Mercurial and Teamwork

As this course has a team project, this lecture is about how to work as a team and make effective use of the version control system, Mercurial, that we have been using throughout the semester. While the focus is on the use of Mercurial, the principles we are introducing here can be adapted to other situations (and alternate version control systems).

Planning and Communication

Two of the most important aspects when it comes to teamwork are planning and communication. In the real world, planning is often referred to as project management. And communication often takes the form of regular team meetings.

In later courses (e.g. software engineering) there is greater emphasis placed on thinking more broadly about software process. We’re not going to cover SE in depth here but want you to be aware that software process is an important topic. Planning and communication are always supporting ingredients of a good software process.

At the level of actual programming, when two folks are working on the same project, it is important to think about how you can organize your work so each person on the team can get something done. As we’ve been learning throughout the semester, the C# language gives us a way to organize our code using projects (with Xamarin Studio). Within a project we can organize it as a collection of files, each of which maintains part of the solution to a problem. These parts are typically organized using classes within a namespace and methods.

So the key to working together–and apart–is to spend some time, initially, planning out the essential organization of a project and the files within it. Much like writing a term paper, you can create classes and methods that are needed–without writing the actual body of the methods–and then commit your code to the repository. Then each member of the team can work on parts of the code and test them independently. Then you can sit together again as a pair to integrate the work you’ve done independently.

It’s easier said than done, but this is intended as a suggestion for how to collaborate.

In any event, the above suggests that you actually need to communicate if you want to get anything done. You should start by discussing what needs to be done and work in real time to do what has been described above. Then you start coding. As you are coding, you are going to realize that as well as you planned the work to be done, you “forgot” or “misunderstood” some aspect. When this happens, you and your partner(s) need to communicate.

In the modern era of software development, we are richly blessed with synchronous communication methods such as instant messaging, texting, group chat, and other forms of synchronous collaboration. When you and your partner(s) are working on the project, we highly encourage you to keep a chat window open (Google Talk, AIM, Yahoo, IRC, whatever) and use it to communicate any issues as they arise.

Typical Scenario

In general, when working with Mercurial, you will find yourself using the following commands in roughly this order:

  • hg incoming: look for incoming changes that were either made by yourself or your partner(s). We say “by yourself” because it is clearly possible that you are working on another computer somewhere else (laptop, home computer) and pushed some changes. So it is a good habit to see whether “anything has changed” since the last time you looked, even if you looked recently.

  • hg pull: If there are incoming changes showing up as incoming, then you should in fact pull them in. This will stage the changes locally but they will not be incorporated until you type hg update.

  • hg update: Absorbs all changes that were pulled from your remote repository. This operation has the potential to overwrite changes you are currently making, so you should make sure that the incoming changes that you observed above are sensible. For example, if you are editing a file named examples/MyProject/my_project.cs and the incoming log suggests that the same file has been modified, you’re likely to end up with a conflict when performing the update. (We’ll cover conflict resolution shortly.)

  • hg add: Whenever you add files to your folder, if you want them to be in the repository, you always need to add them. It is very easy to forget to do so. The hg status command can be used to figure out files that might need to be added.

  • hg status: This command will become one of your best friends. You This typically tells you what has happened since you started your work in this directory (and since the last commit/push cycle). You especially want to keep on the lookout for the following:

    • ?: This means that a file is untracked in the repository. If you see a file with the ? status that is important, you’ll want to add it using hg add as explained above.

    • M: This means that a file has been modified.

    • A: This means that a file has been added.

  • hg commit: In general, you should commit all changes to your repository, especially before you leave the lab for the day. It is important to note that committing is a local operation and does not affect the remote repository (at until a corresponding push has been done.

  • hg push: Almost immediately after a commit, it makes sense to do a push, especially if you are in the lab and will be continuing your work on other computers. In addition, as you’ve observed with previous homework, it is the only way to ensure that you can view the code on BitBucket (our hosting site for Mercurial projects).

Conflict Avoidance

Inevitably, when you are working on project, you are likely to end up in a situation where you and your partner(s) make changes that conflict with one another. In many cases, the version control software can automatically merge the changes. Here are just a few examples of where it is possible to do so:

  • You make changes to different files.

  • You make additions to the repository.

  • You make changes to a common file that do not overlap. An example of this might be where you have two functions in your program and both you and your partner are careful not to modify them.

In general, we encourage you to coordinate your efforts, especially when you are doing something like the third situation.

Where you get into trouble is when there are changes to a common file that conflict with one another. When this happens, you have two choices in practice:

  • use hg merge and hg resolve to merge your changes.

  • make a copy of the conflicting files (e.g. Copy hello.cs to hello.cs-backup and use hg update --clean (changing your copy to match the current version in the remote repository) to just accept the latest versions of all files from the repository.

In our experience, the first option is tricky. You are given the option to perform the merge anyway or use a merge tool to select the changes of interest (and decide between them).

The second option basically results in two copies of the file. You can open up your editor to compare the files side-by-side or use a tool like diff on Unix or a Mac, which gives a side-by-side comparison:

diff -y hello.cs my_hello.cs

(You will need to expand the width of your console window to see clearly!)

This tool is not built into Windows. In the Windows lab, you can use the much less visually helpful

fc hello.cs my_hello.cs

to show differing segments from each file. You can also download difference display tools for Windows that are more visually helpful. One of many choices is at

E-mail Notifications

One of the best ways to avoid conflicts when working on a team is to enable e-mail notification on your repository.

Bitbucket, the hosting service we are using and recommending for our students, provides full support for e-mail notification. Whenever you or your partner(s) push changes to the hosted repository, an e-mail will be generated.

These are the steps to set it up. (Owing to the changing nature of web interfaces, we are providing generic instructions that should be adaptable if the Bitbucket service decides to change its web user interface.)

  1. Make sure your repository is selected. This is always the especially when you visit your repository by URL.

  2. Select the administrative (Admin) tab.

  3. Select Services (left-hand-side navigation).

  4. Add the Email or Email Diff service. These services are basically equivalent, but one will generate links so you can view the differences that were just pushed. We recommend Email Diff.

  5. Add the email notification address. You can only have one address. A good way to overcome this limitation is to set up a group service, say, at Google Groups.

Communication is Key to Success

At the risk of repeating ourselves, we close by reminding you of the central importance of good communication. The authors of this book communicate when it comes to their changes–even before we make them. Yet we occasionally trip over each other, and there is usually a fair amount of manual reconciliation required to deal with conflicts when we end up touching the same file by mistake.

When you absolutely and positively need to change a common file, it is important to ask yourself the important question: Shouldn’t we be sitting together to make these changes? It’s a rhetorical question, but working closely together, either in the same room or through a chat session/phone call, can result in significantly fewer headaches, especially during the early stages of a project.

So please take this time to stop what you are doing and communicate. You’ll know your communication is good if you never need to do anything that has been described on this page. Then again, we’re human. So you it is likely to happen at least once. (We know from experience but are doing everything possible to avoid conflicts in our work!)