Add Direct Lambda Resolver Event Format (AppSync) to Event Definitions

[46ki75]

We’d like to propose adding support for the event schema used by Direct Lambda Resolvers in AWS AppSync.

Background

When using AppSync with AWS Lambda as a data source, users often create a "Function" (VTL mapping template) that transforms the request before it's passed to Lambda. This introduces inconsistent event shapes, requiring manual schema tracking per data source.

To avoid this, Direct Lambda Resolvers allow bypassing VTL altogether. With this, AppSync sends a fixed JSON structure to Lambda. However, the current event definitions do not seem to support this pattern explicitly.

The format is described in the official documentation:

https://docs.aws.amazon.com/appsync/latest/devguide/resolver-context-reference.html

Sample Payload

{
  "arguments": { "input": "foo" },
  "identity": {
    "sourceIp": ["x.x.x.x"],
    "userArn": "arn:aws:iam::123456789012:user/appsync",
    "accountId": "666666666666",
    "user": "AIDAAAAAAAAAAAAAAAAAA"
  },
  "info": {
    "fieldName": "greet",
    "parentTypeName": "Query",
    "selectionSetGraphQL": "",
    "selectionSetList": [],
    "variables": {
      "inputVar": "foo"
    }
  },
  "prev": null,
  "request": {
    "headers": {
      "x-api-key": "***",
      "user-agent": "...",
      "...": "..."
    }
  },
  "source": null,
  "stash": {}
}

Setup Steps (Console-based)

To help with future integration testing or reproducibility, here are the steps to configure Direct Lambda Resolvers via the AWS Management Console:

1. Create a GraphQL API in AppSync

2. Deploy the target Lambda function

3. Add a new Data Source with type AWS Lambda

4. Define the schema, then choose Attach to link a field to the data source

5. In the "Attach Data Source" screen, select VTL (not AppSync JS!)

6. After attaching, disable both:

   • "Enable request mapping template"
   • "Enable response mapping template"

7. Save the resolver

📝 Note:
AppSync JS does not currently support Direct Lambda Resolvers. You must select VTL for this setup to work.

[jlizen]

Checking my understanding - is the proposal to add new types to aws_lambda_events to reflect the "raw" JSON schema when using direct resolvers? Rather than something more complicated that involves generating types based on queries against the raw schema using eg proc macros (like with https://github.com/JeremieRodon/lambda-appsync)?

If so, that seems very reasonable to me. We could keep this under the existing appsync feature flag rather than adding a new one.

Were you interesting in making the contribution adding this support?

[46ki75]

Thank you for the clarification — yes, your understanding is correct.

My intention is to add static Rust types to aws_lambda_events to represent the standardized JSON schema used by Direct Lambda Resolvers in AWS AppSync, not anything dynamic or macro-based like lambda-appsync.

I apologize for the delay in replying — I had already created the PR by the time I saw your response.
As you suggested, I’ve implemented the changes under the existing appsync feature flag.

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