Evil Twins in SCM, Not Hollywood

I keep expecting customers to ask me “Is it contagious Doc?” Or state, “but I am an only child!!!!!” What am I referring to? Evil Twins!!! Sounds menacing right? It really is not, but to a first-time user or someone who has never encountered the warning, “Name already exists in backing stream,” it can be rather perplexing.

So what exactly is an Evil Twin you ask? It is a situation where two completely separate file elements in the same directory structure have the same name. It is actually a common problem across many software configuration management (SCM) systems. So how does this happen exactly? All it takes are two well intended individuals with their own workspaces. Lets use the following scenario, User A and User B both have their own workspaces attached to the same development stream. Currently we have the following directory structure;

\.\dir1\child_dir\

with the following files already under source control:

file_a.txt

file_b.txt

As part of our software project, we also need to create one additional file:

file_c.txt.

User_A creates and adds file_c.txt to source control in the child_dir directory and promotes this newly added file into the development stream.

At the same time, User_A was creating and adding file_c.txt, User_B did the exact same steps. However, A was first to promote so in effect User_A won the race. When User_B attempts to promote his/her file_c.txt the “Name already exists” error pops up front and center and does not allow the promote to continue. Evil Twin prevention at it’s best.

AccuRev tracks an element by its eid (element identification number) and not its name. This is how an Evil Twin is detected. When User_B attempts to promote File_c.txt AccuRev knows that there is already a File_c.txt in the development stream but notices that the eid’s do not match. They are truly separate files since they were independently created and added by different users. They are Evil Twins because they look identical but are different. Really only one of the two is Evil and that is typically the file that ends up getting removed/renamed. But that is another story for another day.

So what should an AccuRev customer do when encountering this problem? Give support@accurev.com a shout and title your email “Need Evil Twin resolution help” or “Name Already exists in backing stream error.” Or you may also take a look at the AccuRev Online FAQ’s in the Customer Support Forum.

Please feel free to let me know if this helps you out!