Monday, June 25, 2007

Distributed Bug Tracking - Again

This is a bit of a clarification and expansion on a previous topic of this blog. To recap what I was talking about was a distributed issue tracking system making use of an underlying distributed version control system for its versioning, but augmented by command line tools that support necessary issue tracking features; like searching, merging, etc. Distributed issue tracking is a very new thing and has a few hurtles to overcome. I am going to talk about what these hurtles are and offer some ideas on how to overcome them.

In the comments of the last blog post on this topic Alex and I had a reasonably long conversation around merging. He ended up posting his thoughts here. Alex has some interesting and useful ideas, though we differ in some specifics.

Merging and Discovery

In the last article I used the term 'merging' in a ambiguous way. I used it to refer to both merging two issues into one another and finding the duplicate issues. For the rest of this article I am going to refer to merging as merging two issues and discovery as finding issue duplications. This should reduce the ambiguity a bit.

Merging Multiple Changes To The Same Issue

Merging is actually a pretty strait forward concept. I think you can treat issues the same way you treat a source file when a merge conflict occurs. By automatically merging what you can and allowing the user to resolve conflicts manually you get reasonable merge behavior with a high probability of a correct result. There is some overhead for the user but, as with source changes, it shouldn't be onerous.

Merging Two Issues Into A Single Issue

This problem is slightly more complex but its really just an extension of the last topic we talked about. In this case we just apply that merge algorithm to two disparate issues instead of two versions of a single issue. There may be some ambiguity around which issue becomes the canonical issue and how to merge history for these two files, however, these issues are mostly solved in distributed version control systems and those solutions would work just fine in this instance as well.


Discovery is by far the most complex issue here and its a problem that occurs in any issue tracking system. Unfortunately, in a distributed issue tracking system the problem has the potential to be much much worse then in an issue tracking system with a central repository. This is due to the fact that each and every user has his own canonical version of the issue repository. For example: User Y sees a bug in the system and enters Issue X to describe it and User Z sees the exact same bug at a similar time and enters Issue W to describe it. Because User X and Z both have canonical versions of the issue repository and they have yet to sync their repositories there is no way for either user to detect that a issue has already been created for that bug. So when they replicate suddenly there are multiple issues in both repositories.

In more normal issue tracking systems this can be mitigated to some extent by encouraging your users to search for existing bugs first and having people familiar with the issue repository reviewing new issues as they are entered. However, this approach wont work with a distributed issue tracking system because each user has a private canonical set until he syncs with some other user. I believe that this problem will be one of the fundamental problems that will plague new distributed issue tracking system for some time.

There are ways to mitigate this. There are very good document similarity algorithms out there and applying them to this problem wouldn't be too difficult. Unfortunately, the text associated with issues tends to be very short and this doesn't give these similarity algorithms much room to work. There are ways we can mitigate these problems though.

First we can reduce the total document corpus by using attributes of the issue to subset the issues for similarity searching. For example, we might only search for similarities within issues that have a specific component tag. Actually, generalizing this statement we can just use emantic properties of the issue to subset the issues that we need to process for the similarity search.

Second we need to give the user a fast and easy way to run through the output of a similarity search and approve/disapprove the merge. This should be something that allows the user to view the issues side by side and hit a single button or key combination to approve or disprove the merge, then the next set pops up. This cycle would allow the user to quickly move through all possible matches. If this worked well we may be able to loosen the similarity constraints a bit to allow for more matches.

Hopefully, a combination of approaches will make the discovery issue more tractable.


The third and last major problem (there are undoubtedly others that I can't think of right now) is simple referencing. There needs to be a way to reference an issue regardless of the repository its on or where it was created. The easiest way to do that would be to make use of simple UUIDs for issue identifiers. They are a bit unwieldy but their inherent uniqueness makes them usable for our purposes. We can reduce the level of pain in using UUIDs manually by allowing the user to specify the unique part of a UUID in the tools that support this distributed issue tracking system. I think monotone and, maybe git, allow something similar for change set identifiers.