How are `union` shapes bound with `httpPayload` serialized in JSON-based protocols?

[david-perez]

The httpPayload trait:

can be applied to structure members that target a string, blob, structure, union, document, map, or list.

Of note is that no AWS protocol supports list or map, but presumably union is supported in e.g. restJson1. How is it serialized?

Consider:

structure OperationInputOutput {
    @httpPayload
    union: Union
}

union Union {
    good: String,
    bad: String,
}

Surely the good/bad tag needs to be serialized in the payload for a deserializer to know what exact variant it's deserializing.

If you don't bind with httpPayload, you'd serialize:

{
    "good": "a good string"
}

[mtdowling]

It's serialized exactly like a structure. It's supported, I just don't think it's been used much in practice yet. We also should add a protocol test.

With an HTTP payload like in your example, it's serialized as the full payload:

{
    "good": "hi"
}

If you don't bind with httpPayload, you'd serialize [...]

No, you'd serialize the union value inside the outer structure. If the OperationInputOutput$union member had no @httpPayload trait, then it's serialized inside a member of the input/output structure:

{
    "union": {
        "good": "hi"
    }
}

[david-perez]

No, you'd serialize the union value inside the outer structure.

Ah, my bad. This is how we're doing it. How do we serialize the union when the user has not set it (it's not required)?. Without httpPayload we'd do:

{
    "union": null
}

So it seems like binding the union shape with httpPayload to the HTTP body should serialize an unset union as just the null value:

null

And for protocols that don't serialize null values, it would just be an empty payload.

It's doable, but implementation-wise tricky, since the deserializer has to account for it expecting a JSON object or a value (or nothing) at the top-level.

Just to confirm, is this behavior what we want?

[mtdowling]

Unions can’t be assigned a default value, ~~so to bind them as httpPayload, they must be required. So I don’t think we need to account for a root level null.~~

[david-perez]

Unions can’t be assigned a default value, so to bind them as httpPayload, they must be required.

I'm confused, is this implying that httpPayload implies the member shape must be default or required?

I thought IDL v2 only made that provision for streaming blobs:

Members targeting streaming blobs MUST be marked with the required trait or default trait.

[mtdowling]

Oh whoops. You’re right. Hm, I don’t know if we need to support an explicit null or can just rely on there being no payload at all.

[david-perez]

Just to document the decision that was made, the added tests show that no payload is sent if the union has no value.