Field Links

Overview

There will be instances where data from different Service Providers are related, like how tables are related in relational databases. As an example, let’s look at these GraphQL Schemas from two different services:

Prediction Service

type Query {
    topThreeCustomers: [RatedCustomer]
}

type RatedCustomer {
    customerId: String
    ratingResult: Int
}

Customer Service

type Query {
    customerById(id: String): Customer
}

type Customer {
    id: String
    name: String
}

The problem

Let’s say an Application wants to display the top three customers including the name of each customer. There will be several calls to GraphQL Orchestrator:

1. Query prediction service:

   query {
       topThreeCustomers: {
           customerId
           ratingResult
       }
   }
   

2. Query customer service for each rated customer: ` customerById(id: $CustomerId) `

The application needs multiple calls and defeats the purpose of having a single call to a graphql endpoint.

Solution

extend type RatedCustomer {
    # $customerId is a reference to a sibling field
    customerInfo : Customer @resolver(field: "customerById" arguments: [{name : "id", value: "$customerId"}])    
}

@resolver directive section further explains each element in detail.

With the extended RatedCustomer type, the application can make SINGLE CALL to achieve the objective:

query {
    topThreeCustomers: {
        customerId
        ratingResult
        customerInfo: { name }
    }
}

Can you guess what happen under the hood?

1. For field topThreeCustomers, orchestrator will call the Prediction Service first which shall return a list of RatedCustomer
2. Then it will resolve topThreeCustomers.customerInfo by executing the @resolver directive which basically calls the Customer Service passing the value of $customerId. Even if the list is of size 3, the orchestrator will only make one call to the Customer Service and batch the list together using an alias as shown below

query {
    id0_O: customerInfo: { name }
    id01_1: customerInfo: { name }
    id02_2: customerInfo: { name }
}

@resolver directive

resolver directive on field definition defines how to resolve a field that belongs to a different provider.