Node Resolvers

Schema

Nodes are types that are resolvable by ID and implement the Node interface. Every object type that implements the Node interface has a corresponding node resolver.

interface Node {
  id: ID!
}

type User implements Node {
  id: ID!
  firstName: String
  lastName: String
  displayName: String @resolver
}

Generated base class

Viaduct generates an abstract base class for all object types that implement Node. For the User example above, Viaduct generates the following code:

object NodeResolvers {
  abstract class User {
    open suspend fun resolve(ctx: Context): viaduct.api.grts.User =
      throw NotImplementedError()

    open suspend fun batchResolve(contexts: List<Context>): List<FieldValue<viaduct.api.grts.User>> =
      throw NotImplementedError()

    class Context: NodeExecutionContext<viaduct.api.grts.User>
  }

  // If there were more nodes, their base classes would be generated here
}

The nested Context class is described in more detail below.

Implementation

Implement a node resolver by subclassing the generated base class and overriding exactly one of either resolve or batchResolve.

Here’s an example of a non-batching resolver for User that calls a user service to get data for a single user:

class UserNodeResolver @Inject constructor(
  val userService: UserServiceClient
): NodeResolvers.User() {
  override suspend fun resolve(ctx: Context): User {
    // Fetches data for a single User ID
    val data = userService.fetch(ctx.id.internalId)
    return User.builder(ctx)
      .firstName(data.firstName)
      .lastName(data.lastName)
      .build()
  }
}

Points illustrated by this example:

• Dependency injection can be used to provide access to values beyond what’s in the execution context.
• You should not provide values for fields outside the resolver’s responsibility set. In the example above, we do not set displayName when building the User GRT.

Alternatively, if the user service provides a batch endpoint, you should implement a batch node resolver. Node resolvers typically implement batchResolve to avoid the N+1 problem. Learn more about batch resolution here.

Context

Both resolve and batchResolve take Context objects as input. This class is an instance of NodeExecutionContext :

interface NodeExecutionContext<T: NodeObject>: ResolverExecutionContext {
  val id: GlobalID<T>
  fun selections(): SelectionSet<T>
}

For the example User type, the T type would be the User GRT.

NodeExecutionContext includes the ID of the node to be resolved, and the selection set for the node being requested by the query. Most node resolvers are not “selective,” i.e., they ignore this selection set and thus don’t call this function. In this case, as discussed above, it’s important that the node resolver returns its entire responsibility set.

Advanced users: If the selections function is not called by an invocation of a resolver, then the engine will assume that invocation will return the full responsibility set of the resolver and may take actions based on that assumption. If a resolver is going to be selective, then it must call this function to get its selection set rather than obtain it through some other means.

Since NodeExecutionContext implements ResolverExecutionContext, it also includes the utilities provided there, which allow you to:

• Execute subqueries
• Construct node references
• Construct GlobalIDs

Responsibility set

The node resolver is responsible for resolving all fields, including nested fields, without its own resolver. These are typically core fields that are stored together and can be efficiently retrieved together.

In the example above, the node resolver for User is responsible for returning the firstName and lastName fields, but not the displayName field, which has its own resolver. Note that node resolvers are not responsible for the id field, since the ID is an input to the node resolver.

Node resolvers are also responsible for determining whether the node exists. If a node resolver returns an error value, the entire node in the GraphQL response will be null, not just the fields in the node resolver’s responsibility set.