This is a Java library to combine several graphql services together.

This is achieved by declaring the services, which should be combined, in a DSL which is build on top of the GraphQL SDL (Schema Definition Language).

While this library is in Java, the underlying services only need to provide a standard GraphQL API and no Java Code is needed to combine services together. It is all done declarative as part of the DSL.

Lets assume we have two Services: "Issues" and "Users". One has Issues and one has Users. An Issue has a reference to its authors.

This is the schema for the Issues service:

type Query {
    issues: [Issue]
}
type Issue {
    id: ID
    authorIds: [ID]
}

And this is the Users service:

type Query {
    usersById(id: [ID]): [User]
}
type User {
    id: ID
    fullName: String
}

This is how you would combine these two services together with Nadel while also renaming a field at the same time.

service Issues {
    type Query {
        issues: [Issue]
    }
    type Issue {
        id: ID
        authors: [User] => hydrated from Users.usersById(id: $source.authorIds) batch size 10
    }
}

service Users {
    type Query {
        usersById(id: [ID]): [User]
    }
    type User {
        id: ID
        name: String => renamed from $source.fullName
    }
}

The result is a new GraphQL API which combines the two services in one and has the following schema:

type Query {
    issues: [Issue]
    usersById(id: [ID]): [User]
}       
type Issue {
    id: ID
    authors: [User]
}
type User {
    id: ID
    name: String
}

• Explicit overall schema design: every field for the overall schema needs to be declared in the Nadel DSL. If it is not declared it will not be exposed
• Hydration: resolving referencing to other objects by calling another service
• Field rename: renaming field from the underlying schema to a new name in the overall schema
• Type rename: renaming a type from the underlying schema to a new name in the overall schema
• Batching: Hydration calls are automatically batched with a configurable max batch size per call