A revision idea is a special token which generates each time when data saves into the data base. When you're safe again, it will check if this idea is the same or it's already ablated. I think mongod b does have transactions now, but sti haven't used them. It's also worth discussing like set and increment and those types of things. So if i want to add a category to a product, i could do like, add to set on that thing, and pass the just put this thing in its category list. Will that also increment revision? So only set.