I am going to build a grape arbor and I have some nice 8 foot cedar logs - about 8-10" in diameter for posts. Should I go with concrete or gravel? How should I attach the log to the footing? I was thinking about sinking some PT 2 by 4's in the concrete or gravel and then lag screwing those to the logs. Should there be something between the concrete and the log?

Unless this is a really big heavy arbor, why do you need a footing? If you are going to just set the logs on a pad(footing?), sink a pin in the concrete, rebar works well, drill a hole in the cedar and then set the log over the pin. If you can find big standoffs, put one under the log or 3-4 layers of tarpaper to elevate the log off the concrete. Of course, you will have to stabilize the logs. Can you direct bury the cedar posts? Depending on the species, they should last 20 years. You lagging alternative can work well too. I just stabilized a fence for someone by using 3 ft. 2X4 PT, buried 2' and lagged with 6" X5/16 galvanized lags.

Thanks for the advice. I only have 8 foot logs so I don't want to bury them so people can easily walk under it and pick grapes some day. I like the rebar idea - maybe do both that and lag. How much rebar should go in the log?

You should get the rebar into the cement at least a foot and then about the same up into your log, gravity will do the rest. Tar paper would break the cement wood contact but I prefer to make a 2x10 PT cut into a circle to use as a washer, te can be stained to look similar to the log