Coordinating Distributed Development in Projects

I quickly wanted to kick around an idea about how to make distributed development within a project easier. Project for the purpose of this writing is not a typical open source project but something more along the lines of a commercial environment. So you will usually have 10-50 people working on a solution with access to common infrastructure (corporate network, central Version Control System, etc.).

So far we basically tell people that they can solve this either on an organizational or technical level. Organizational more or less means phone or email, while technical means pessimistic locking in the Version Control System (VCS). Both have a number of disadvantages and will increasingly be challenged.

I will start with looking at the VCS-based approach.  This can be seen as doing the coordination ex post, meaning that upon check-in people realize that someone else made conflicting changes or objects are locked in VCS. The result is unnecessary additional work using diff/merge functionality. So we can happily start a diff/merge exercise and try to make our work fit into the other changes that were done. The common view is that this is perfectly acceptable. What is often overlooked though, is that many such statements are/were made in the context of open source projects, where there is no chance to coordinate upfront. So I look at it as a last resort and not as something that should occur on a regular base because two people accidentally worked on the same thing. Rather diff/merge would be needed for porting stuff between branches and apart from that or similar scenarios should in general be avoided.

With the pessimistic locking approach we are (ab)using a VCS on a conceptual level to coordinate work. And technically not all VCS support pessimistic locking. This becomes increasingly important when we speak about distributed VCS like Git or Mercurial. One other issue with pessimistic locking is that it usually does not prevent you from checking out but from checking in again. And if you forget to acquire a lock, you will have spent time on some work only to find out later that you are basically screwed because someone else also worked on it and did not forget to acquire the lock.

So when you look at all these points, the question raises why the VCS locking approach is still favored by most folks compared to the upfront coordination. The latter would be done by letting everyone know that I will now start working on something. My educated guess is that the most important factor for preferring the VCS locking is that it’s integrated in the toolchain and hence automated. Also, I don’t need to think about who needs the information to populate my email’s To: field nor do I have to switch to another program.

So what if we could combine the ex ante aspect of informing people with the  integrated and publish-subscribe nature of the VCS lock? You actually often find this in server-based development environments, where people do not work against a local workspace but a central development instance of the system. These environments usually offer a command to lock certain objects on the server; the lock request is not expressed towards the VCS but the “live” system and I cannot perform any change without having acquired such a lock. I have worked with such a system for many years and while there are certain drawbacks to the shared nature of it, in most circumstances and from a productivity point of view it is just great.

So we need to find a way to incorporate this “live locking” into a setup with many disparate development environments.  In terms of implementation we could probably leverage the Eclipse Communication Framework (ECF) and e.g. an XMPP-based IM server. The workflow would roughly look like this:

  • After installation the user configures a “Coordination Server” (which would be an XMPP server) and his/her account there
  • For each Eclipse project there is a “chat room” or something similar that basically plays the role of a topic (in JMS terms). Whenever someone opens a project of that name, the respective Eclipse instance will be added to that chat room.
  • There is a “lock/unlock” entry in the context menu of all objects (e.g. classes). Whenever someone clicks on of those a respective message is sent to the chat room and picked up by all subscribers.
  • The open question for me is how to persist that message in a stateful manner and all the associated questions around conflict resolution etc. In general I would favour a mostly manual approach here, because it would make the design/implementation a whole lot easier.
  • These machine-generated messages adhere to some naming convention, so that they can be processed/filtered easily. All other messages go through and can be used for human-to-human communication.

These are my initial thoughts and I look forward to your comments, so please feel free to share them.

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