Adding Middleware/Plugin before every Resolver Calls in GraphQL Yoga - Federation Supergraph

[rrguhan6]

import { readFileSync } from 'fs'
import { createServer } from 'node:http'
import { createYoga } from 'graphql-yoga'
import { getStitchedSchemaFromSupergraphSdl } from '@graphql-tools/federation'

// This doesn't have to be from a file system, it can be fetched via HTTP from a schema registry
const supergraphSdl = readFileSync('./supergraph.graphql').toString()
const schema = await getStitchedSchemaFromSupergraphSdl({ supergraphSdl })

// TODO: How can I add middleware that should be called before every resolver call in the GraphQL Yoga Gateway?
// I want to perform some actions before each resolver is executed.

const yoga = createYoga({ schema })
const server = createServer(yoga)

server.listen(4000, () => {
  console.log(`🚀 Server ready at http://localhost:4000/graphql`)
})

I would like to add middleware that gets executed before every resolver call in above code. (like this: https://the-guild.dev/graphql/mesh/docs/transforms/resolvers-composition)

This middleware should allow to perform custom actions before the resolver is invoked. If anyone has insights into how to do this with given code using GraphQL Yoga, I would greatly appreciate your guidance.

is there any plugin hooks that might help here?

Thank you! 🙏

[EmrysMyrddin]

One way to do this is to actually modify the schema and wrap all resolver into a new function, which will perform the actions you need before executing the actual resolver.

Here an example:

import { readFileSync } from 'fs'
import { createServer } from 'node:http'
import { createYoga } from 'graphql-yoga'
import { getStitchedSchemaFromSupergraphSdl } from '@graphql-tools/federation'
import { mapSchema, MapperKind } from '@graphql-tools/utils'
import { defaultFieldResolver } from 'graphql'

// This doesn't have to be from a file system, it can be fetched via HTTP from a schema registry
const supergraphSdl = readFileSync('./supergraph.graphql').toString()
const supergraphSchema = await getStitchedSchemaFromSupergraphSdl({ supergraphSdl })

const schema = mapSchema(supergraphSchema, {
  [MapperKind.OBJECT_FIELD]: (fieldConfig) => {
    const { resolve = defaultFieldResolver } = fieldConfig
    return {
      ...fieldConfig,
      async resolve(obj, args, ctx, info) {
        // Perform any action you want here.
        const resolvedValue = await resolve(obj, args, ctx, info)
        // You can even do things after resolver execution and manipulate the resolved value
        return resolvedValue
      },
    }
  },
})

const yoga = createYoga({ schema:  })
const server = createServer(yoga)

server.listen(4000, () => {
  console.log(`🚀 Server ready at http://localhost:4000/graphql`)
})

[EmrysMyrddin]

Just be aware of the possible performance impact this can. Since this runs on every resolver, any non trivial code here can really slowdown your gateway.

[rrguhan6]

@EmrysMyrddin thanks!, is it possible to call middleware before query and mutation resolvers alone instead of calling them for every fields in the query

EX: for this query middleware should be called twice (for locations and latestReviews)

{
  locations {
    id
    name  
  }
  latestReviews {
      comment
  }
}

[EmrysMyrddin]

Yes sure, you can use the MapperKind.ROOT_FIELD mapper for this. It allows to modify each field (regardless of being query, mutation or subscriptions) with the same mapper. You can also use MapperKind.QUERY_FIELD_ROOT, MapperKind.MUTATION_FIELD_ROOT or MapperKind.SUBSCRIPTION_FIELD_ROOT if you want to modify each root field type separately.

Here and example:

const schema = mapSchema(supergraphSchema, {
  [MapperKind.ROOT_FIELD]: (fieldConfig) => {
    const { resolve = defaultFieldResolver } = fieldConfig
    return {
      ...fieldConfig,
      async resolve(obj, args, ctx, info) {
        // Perform any action you want here.
        const resolvedValue = await resolve(obj, args, ctx, info)
        // You can even do things after resolver execution and manipulate the resolved value
        return resolvedValue
      },
    }
  },
})