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Schema

To set up a lambda webhook, you need to define it in your GraphQL schema by using the @lambdaOnMutate directive along with the mutation events (add/update/delete) you want to listen on.
Lambda webhooks only listen for events from the root mutation. You can create a schema that’s capable of creating deeply nested objects, but only the parent level webhooks are invoked for the mutation.
For example, to define a lambda webhook for all mutation events (add/update/delete) on any Author object: 
type Author @lambdaOnMutate(add: true, update: true, delete: true) {
  id: ID!
  name: String! @search(by: [hash, trigram])
  dob: DateTime
  reputation: Float
}

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Resolver

Once the schema is ready, you can define your JavaScript functions and add those as resolvers in your JS source code. To add the resolvers you should use the addWebHookResolversmethod.
A Lambda Webhook resolver can use a combination of event, dql, graphql or authHeader inside the function.

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Event object

You also have access to the event object within the resolver. Depending on the value of operation field, only one of the fields (add/update/delete) is part of the event object. The definition of event is as follows: 
"event": {
    "__typename": "<Typename>",
    "operation": "<one-of: add/update/delete>",
    "commitTs": <uint64, the commitTs of the mutation>
    "add": {
      "rootUIDs": [<list-of-UIDs-that-were-created-for-root-nodes-in-this-mutation>],
      "input": [<AddTypeInput: i.e. all the data that was received as part of the `input` argument>]
    },
    "update": {
      "rootUIDs": [<list-of-UIDs-of-root-nodes-for-which-something-was-set/removed-in-this-mutation>],
      "setPatch": <TypePatch: the object that was received as the patch for set>,
      "removePatch": <TypePatch: the object that was received as the patch for remove>
    },
    "delete": {
      "rootUIDs": [<list-of-UIDs-of-root-nodes-which-were-deleted-in-this-mutation>]
    }

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Resolver examples

For example, to define JavaScript lambda functions for each mutation event for which @lambdaOnMutate is enabled and add those as resolvers: 
async function addAuthorWebhook({ event, dql, graphql, authHeader }) {
  // execute what you want on addition of an author
  // maybe send a welcome mail to the author
}

async function updateAuthorWebhook({ event, dql, graphql, authHeader }) {
  // execute what you want on update of an author
  // maybe send a mail to the author informing that few details have been updated
}

async function deleteAuthorWebhook({ event, dql, graphql, authHeader }) {
  // execute what you want on deletion of an author
  // maybe mail the author saying they have been removed from the platform
}

self.addWebHookResolvers({
  "Author.add": addAuthorWebhook,
  "Author.update": updateAuthorWebhook,
  "Author.delete": deleteAuthorWebhook,
})

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Example

Finally, if you execute an addAuthor mutation, the add operation mapped to the addAuthorWebhook resolver is triggered: 
mutation {
  addAuthor(input: [{ name: "Ken Addams" }]) {
    author {
      id
      name
    }
  }
}