Some kind of compare-and-set! operator which occurs at the transactor perhaps.

Update:

1. you can do synchronous transactions.

http://datomic.com/docs/javadoc/datomic/Connection.html#tran...

2. transactions can include data functions.

"The database can be extended with data functions that expand into other data functions, or eventually bottom out as assertions and retractions. A set of assertions/ retractions/functions, represented as data structures, is sent to the transactor as a transaction, and either succeeds or fails all together, as one would expect."