From some pictures I saw, it appeared the purlins support the rafters as well as the "A" shaped side wall. Some of the logs on the side wall did not appear to have purlins or any connection to the adjacent wall which to me would seem undesirable. It seems a bit tricky to line up everything since there are several things going on. I have sort of a vague idea of how you might go about it.
A purlin can be made level with itself and the purlin on the other side of the "A".
A straight board can be used to position the purlins as you go so that they line up with where the roof slant would go .. as well as some sort of angle mechanism such as make sure it 45 degrees etc ..
I would imagine every side log should be notched so that it either sits on a purlin, or a purlin sits on it. If you had 8 inch logs, that would mean you might have close to one purlin for every 1 foot of vertical rise .. The cabin pic I saw I think the "A" part rose up 4 feet from the base wall (for a 12x9 cabin). They showed 2 pairs of purlins plus the rafter so it was a lless than a purlin per one foot, but they showed one log that was not connected to a purlin in anyway which I'm not sure about ..