mercurius).
How the gateway lifecycle works integrated with the Mercurius lifecycle.
Incoming GraphQL Request
│
└─▶ Routing
│
errors ◀─┴─▶ preParsing Hook
│
errors ◀─┴─▶ Parsing
│
errors ◀─┴─▶ preValidation Hook
│
errors ◀─┴─▶ Validation
│
errors ◀─┴─▶ preExecution Hook
│
errors ◀─┴─▶ Execution
│
└─▶ preGatewayExecution Hook(s) (appends errors only)
│
errors ◀─┴─▶ GatewayExecution(s)
│
errors ◀─┴─▶ Resolution (once all GatewayExecutions have finished)
│
└─▶ onResolution Hook
How the gateway subscription lifecycle works integrated with the Mercurius lifecycle.
Incoming GraphQL Websocket subscription data
│
└─▶ Routing
│
errors ◀─┴─▶ preSubscriptionParsing Hook
│
errors ◀─┴─▶ Subscription Parsing
│
errors ◀─┴─▶ preSubscriptionExecution Hook
│
errors ◀─┴─▶ Subscription Execution
│
│
└─▶ preGatewaySubscriptionExecution Hook(s)
│
errors ◀─┴─▶ Gateway Subscription Execution(s)
│
wait for subscription data
│
subscription closed on error ◀─┴─▶ Subscription Resolution (when subscription data is received)
│
└─▶ onSubscriptionResolution Hook
│
keeping processing until subscription ended
│
subscription closed on error ◀─┴─▶ Subscription End (when subscription stop is received)
│
└─▶ onSubscriptionEnd Hook
It is pretty easy to understand where each hook is executed by looking at the lifecycle definition.
In the preGatewayExecution hook, you can modify the following items by returning them in the hook definition:
documenterrors
This hook will only be triggered in gateway mode. When in gateway mode, each hook definition will trigger multiple times in a single request just before executing remote GraphQL queries on the federated services.
Note, this hook contains service metadata in the service parameter:
name: service name
fastify.graphqlGateway.addHook('preGatewayExecution', async (schema, document, context, service) => {
const { modifiedDocument, errors } = await asyncMethod(document)
return {
document: modifiedDocument,
errors
}
})If you get an error during the execution of your hook, you can just throw an error.
The preGatewayExecution hook, which will continue execution of the rest of the query and append the error to the errors array in the response.
fastify.graphqlGateway.addHook('preGatewayExecution', async (schema, document, context, service) => {
throw new Error('Some error')
})The preGatewayExecution hook support adding errors to the GraphQL response.
fastify.graphqlGateway.addHook('preGatewayExecution', async (schema, document, context) => {
return {
errors: [new Error('foo')]
}
})Note, the original query will still execute. Adding the above will result in the following response:
{
"data": {
"foo": "bar"
},
"errors": [
{
"message": "foo"
}
]
}It is pretty easy to understand where each hook is executed by looking at the lifecycle definition.
This hook will only be triggered in gateway mode. When in gateway mode, each hook definition will trigger when creating a subscription with a federated service.
Note, this hook contains service metadata in the service parameter:
name: service name
fastify.graphqlGateway.addHook('preGatewaySubscriptionExecution', async (schema, document, context, service) => {
await asyncMethod()
})If you get an error during the execution of your subscription hook, you can just throw an error and Mercurius will send the appropriate errors to the user along the websocket.`
fastify.graphqlGateway.addHook('preSubscriptionParsing', async (schema, source, context) => {
throw new Error('Some error')
})There is one hook that you can use in a GraphQL application.
When the Gateway service obtains new versions of federated schemas within a defined polling interval, the onGatewayReplaceSchema hook will be triggered every time a new schema is built. It is called just before the old schema is replaced with the new one.
It has the following parameters:
instance- The gateway serverFastifyInstance(this contains the old schema).schema- The new schema that has been built from the gateway refresh.
fastify.graphqlGateway.addHook('onGatewayReplaceSchema', async (instance, schema) => {
await someSchemaTraversalFn()
})If this hook throws, the error will be caught and logged using the FastifyInstance logger. Subsequent onGatewayReplaceSchema hooks that are registered will not be run for this interval.
Collectors gather additional information about the response from the services that are part of the gateway and adds them to the context.collectors object.
Depending on the configuration it may have the following fields:
responseHeadersstatusCodesextensions
Each collector stores data in the same format:
{
"foo": { // query id
"service": "bar", // name of the service
"data": {
"foo": "bar" // example: statusCode: 500
}
}
}It is possible to access and manipulate context.collectors via onResolution Hook
app.graphql.addHook('onResolution', async function (execution, context) {
console.log(context.collectors)
// {
// statusCodes: {
// topPosts: {
// service: 'post',
// data: {
// statusCode: 500
// }
// },
// me: {
// service: 'user',
// data: {
// statusCode: 404
// }
// }
// },
// responseHeaders: {...},
// extensions: {...},
// }
execution.collectors = context.collectors // add collectors to response
})
