feature: Support the Subject property when publishing to SNS

[jtsalva]

Use Message.Header.Subject as SNS subject

[dhickie]

The naming here suggests that this is actual value for the subject, rather than something that generates subjects for a given message. I would suggest something like SnsSubjectGenerator, with a description of "An optional delegate for generating the SNS subject for a given message".

[iancooper]

I have a few questions here.

Typically we compose the body of a message from the properties of the request that is being serialized. We don't tend to add metadata in via the publish process, but via that message mapping pipeline (either the sink or intermediary middleware).

The publication is accessible via middleware, so it might make sense to add a subject, particularly if we are adding metadata into the body of the message, via that middleware and not via the serialization of the request in the sink. This allows you to not have to remember to add in subject in every mapper, just add the middleware (especially as V10 wants to add default mappers).

As subject is meant for an email header, it would feel that you are doing something specific here i.e. composing a message that will be sent as an email, so adding middleware that supports an SNS subject may be the right approach.

We may need to think about our goal here. I am not sure this is the right approach. I can see why you would opt for it, without knowing about the mapping pipeline however. So I do get that.

[jtsalva]

Summary of chat with @iancooper

Ideally, Brighter would avoid this work because the SNS Subject is intended for email and is arguably being misused by JustSaying. Nevertheless we agreed on the following:

• Message.Subject, currently only used for cloud events will be reused to carry the SNS subject in v10. In v9, the SNS subject will be contained in the message bag. The message producer will pass any provided subject onto SNS and the consumer will make available the SNS subject in Message
• Brighter library users are recommended to use tranformers to map the subject from and to Message but also have the option of just doing that in the mapper

[iancooper]

Do we know what happens if the subject is null or empty? We probably need a test for that, as modifying the existing test won't tell us if it is breaking for existing consumers. It just needs a conditional if it breaks anyone.

[iancooper]

As above, we need a test for when there is no subject.

[iancooper]

@jtsalva merged