To use with local development services, add LOCAL=true to .env and list your services in services.json

Pull data from another service

Users pulls books by adding book to a user:

# BooksBook MUST have the same structure as external type
# reduced to key fields (e.g. {id} or {topic: {id}}) and, optionally, denrmalized fields 
type User @key(fields: "id") {
    id: ID!
    name: String
    book: BooksBook
}

# This will use federation _entities(representations: $representations) query 
# using external type's key 
type BooksBook @external {
    id: ID!
    # optional back link. `fields` supports graphql syntax with aliases, can be use fields, leaf properties will be used as vars in  `representation`
    # e.g.  @inverse(fields: "title publisher {publisherId: id}", representation: "book: {title: $title, publisher_id: $publisherId}")
    # also supports custom queries via `query` prop
    # e.g.  @inverse(fields: "boookId: id", representation: "$boookId", query: "users(where: {book_id_in: $representations})")
    user: User @inverse(fields: "id", representation: "book: {id: $id}") 
}

# This will use federation _entities(representations: $representations) query 
# using explicity set key in `fields` (can be complex, such as "topic {id}, uses graphql selection syntax")
type BooksBook @external(fields: "id") {
    id: ID!
}

# This will use custom query from  related service and pass it's arguments to this service
# query has two predefined variables  that can be used: $representations and $typename
# $typename will be replaced with nrmalized type (`Book` in this example)
# in $representations will be injected result of querying `fields` from stored representations
# e.g. parent user `{"id": 1. "book": {"id": 1}}`, `fields: "id"` will query `book` and result in `{"id": 1}`
# optional `onlyKeys: true` will extract values from representation, and result in `1`
# field arguments will be smartly merged with relation constraint  
# LIMITATIONS:
# * query must be on top level
# * `$representations` will always be an array
type BooksBook @external(query: "Books(where: {id_in: $representations})", fields: "id", onlyKeys: true) {
    id: ID!
}

Push data to another service

Books pushes itself to users by adding book to a user from itself

type Book @key(fields: "id") {
   id: ID!
   title: String
   announce: String
}
extend type UsersUser @key(fields: "id") {
   id: ID! @external
   book: Book
}

dev tips

Homestead included. To run without php copy Homestead.yaml.example to Homestead.yaml and replace $PWD with current directory and change provider if needed`. In bash:

cp Homestead.yaml.example Homestead.yaml && sed -i '' "s|\$PWD|$PWD|g" Homestead.yaml