[TYPO3-mvc] Using external APIs in DDD

[Christine Gerpheide]

Hello list :)

I have a conceptual question about using APIs within extbase and DDD.  I
have an API that I will need to use within the Repository or Controllers of
a couple different objects, and I'm not sure where to put it.  Specifically
I'm using an email marketing API, so I need to be able to for instance
retrieve both Campaigns and RecipientLists and send emails to
RecipientLists.  Is a service appropriate for this because it spans multiple
domains?  Conceptually the only problem I see there is that this API object
cannot be a singleton, because the API key I'll be sending will change
depending on the logged in user, and I had thought services were meant to be
singletons.  Is there somewhere more appropriate, maybe something like
"Library"?

Thanks,
Christine