dynamodb/attributevalue: allow registration of custom type marshal/unmarshal functions

[ztison]

Describe the feature

As a DynamoDB user, I want to be able to register custom marshaler/unmarshaler functions.

Use Case

• I want to have common API types not coupled with the repository layer. So I don't want to implement MarshalDynamoDBAttributeValue and UnmarshalDynamoDBAttributeValue functions with dependency on AttributeValue in my high-level code.

• I want to implement custom marshaler/unmarshaler for types that I don't own. It can be required because of data masking or to omit empty values.

Proposed Solution

func (e *Encoder) RegisterMarshaller(t reflect.Type, f func(interface{}) (types.AttributeValue, error)) error
func (d *Decoder) RegisterUnmarshaller(t reflect.Type, f func(types.AttributeValue) (interface{}, error)) error

type Foo string

func StringMarshaller(value interface{}) (types.AttributeValue, error) {
	if foo, ok := value.(Foo); ok {
		return &types.AttributeValueMemberS{Value: string(foo)}, nil
	}
	return nil, fmt.Errorf("type error")
}

var foo = "foo"
enc.RegisterMarshaller(reflect.TypeOf(foo), StringMarshaller)

Other Information

Based on customer marshaler/unmarshaler for json

Rejected Alternatives

API without exposing reflect:

func (e *Encoder) RegisterFunc(f interface{})
func (d *Decoder) RegisterFunc(f interface{})

API based on generics:

func (e *Encoder)  Register[T any](f func(T) (types.AttributeValue, error)) { ... }
func (d *Decoder)  Register[T any](f func(types.AttributeValue) (T, error)) { ... }

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

AWS Go SDK V2 Module Versions Used

github.com/aws/aws-sdk-go-v2/feature/dynamodb/attributevalue@v1.10.14
github.com/aws/aws-sdk-go-v2/feature/dynamodb/expression@v1.4.39
github.com/aws/aws-sdk-go-v2/service/dynamodb@v1.18.4
github.com/aws/aws-sdk-go-v2/service/dynamodbstreams@v1.14.4

Go version used

go version go1.19.5 darwin/arm64

[raulvc]

so.. turns out this still isn't a thing, could you point me to a fork/custom implementation of an attributevalue encoder/decoder that allows me to register a uuid type?

[ztison]

Just closed PR exists https://github.com/aws/aws-sdk-go-v2/pull/2060/files

[RanVaknin]

Hi all, thank you for your patience. At this point in time, there is no plan to implement this. We are aware that there discussions happening on the standard library level, and we want to see how this is developing before taking any action on it from our side.

I hope we can revisit this sooner than later.
Thanks,
Ran~

[github-actions]

This issue is now closed. Comments on closed issues are hard for our team to see.
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[ashwinrs]

+1 to @ztison proposal on how to ideally expose custom marshaler/unmarshaller. The existing interface makes the code extremely verbose and complex.

For example, we use go generics when interacting with DynamoDB. DynamoDB does not handle structpb.Struct well. So the way we now have to handle it is, iterate over every field in all the structs to check if its of type structpb.Struct and then do the custom handling. It would be a lot cleaner with the approach @ztison proposed.

[austince]

I work with @ztison, we were ultimately forced to fork