The same thread that is making the data change. If it is a pre-emptive DB4D thread, the same thread writes to the journal. If it is the cooperative 4D language thread (e.g. stored procedures or Web processes) only a single thread is accessing the journal. It can be a combination of both of course. Further explanation here: https://www.youtube.com/watch?v=vVej7SwI-uM&t=38m14s