# 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 guide students through the process of cloning out git repositories. As an example, an instructor might maintain a collection of notebooks, which they will update and extend throughout 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.

To use it, you need to construct long and complicated URLs, but fortunately there is a handy nbgitpull link generator to help you. The generator will ask for your JupyterHub server (use https://uxxx.syzygy.ca/jupyter/ where uxxx is the name of your university), a github repository, and (optionally) file you would like to open once the repository has been cloned. After you fill all that information in, the generator will produce a long complicated link which might look like…

https://uxxx.syzygy.ca/jupyter/hub/user-redirect/git-pull?repo=https%3A%2F%2Fgithub.com%2Fpimsmath%2Fpublic-notebooks&urlpath=tree%2Fpublic-notebooks%2Fpath%2Fto%2Ffile.ipynb&branch=master


When the student visits the link for the first time the repository will be cloned out into their home directory. When they visit the same link later in the term git will “pull” in any changes from the repository and try to merge them safely with the student’s existing work (see below for more details of how the merge works).

The strange characters in query string are HTML encoded versions of the familiar ascii characters. For instance, http:// becomes http%3A%2F%2F. It is important that these values are properly encoded or they might be ignored by the hub. Some URL shortening services (e.g. bit.ly) will strip the encoding and may break the links, so where possible please distribute the full (encoded) link produced by the generator or use a shortening service which doesn’t strip the encoding (e.g. tinyurl). The parameters in the query string are…

• repo=https:%3A%2F%2Fgithub.com%2Fpimsubc%2Fpublic-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”)
• urlpath=tree%2Fpublic-notebooks%2Fpath%2Fto%2Ffile.ipynb is an optional argument that specifies the path of the directory / notebook inside the repo to launch after cloning. By default, the base directory of the linked Git repository is opened. It’s recommended that you specify a path to a specific file if you want that file to open after users click the link.

NBGitPuller 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/hub/user-redirect/git-pull?repo=https%3A%2F%2Fgithub.com%2Fpimsmath%2Fpublic-notebooks&urlpath=tree%2Fpublic-notebooks%2Fnbpuller-example1.ipynb&branch=master


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

• https://pims.syzygy.ca/jupyter/hub/user-redirect/git-pull refers to the syzygy instance (pims.syzygy.ca).
• https%3A%2F%2Fgithub.com%2Fpimsmath%2Fpublic-notebooks tells nbpuller which repository to use (https://github.com/pimsmath/public-notebooks).
• branch=master specifies the master branch within the repository.
• urlpath=tree%2Fpublic-notebooks%2Fnbpuller-example1.ipynb tells nbpuller to open that file (nbpuller-example1.ipynb`) automatically after cloning.

### 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.”