We will use Git and GitHub extensively in CS 5220: for version control (of course) across multiple machines, for collaborative work, for peer review, and for submitting homework and projects. This post serves mostly to document the rationale behind the Git workflow we’ll use in class. Some parts of this process match my standard workflow; see, for example, my repository for CS 4220 in Spring 2015. Other parts of the process – specifically, using pull requests to manage homework – are new to me. I will document my thoughts on what worked well and what didn’t at the end of the semester, but for now let’s start with the plan.

Rationale for Git

I’ve strongly encouraged students in past classes to use Git (or some other version control system), but I have never enforced it. But there are at least three broad reasons why it makes sense to enforce the use of Git in a class like CS 5220:

  1. Most work in the class is done in teams. I know better than to use the anarchy method of software team coordination, but for a student who has never used version control, that has to be a tempting thought. Better to enforce some basic good practices up front.
  2. Work will inevitably involve multiple machines and accounts: at least the accounts of collaborating students on the class cluster, and probably also the laptops or desktop machines of anyone in a project group. Things are additionally complicated by the fact that students might reasonably want to work on code when disconnected from the campus network (though the campus VPN is always an option). Again, it makes a lot more sense to handle these copies systematically (via Git) than to use the copy/FTP method.
  3. Distributed version control systems (DVCS) are industry standard tools at this point. There are DVCS systems other than Git, and in the absence of other context it might make sense to use a system like Mercurial instead. But given the popularity of GitHub as a hosting solution, skills in Git are probably more relevant for most open source projects.

Also, partly because if has become so popular, some of the more annoying issues with Git (e.g. poor Windows support) have largely been addressed by third-party tools.

Rationale for GitHub

There are two popular options for hosting Git repositories:

  • GitHub is the most popular Git hosting service. Creating public repositories is free, and there are also student accounts with five free private repositories. In addition, it is possible to apply for private repositories for a class, which is an option I considered.
  • BitBucket is a direct competitor with GitHub. Like GitHub, BitBucket comes with an impressive array of web-based tools for tracking issues, building wikis, etc. An attractive feature of BitBucket is that it offers unlimited private repositories for academic users; this is one of the selling points for me in my own work!

In addition to BitBucket and GitHub, there are also good options that would not involve cloud hosting. Cornell has an institutional subscription to TeamForge, which now supports Git repositories; and there are also several groups around campus using private GitLab servers. I have accounts on GitHub and BitBucket (actively used), and GitLab.com (mostly dormant).

Using an externally hosted service like GitHub is attractive for reliability reasons (enough people depend on GitHub that issues and service interruptions tend to be addressed promptly). More to the point, though, GitHub is really popular. That means learning to use GitHub will probably be valuable for students in the class, both as a skill in itself and (particularly for the CS undergrad and MEng population) as a way of building a portfolio for prospective employers. The popularity of GitHub also means there are a lot of resources online in case students get turned about. In addition, GitHub has begun to make active efforts to support educational use as well as supporting citable scientific code bases.

Rationale for public repositories

By default, student work for the class will go into public repositories. The advantage of this arrangement is that students have more opportunity to provide each other with inspiration and feedback. The chief pedagogical disadvantage (as much perceived as real, I think) is that a student who sees someone else’s solutions before solving a problem on their own might not learn as much. The chief logistical disadvantage comes down to grading: grades based on solutions to a standardized assignment, particularly when the solutions are graded in a semi-automated fashion, clearly won’t work well if the entire class can collaborate on the solution.

While I will use public repositories by default for this class, I will allow alternatives. This is based on my understanding of two competing concerns: the utility of public feedback, and the privacy concerns that students may have.

Public repositories and feedback

It’s likely that I will continue to use private repositories in other classes in the future, but CS 5220 seems like an ideal class with which to try making all repositories public. From past experience, the students in this class tend to be very motivated and to come from highly diverse backgrounds. They have a lot that they can teach each other, and I expect that providing incentives for public discussion of class work can only help things. Private classwork, while logistically convenient, blocks students from many of the ways in which they might otherwise teach each other. Not only that, but course policies that forbid collaboration do not teach the real skills needed for appropriate re-use of code and ideas. Making all repositories public provides everyone with practice in this skill. It also means that any poor behavior (e.g. bulk copying of someone else’s code base) will be public, which addresses what I think is one of the primary motivations for cheating in CS courses: not only is it easy, but students have the impression that nobody will ever find out.

Just because it is possible for students to view work that others are doing does not mean that everyone will take advantage of the opportunity, even when it is appropriate and encouraged. So I plan to use two mechanisms to encourage this:

  1. All projects will be peer reviewed (with the option for students to address reviewer concerns) prior to final submission.
  2. Students who productively contribute outside their group (e.g. by suggesting a bug fix to another group) should get credit. This policy gets explicit mention on the syllabus when it comes to the main course repository (if I accept a pull request, I’ll give some measure of extra credit), but I will also find ways to make it count when students help each other.

The goal of using public repositories is to provide more rapid feedback (from teaching staff and peers) and to improve student learning. But it’s true that this doesn’t make evaluation any simpler. On the other hand, groups this time will be assigned, with peer evaluation within each group of the contributions made by each of the group members (assigning the group isn’t meant to defend against gaming this evaluation, but it may be a nice side effect).

Privacy considerations

FERPA gives students certain rights involving access to their educational records and restricts schools from releasing personally identifying information about students except under specific circumstances. While the guidelines on some things are clear – it’s not okay to publish grades, for example – the guidelines regarding a social coding environment like GitHub are less clear. Universities take this stuff seriously, and sometimes perhaps overreact – as, for example, in 2011 when Georgia Tech killed their wiki system under the interpretation that it leaked information about which classes students took.

There are many examples online of class policies that explicitly address FERPA in the context of social media: the pages at U Central Florida and Indiana University are particularly useful, and I’ve found other examples of forms regarding FERPA from faculty at Notre Dame and Vanderbilt. GitHub repositories, like public blogs, are never “in our keeping” and so do not become educational records; see also here and here. Consensus seems to be that public assignments are fine under FERPA; and I like the comment that “FERPA is not an excuse for bad pedagogy.” At the same time, it is important to take care with student privacy – both because of FERPA and because it’s just the right thing to do. Mostly, I think that means taking steps to ensure that neither the instructional staff nor the students divulge personally identifying information about other students (what a student chooses to publish about herself is her own business). But a student still might have legitimate privacy concerns.

Given this context, using GitHub for class is encouraged, but not required (and there is no grade penalty for opting out). It’s possible to make a pseudonymous account on GitHub, and this is one option. But nothing about the general abstraction of Git pull requests and patches is specific to GitHub. Indeed, the Linux kernel project famously does not use the GitHub pull request mechanism. Students who prefer not to use GitHub can still use Git, and may submit their assignments through CMS in the form of ordinary Git (vs GitHub) request pulls (either against a repository to which the instructors have read permission or using the -p option). It remains to be seen whether any students actually choose this option, but the mechanism is at least straightforward (and shouldn’t make the grading workflow overly complicated).

Rationale for pull request submission

There are two standard models; the GitHub education guide refers to these respectively as the “fork” and “sandbox” strategies. In the fork strategy, students fork an instructor-created repository containing the basic assignment skeleton, and submit solutions via GitHub pull requests. In the sandbox strategy, the instructor drives the creation and population of student repositories.

As usual, one of the major advantages of using the pull request (PR) model is that it trains students to use pull requests – a valuable skill in many modern development environments. It is also true that GitHub provides excellent online tools for reviewing pull requests (and tools like hub can serve to automate processes where the online tools would get clunky for large numbers of students). A second, independent reason to use pull requests is that CS 5220 often attracts highly engaged auditors. The fork/PR model makes the students (fully enrolled or auditing) responsible for deciding whether they want to submit.

A potential disadvantage to using pull requests for homework submission is that there is no intrinsic way to distinguish genuine pull requests (e.g. to address a bug in project skeleton code) from submission pull requests. For this reason, we recommend pull requests with “HW” at the start of the title as a disambiguation convention. This is much like the “WIP” tag often used for work-in-progress pull requests.