Question about writing a Cognito post-auth lambda in go

Question

I wrote a really simple post-auth lambda for cognito like this:

```go
func HandleRequest(ctx context.Context, e events.CognitoEventUserPoolsPostAuthentication ) error {
	eventJson, _ := json.MarshalIndent(e, "", "  ")
	eventString := strings.ReplaceAll(string(eventJson), "
", "")

log.Printf("EVENT: %s", eventString)
	return nil
}

func main() {
	lambda.Start(HandleRequest)
}

```

It's only there - for now - to log activity.  But when I actually attach it to the user pool as a post-authentication trigger it makes OAuth2 not work.  The error message is "contact administrator."

I'm thinking that I need to actually do something besides return error = nil.  I'm not having much luck finding any examples of this in go, though.

The javascript example ends with:
```
    // Return to Amazon Cognito
    callback(null, event);
```
Is that what the expectation is in 'go' as well - that I return `(events.CognitoEventUserPoolsPostAuthentication, error)` and just send it back the original event?

wz2b · Accepted Answer

I figured this out.  I want more control over things so I'm using this:

```func HandleRequest(ctx context.Context, request events.APIGatewayV2HTTPRequest) (events.APIGatewayV2HTTPResponse, error) { ... }```

which works great.  It was kind of nice before that it would take care of marshaling/unmarshaling and now I have to do it myself, but that's not bad.  I do kind of wish the Response "Body" was just `interface{}` and that it would automatically marshal/unmarshal based on `Content-Type` but it's fine.

One thing I do miss from doing this in Kotlin and Spring is the ability to just throw an Exception, and write an ExceptionMapper that would set the appropriate http status code.

ADI-Ashish · Answer

I tried following

```go
package main

import (
	"fmt"

"github.com/aws/aws-lambda-go/events"
	"github.com/aws/aws-lambda-go/lambda"
)

func handler(event events.CognitoEventUserPoolsPostAuthentication) (events.CognitoEventUserPoolsPostAuthentication, error) {
	fmt.Printf("PreAuthentication of user: %s
", event.UserName)

// Return to Amazon Cognito
	return event, nil
	// Try raising an error
	// return event, fmt.Errorf("TESTING LAMBDA ERRORS WITH GOLANG")
}

func main() {
	lambda.Start(handler)
}
```

Question about writing a Cognito post-auth lambda in go

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