Link Types Using @materializer

Combine subgraphs using the @materializer directive

Let us say that you have built up two subgraphs--one for Customer (against a REST backend), and another for Order against a MySQL database. At the simplest, your schema might look like:

1. customer.graphql:

type Customer {
  name: String
  id: ID
  email: String
}
type Query {
  customer(id: ID): Customer @rest(endpoint: "https://api.acme.com/customers/$id")

  customerByEmail(email: String): Customer
    @rest(endpoint: "https://api.acme.com/customers?email=$email")
}

2. order.graphql:

type Order {
  createOn: Date
  amount: Float
}
type Query {
  orders(customerId: ID): [Order] @dbquery(type: "mysql", table: "orders")
}

For building the supergraph, it does not matter how each subgraph is built. They are stitched together at the schema level, not at the implementation level. The stitching syntax is simple: data from one subgraph is used to provide arguments to a field selection in the same or different subgraph, which is then used to resolve a field in the originating subgraph through a series of transformations.

At its simplest, this might look like: a new file customer-orders.graphql:

extend type Customer {
  orders: [Order]
  @materializer (query: "orders", arguments: [{name: "customerId", field: "id"}]
}

Let us examine what the preceding command is:

• The extend clause extends a subgraph with new fields. In this case, we are adding a new field to type Customer. We can achieve the same purpose by editing the customer.graphql file, but using extend keeps the concerns of the supergraph separate from those of the subgraphs, allowing each to evolve and be maintained with minimal impact to the other.
• The new field declaration, that defines the name of the new field (orders), its type ([Order]), and a @materializer directive.
• The @materializer directive specifies a field selection that is used to resolve the annotated field. It has two arguments:
  • A field selection, specified by the query argument. This field selection must have the same type as the the new field. In this case, both have the same type [Orders]. The field selection need not be a top-level field selection. It can be a nested field selection, as long as the type of the field selection matches the type of the new field. For example, you could add a customerName: String field to a type, and populate it with a materializer like this: customerName: String @materializer(query: "customerByEmail { name }")
  • A list of argument mappings, specified by the arguments argument. Basically, you are telling StepZen how to map the arguments and fields of the annotated field and its enclosing type, respectively, to the arguments of the field selection specified by query. So here, the query: "orders" gets the customerId argument from the field id of Customer.

As you can see in the preceding section, the way in which orders is implemented is irrelevant for the stitching. That is the beauty of the StepZen graph of subgraphs concepts. You can stitch a layer.

Sometimes, you have to do a series of transformations to massage the data from the first subgraph to provide the right arguments for the second subgraph. For example, to extend customers with weather information, you might have to geocode the field address into lat,lon before resolving weather. You can do that easily in StepZen using the @sequence construct:

extend type Customer {
  weather: Weather @materializer(query: "customerWeather")
}
type Coord {
  lat: Float
  lon: Float
}
type Query {
  customerWeather(address: String): Weather
    @sequence(steps: [{ query: "geocode" }, { query: "weather" }])

  geocode(address: String): Coord
  # any implementation, using @rest, @graphql, @dbquery
}

The StepZen Federation model

As you can see, the stitching (using @materializer and when needed @sequence) is independent of how that subgraph is formed.

When all subgraphs are formed using @graphql, we are federating across a set of 'subgraphs'. Whether each subgraph is built in StepZen or not, the subgraphs your teams use can all be made into a federated supergraph in StepZen with @materializer and @sequence.