Change visibility in the directory...or lack there of (aka "what's the point of aging?")
I’m often asked about aging in adamsync so I thought I’d present the more general problem here for people to ponder. Hopefully this gives some context around the problem which aging in adamsync is supposed to address.
Imagine you are writing a tool which sync’s changes out of AD. You (the person running this tool) have some set of permissions…whatever they may be. You are syncing along happily.
One day you get a phone call…”My user was moved from OU=bar in to OU=foo yet the sync target still shows me in OU=bar. What gives?” You begin to investigate only to find out that you don’t have permissions to OU=foo. As a result, you don’t have any of the objects in OU=foo in your target location. The reason is straight forward….you don’t have permissions to the target, so when the object moved from bar to foo you never saw this change. You couldn’t see this change! You didn’t have permissions to OU=foo.
This is one of many such cases. If you don’t have the ability to see some object in the target location, it is hard to say anything about your view of it from the source. You could still have the object in the source location and have no idea that it moved out of your view. The reason is of course straight forward….you can’t see the target so you didn’t see that mod and we don’t have any construct where the source can say “out of your purview but not here anymore.” So you simply don’t realize the object has changed.
Historically, this was not nearly as much of a problem. Most people use DirSync to sync changes out of AD. In Win2K, in order to use DirSync you needed to be a domain admin. So, you could see most things that happen (out of the box anyway). In Win2k3 we built a feature for DirSync that made this problem more common….DirSync object security mode. In this mode anyone can use DirSync to sync out of any partition they so choose, and DirSync only shows changes for objects you have access to see. This is a very useful feature.
So now let’s consider adamsync, a simple DirSync client, and the problem I've mentioned above. When we wrote adamsync we wanted to ensure that we could handle the scenario where you are not an admin and want to sync data out. So, we default the tool to object security mode. This is fairly convenient for non-admins that wish to use the tool.
However, consider a very mainstream case. You are using adamsync to sync objects out of some domain NC. Objects are deleted. You don’t have permissions to see the deletions (remember I said that you “lose changes” when you move an object to a place you don’t have perms to? Well normal users don’t have permissions to the deleted objects container out of the box. So it’s a very common mainstream case for this problem….). As a result, you never reflect the deletion in your target container in ADAM. You’re woefully out of date.
One fix for this problem could be that you just give more perms. In the deleted objects case, just give the user who is syncing permissions to read the deleted objects container. But some people might not find that acceptable, depending upon their scenario.
Enter aging. We wrote aging to be a periodic background thread that goes and checks to make sure objects which we haven’t seen change in a while are actually still there. So you can imagine that every now and then you go back and check to ensure that all objects you have in the target are still in the source. This is the aging approach. While the specifics are configurable, that’s the basic idea.
Aging is just one such mechanism. There are lots of approaches to this problem that one could consider. It’s just the one we chose for adamsync.
One minor point I’ll raise before ending this post. Aging in ADAMSync in R2 is unfortunately not working properly. There is a bug that basically breaks it in some cases. It’s hard to say when but you should assume you’ll hit it at some point….no idea if you will, but you never know. So if you need aging pre-LH (ie, you have a compelling scenario where you want to sync as a non-admin) please open a QFE request with PSS. Or just give perms to deleted objects for now (or whatever the container is which you can't see)…a much easier quick-fix.
(just updated some formatting)