Classical Pub/Sub might not be a best fit, some of the reasons Adrian outlined pretty nicely for you in his comment.
May I propose to consider Event Sourcing pattern? You don't really remove or modify any records -- you just append them. Usually this pattern is being used with CQRS pattern -- it brings really nice scaling advantages later on into your project.
I think the requests and the results (process and email) are ephemeral, so they do not need to be kept indefinitely. Event sourcing solves the problem when you need to keep all the previous states of the entities (kind of), I don't think this is the case. The requests are unique and do not change either need to be kept for longer period of time.
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