We’re overhauling Dgraph’s docs to make them clearer and more approachable. If you notice any issues during this transition or have suggestions, please let us know.
Lambda provides a way to write your custom logic in JavaScript, integrate it with your GraphQL schema, and execute it using the GraphQL API in a few easy steps:
  1. Set up a Dgraph cluster with a working lambda server
  2. Declare lambda queries, mutations, and fields in your GraphQL schema as needed
  3. Define lambda resolvers for them in a JavaScript file
This also simplifies the job of developers, as they can build a complex backend that’s rich with business logic, without setting up multiple different services. Also, you can build your backend in JavaScript, which means you can build both your frontend and backend using the same language. Dgraph doesn’t execute your custom logic itself. It makes external HTTP requests to a user-defined lambda server.
If you want to deploy your own lambda server, you can find the implementation of Dgraph Lambda in our open source repository.

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Declaring lambda in a GraphQL schema

There are three places where you can use the @lambda directive and thus tell Dgraph where to apply custom JavaScript logic.
  • You can add lambda fields to your types and interfaces, as follows:
type MyType {
  ...
  customField: String @lambda
}
  • You can add lambda queries to the Query type, as follows:
type Query {
  myCustomQuery(...): QueryResultType @lambda
}
  • You can add lambda mutations to the Mutation type, as follows:
type Mutation {
  myCustomMutation(...): MutationResult @lambda
}

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Defining lambda resolvers in JavaScript

A lambda resolver is a user-defined JavaScript function that performs custom actions over the GraphQL types, interfaces, queries, and mutations. There are two methods to register JavaScript resolvers:
  • self.addGraphQLResolvers
  • self.addMultiParentGraphQLResolvers
Functions self.addGraphQLResolvers and self.addMultiParentGraphQLResolvers can be called multiple times in your resolver code.

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addGraphQLResolvers

The self.addGraphQLResolvers method takes an object as an argument, which maps a resolver name to the resolver function that implements it. The resolver functions registered using self.addGraphQLResolvers receive { parent, args, graphql, dql } as argument:
  • parent, the parent object for which to resolve the current lambda field registered using addGraphQLResolver. The parent receives all immediate fields of that object, whether or not they were actually queried. Available only for types and interfaces (null for queries and mutations)
  • args, the set of arguments for lambda queries and mutations
  • graphql, a function to execute auto-generated GraphQL API calls from the lambda server. The user’s auth header is passed back to the graphql function, so this can be used securely
  • dql, provides an API to execute DQL from the lambda server
  • authHeader, provides the JWT key and value of the auth header passed from the client
The addGraphQLResolvers can be represented with the following TypeScript types: 
type GraphQLResponse {
  data?: Record<string, any>
  errors?: { message: string }[]
}

type AuthHeader {
  key: string
  value: string
}

type GraphQLEventWithParent = {
  parent: Record<string, any> | null
  args: Record<string, any>
  graphql: (query: string, vars?: Record<string, any>, authHeader?: AuthHeader) => Promise<GraphQLResponse>
  dql: {
    query: (dql: string, vars?: Record<string, any>) => Promise<GraphQLResponse>
    mutate: (dql: string) => Promise<GraphQLResponse>
  }
  authHeader: AuthHeader
}

function addGraphQLResolvers(resolvers: {
  [key: string]: (e: GraphQLEventWithParent) => any;
}): void
self.addGraphQLResolvers is the default choice for registering resolvers when the result of the lambda for each parent is independent of other parents.
Each resolver function should return data in the exact format as the return type of GraphQL field, query, or mutation for which it’s being registered. In the following example, the resolver function myTypeResolver registered for the customField field in MyType returns a string because the return type of that field in the GraphQL schema is String: 
const myTypeResolver = ({ parent: { customField } }) =>
  `My value is ${customField}.`

self.addGraphQLResolvers({
  "MyType.customField": myTypeResolver,
})
Another resolver example using a graphql call: 
async function todoTitles({ graphql }) {
  const results = await graphql("{ queryTodo { title } }")
  return results.data.queryTodo.map((t) => t.title)
}

self.addGraphQLResolvers({
  "Query.todoTitles": todoTitles,
})

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addMultiParentGraphQLResolvers

The self.addMultiParentGraphQLResolvers is useful in scenarios where you want to perform computations involving all the parents returned from Dgraph for a lambda field. This is useful in two scenarios:
  • When you want to perform a computation between parents
  • When you want to execute a complex query, and want to optimize it by firing a single query for all the parents
This method takes an object as an argument, which maps a resolver name to the resolver function that implements it. The resolver functions registered using this method receive { parents, args, graphql, dql } as argument:
  • parents, a list of parent objects for which to resolve the current lambda field registered using addMultiParentGraphQLResolvers. Available only for types and interfaces (null for queries and mutations)
  • args, the set of arguments for lambda queries and mutations (null for types and interfaces)
  • graphql, a function to execute auto-generated GraphQL API calls from the lambda server
  • dql, provides an API to execute DQL from the lambda server
  • authHeader, provides the JWT key and value of the auth header passed from the client
The addMultiParentGraphQLResolvers can be represented with the following TypeScript types: 
type GraphQLResponse {
  data?: Record<string, any>
  errors?: { message: string }[]
}

type AuthHeader {
  key: string
  value: string
}

type GraphQLEventWithParents = {
  parents: (Record<string, any>)[] | null
  args: Record<string, any>
  graphql: (query: string, vars?: Record<string, any>, authHeader?: AuthHeader) => Promise<GraphQLResponse>
  dql: {
    query: (dql: string, vars?: Record<string, any>) => Promise<GraphQLResponse>
    mutate: (dql: string) => Promise<GraphQLResponse>
  }
  authHeader: AuthHeader
}

function addMultiParentGraphQLResolvers(resolvers: {
  [key: string]: (e: GraphQLEventWithParents) => any;
}): void
This method shouldn’t be used for lambda queries or lambda mutations.
Each resolver function should return data as a list of the return type of GraphQL field for which it’s being registered. In the following example, the resolver function rank() registered for the rank field in Author, returns a list of integers because the return type of that field in the GraphQL schema is Int: 
type Author {
  id: ID!
  name: String! @search(by: [hash, trigram])
  reputation: Float @search
  rank: Int @lambda
}

import { sortBy } from "lodash"

/* 
This function computes the rank of each author based on the reputation of the author relative to other authors.
*/
async function rank({ parents }) {
  const idRepMap = {}
  sortBy(parents, "reputation").forEach(
    (parent, i) => (idRepMap[parent.id] = parents.length - i),
  )
  return parents.map((p) => idRepMap[p.id])
}

self.addMultiParentGraphQLResolvers({
  "Author.rank": rank,
})
Scripts containing import packages (such as this example) require compilation using Webpack.
The following example resolver uses a dql call: 
async function reallyComplexDql({ parents, dql }) {
  const ids = parents.map((p) => p.id)
  const someComplexResults = await dql.query(
    `really-complex-query-here with ${ids}`,
  )
  return parents.map((parent) => someComplexResults[parent.id])
}

self.addMultiParentGraphQLResolvers({
  "MyType.reallyComplexProperty": reallyComplexDql,
})
The following resolver example uses a graphql call and manually overrides the authHeader provided by the client: 
async function secretGraphQL({ parents, graphql }) {
  const ids = parents.map((p) => p.id);
  const secretResults = await graphql(
    `query myQueryName ($ids: [ID!]) {
      queryMyType(filter: { id: $ids }) {
        id
        controlledEdge {
          myField
        }
      }
    }`,
    { ids },
    {
      key: 'X-My-App-Auth'
      value: 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJodHRwczovL215LmFwcC5pby9qd3QvY2xhaW1zIjp7IlVTRVIiOiJmb28ifSwiZXhwIjoxODAwMDAwMDAwLCJzdWIiOiJ0ZXN0IiwibmFtZSI6IkpvaG4gRG9lIDIiLCJpYXQiOjE1MTYyMzkwMjJ9.wI3857KzwjtZAtOjng6MnzKVhFSqS1vt1SjxUMZF4jc'
    }
  );
  return parents.map((parent) => {
    const secretRes = secretResults.data.find(res => res.id === parent.id)
    parent.secretProperty = null
    if (secretRes) {
      if (secretRes.controlledEdge) {
        parent.secretProperty = secretRes.controlledEdge.myField
      }
    }
    return parent
  });
}
self.addMultiParentGraphQLResolvers({
  "MyType.secretProperty": secretGraphQL,
});

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Example

For example, if you execute the following lambda query: 
query {
  queryMyType {
    customField
  }
}
You should see a response such as the following: 
{
  "queryMyType": [
    {
      "customField": "My value is Lambda Example"
    }
  ]
}

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Learn more

To learn more about the @lambda directive, see: