Say, if I have a transaction that includes :db.fn/cas and some other data, like [[:db.fn/cas [:some/id 234] :my/attr 1 2] {:db/id 123 :something/else 45}], will the entire transaction be aborted if the compare-and-swap fails?

yep

http://docs.datomic.com/database-functions.html#processing-transaction-functions

Awesome, thanks @marshall

where can i find more info about using multiple databases?

I can't really find much information about it. I want to start preparing for 10-20 billion datoms and don't want any major mistakes in my architecture.

Let's say for example i had a photos application. If I say every photo has ~ 10 attributes that get edited on average 10 times each, that's 100 datoms which means i can store 100M photos before the DB starts to choke. Eventually I'd have to start putting photos in a second database. Then when I do a lookup for a photo, I'd need to know which database it's in before I try to retrieve it. It's sounding pretty complicated.