As always with these “best” practice type of questions, the answer is “it depends.” There is no single best way of doing it. it really depends upon what your payload is. If, for example, you were sending pdf files or images, then json is a horrible choice. If you’re sending some structured data payload, then json is a fine choice (though there’s cases where csv or xml or protobuf or other might be the best for that situation).

My personal use involves integration with a 3rd party company, so they dictated the message format. They have a message attribute (I asked the Broadway team to expose those) that is a cryptographic IV (initialization vector) that I have to read and combine with a shared secret to decrypt the json payload.

I would not use message attributes for domain knowledge purposes though. I see them more as infrastructural metadata. However, I tend to be cautious about buying too deeply into one vendor’s implementation. The less you depend upon their special features the easier it will be for you to migrate to some other queueing service. Tradeoffs apply and your situation may not match mine though.