Graphql federation with kickstart

See basic concepts in Apollo doc.

How to for back-end services:

One service declare GQL type, which other services may want to extend (add its fields onto type). How to do it on back-end side:

Service account-service provide type Account in its schema see example bellow.

type Query {
    userAccount(userId: String!): Account!
}

type Account @key(fields: "iban") {
    iban: String!
    currency: String!
}

So frond end could execute query like:

query GetUserAccount($userId: String!) {
    userAccount(userId: $userId) {
        iban
        currency
    }
}

With federation another back-end service can provide additional fields for Account type and provide richer API for frond end. To do this GQL offer key word extend or alternatively for libs, which does not support federation syntax yet directive @extends placed on type definition.

Extended types has to provide @key directive - at least one field but can provide more or composite see doc. Key is used to identify entity. Entity is object, which is declared in subschema and can be referenced via keys in other subschemas - basically allow to resolve entity instance with key and then add their own fields via extension mechanism.

Example Account type extension by balance-service. Service balance-service can extend type Account and add new fields in this case field balance.

scalar BigDecimal

type Account @key(fields: "iban") @extends {
    iban: String! @external
    currency: String!
    # lets say, that balance is returned in currency provided by account - so currency is required    
    balance: BigDecimal! @requires(fields: "currency")
}

Notable points in example:

In example is used @extends directive instead of extend key word. Kickstart has problem with extend and Apollo support both ways - behaves same. Fields with directive @external - those fields are declared on extended type. So it is not redeclaration, but info for Apollo, that we expect those fields as incoming data required for resolving our added fields. Field balance is only newly declared field here. Directive requires instrument Apollo federation, that currency from extended type has to be send with data. Field iban is key, thus provided automatically.

So balance-service can now enrich super schema. Frond end can perform calls like

# user id is usually taken from JWT :)
query GetUserAccount($userId: String!) {
    userAccount(userId: $userId) {
        iban
        currency
        balance
    }
}

after all provided sub schemas in this case schemas from account-service and balance-service schemas are composed together using federation supporting lib like Apollo Federation.

Implementing Federation on BE

Enable federation for Spring context - place following annotation on some config class.

@EnableGraphQlFederation
@Configuration
class SomeConfigClass

Add implementation for extended types.

@Component
class AccountResolver : GraphQLResolver<Account>, FederationReferenceResolver<Account> {
    //extending Account with balance field - hopefully I am rich
    fun balance(context: Account): BigDecimal = returnedBalance

    companion object {
        val returnedBalance: BigDecimal = BigDecimal.TEN
    }
}

data class Account(
    val iban: String,
    val currency: String
)

In example GraphQLResolver<Account> allow to add field balance on GQL type Account. Interface FederationReferenceResolver<Account> handle resolution of federation related operations. On interface are present 3 functions, which has default values. Those may need some overrides so pleas read doc on this interface.

Behind the scenes. With Federation approach may happen, that some types in schema are unreachable until whole federation schema is created. Kickstart provide SchemaParserDictionary class which allow to add types into schema manually which may be needed for some types. Federation lib require use of SchemaParserDictionaryCustomizer which provide mechanism for those overriding. So use this please, because injection SchemaParserDictionary and its adjust may be performed after schema is created and app won`t start.