Deferred Resolvers

Deferred Resolvers and Fetchers

Fetchers and Deferred Resolvers are mechanisms for batch retrieval of objects from their sources like database or external API. Deferred Resolver provides an efficient, low-level API but, on the other hand, it’s more complicated to use and less secure. Fetcher is a specialized version of Deferred Resolver. It provides a high-level API, it’s easier to use and usually provides all you need. It optimizes resolution of fetched entities based on ID or relation. Also, it deduplicates entities and caches the results and much more.

Let’s implement one for the Link entity:

Now we have to change fields in QueryType to use that Fetcher:

We’re still using dao.getLinks to fetch links from database, but now it’s wrapped in Fetcher. It optimizes the query before call. Firstly it gathers all data it should fetch and then it executes queries. Caching and deduplication mechanisms allow to avoid duplicated queries and give results faster.

As you can see, we use the same fetcher in two fields, in the first example we’re providing only a single id and expecting one entity in response, optional object (deferOpt function). In the second case we’re providing a list of ids and expecting a sequence of objects (deferSeq).

After defining a resolver we have to inform executor to use it. Firstly, push it to the lower level by using proper DeferredResolver function which rebuild it:

Such resolver have to be passed into the Executor to make it available for use.

Since, we’re using DAO.getLinks to fetch a single entity or an entire list, we don’t need the getLink function anymore.

HasId type class

If you tried to execute a query, you got an error at this point. The reason is that Fetcher needs ‘something’ what extracts ids from entities. This thing is HasId type class. You have a few choices on how to provide such class for your model. Firstly you can pass it explicitly, like this:

val linksFetcher = Fetcher(
  (ctx: MyContext, ids: Seq[Int]) => ctx.dao.getLinks(ids)
)(HasId(_.id))

On the other hand, you can declare an implicit constant in the same context so the fetcher will take it implicitly. The third way is to provide HasId implicitly inside the companion object of our model, like this:

object Link {
    implicit val hasId = HasId[Link, Int](_.id)
}

For now we’re going the second path, but in the Interfaces chapter it will change.

After the last changes, our GraphQLSchema file content should looks like the this:

package com.howtographql.scala.sangria

import sangria.schema.{ListType, ObjectType}
import models._
import sangria.execution.deferred.{DeferredResolver, Fetcher, HasId}
import sangria.schema._
import sangria.macros.derive._

object GraphQLSchema {

  implicit val LinkType = deriveObjectType[Unit, Link]()
  implicit val linkHasId = HasId[Link, Int](_.id)

  val linksFetcher = Fetcher(
    (ctx: MyContext, ids: Seq[Int]) => ctx.dao.getLinks(ids)
  )

  val Resolver = DeferredResolver.fetchers(linksFetcher)


  val Id = Argument("id", IntType)
  val Ids = Argument("ids", ListInputType(IntType))

  val QueryType = ObjectType(
    "Query",
    fields[MyContext, Unit](
      Field("allLinks", ListType(LinkType), resolve = c => c.ctx.dao.allLinks),
      Field("link",
        OptionType(LinkType),
        arguments = Id :: Nil,
        resolve = c => linksFetcher.deferOpt(c.arg(Id))
      ),
      Field("links",
        ListType(LinkType),
        arguments = Ids :: Nil,
        resolve = c => linksFetcher.deferSeq(c.arg(Ids))
      )
    )
  )

  val SchemaDefinition = Schema(QueryType)
}

Test it

You can debug the DAO.getLinks function in any way and execute the following query:

query {

    l1: link(id: 1){
      id
      url
    }

  l2: link(id: 1){
      id
      url
    }

    links(ids: [2,3]){
      id
      url
    }

}

Even though the query request the same link twice, when you debug it, you can see that getLinks is called only once for that ID!

What’s next?

In the next chapter we will add an additional field to the Link class. We need to add information about the moment when the link is added. The best type to cover such needs is datetime. H2 doesn’t support the type we need so we have to manage it somehow manually. In Sangria you can define your own scalar types and this is what we will learn about.