See mailing list thread:

http://www.midgard-project.org/discussion/developer-forum/midgard2-_transactions-and_signals/

In short:

When transaction is active signals must be buffered and only sent on commit (and then accompanied by transaction id), if GDA backend does not support transactions then we still buffer signals but rollback() triggers commit signal (with a flag set to identify that it should have been a rollback).

Some IPC signals will probably need config option for turning them on/off in constrained environments (for example we probably do not want to send IPC signals while inside a transaction on mobile platforms like Maemo), see Trac ticket 720 for an idea for more modular approach.

(copied from http://trac.midgard-project.org/ticket/1585)