# Teaching with Syzygy

JupyterHub can be a great resource for teaching and we we encourage people to use syzygy to help teach their classes, but you should keep the following things in mind:

• syzygy.ca hosts are shared services. Asking a large group of students to do the same task simultaneously may exhaust the available resources and make your experience very slow. When machines are under extreme resource pressure, they may decide to shutdown processes that they don’t have any hope of servicing. Try to plan ahead by splitting up and spreading out tasks.

• We have a very small support team. We try hard to make sure our services are always available, but we still have planned and unplanned outages. Normally we will try to resume the service as soon as possible, but this depends on lots of factors, and we can’t always do it the same day.

For these reasons we recommend against using syzygy for e.g. timed exams and we urge you to be careful when planning submission dates for homework etc.

## Tools

### NBGitPuller

NBGitPuller is a very useful server extension which uses specially constructed URLs to allow students to be guided through the process of cloning out git repositories. As an example, an instructor might maintain a collection of notebooks for their class, which they will update and extend through the term. By giving a specially constructed URL to their students, each student will automatically be given their own copy of the files at the start of term and can keep it in sync (without losing their changes) as the term progresses.

We are in the process of rolling out this extension and making it available by default on all of our hubs so it may already be installed and activated on your syzygy.ca instance, or it will be soon. To use it, you need to construct URLs which specify the syzygy server you want to use and the repository you want to interact with. The URL will look something like

https://uxxx.syzygy.ca/jupyter/hub/user-redirect/git-pull?repo=https://github.com/pimsmath/public-notebooks&branch=master


Where,

• repo=https://github.com/pimsubc/public-notebooks is required and specifies the URL of the git repository you want to clone.
• branch=master is an optional git branch name (default “master”)
• subPath=path/to/file.ipynb is an optional path within the repository to restrict the clone to a specific file or directory of interest.

The functionality is implemented as a notebook extension so doesn’t depend on any specific kernel. It will happily clone out any repository, so you can use it with R, Python or any other kernel installed on your syzygy instance.

### nbgitpuller service demonstration

The following link demonstrates the nbgitpuller service on pims.syzygy.ca by cloning out a simple python3 example notebook. To try the service out, simply click on the following link:

Clicking on the link should trigger the following actions

1. Take you to pims.syzygy.ca
4. Clone out a copy of the notebook to public-notebooks/nbpuller-example1.ipynb
5. Open the notebook in your browser.

The full URL used in the link is

https://pims.syzygy.ca/jupyter/user-redirect/git-pull?repo=https://github.com/pimsmath/public-notebooks&branch=master&subPath=nbpuller-example1.ipynb


Splitting this apart, we can see the pattern described above:

• https://pims.syzygy.ca/jupyter/user-redirect/git-pull refers to the syzygy instance (pims.syzygy.ca).
• repo=https://github.com/pimsmath/repo=public-notebook tells nbpuller which repository to use.
• branch=master specifies the master branch within the repository.
• subPath=nbpuller-example1.ipynb tells nbpuller to clone just that one file.

### How NBGitPuller Works

As its name implies, NBGitPuller works by constructing and executing git commands. Git is a distributed version control system which lets you keep track of changes and merge them with other people’s. The initial click on an NBGitPuller link will take the user to the relevant JupyterHub, log them in (if necessary) then try to grab a clone of the repository. If you’re not familiar with git, this is essentially a copy of the remote files in the repository with the advantage that both the user and the repository owner can continue to modify their own copies of the files then ultimately merge them together (inside the user’s clone). To do all of this NBGitPuller implements a subset of the git commands so that subsequent clicks on a link by the user will try to merge changes according to this plan:

• If content has changed in both places, prefer local (user) changes over remote changes.
• If a file was deleted locally but present in the remote, remote file is restored to local repository. This allows users to get a ‘fresh copy’ of a file by just deleting the file locally & clicking the link again.
• If a file exists locally but is untracked by git (maybe someone uploaded it manually), then rename the file, and pull in remote copy.

Since git is doing all of the heavy lifting here it is also possible to use git directly to do more advanced tasks (e.g. different merging strategies) with the caveat that mixing ordinary git commands and NBGitPuller is not a good idea. Once you start using git “manually” for that repository you should avoid using the NBGitPuller links as the merging patterns will almost certainly be different. As the NBGitPuller documentation says “[this] is going to cause surprises on an ongoing basis, and should be avoided.”