Concepts around collaboration

Objectives

Be able to decide whether to divide work at the branch level or at the repository level.

Instructor note

15 min teaching

Motivation

Someone has given you access to a repository online and you want to contribute?
We will learn how to make a copy and send changes back.
Then, we make a “pull request” that allows a review.
Once we know how code review works, we will be able to propose changes to repositories of others and review changes submitted by external contributors.

Commits, branches, repositories, forks, clones

repository: The project, contains all data and history (commits, branches, tags).
commit: Snapshot of the project, gets a unique identifier (e.g. c7f0e8bfc718be04525847fc7ac237f470add76e).
branch: Independent development line. The main development line is often called main.
tag: A pointer to one commit, to be able to refer to it later. Like a commemorative plaque that you attach to a particular commit (e.g. phd-printed or paper-submitted).
cloning: Copying the whole repository to your laptop - the first time. It is not necessary to download each file one by one.
forking: Taking a copy of a repository (which is typically not yours) - your copy (fork) stays on GitHub/GitLab and you can make changes to your copy.

Cloning a repository

In order to make a complete copy a whole repository, the git clone command can be used. When cloning, all the files, of all or selected branches, of a repository are copied in one operation. Cloning of a repository is of relevance in a few different situations:

Working on your own, cloning is the operation that you can use to create multiple instances of a repository on, for instance, a personal computer, a server, and a supercomputer.
The parent repository could be a repository that you or your colleague own. A common use case for cloning is when working together within a smaller team where everyone has read and write access to the same git repository.
Alternatively, cloning can be made from a public repository of a code that you would like to use. Perhaps you have no intention to work on the code, but would like to stay in tune with the latest developments, also in-between releases of new versions of the code.

Forking a repository

When a fork is made on GitHub/GitLab a complete copy, of all or selected branches, of the repository is made. The copy will reside under a different account on GitHub/GitLab. Forking of a repository is of high relevance when working with a git repository to which you do not have write access.

In the fork repository commits can be made to the base branch (main or master), and to other branches.
The commits that are made within the branches of the fork repository can be contributed back to the parent repository by means of pull or merge requests.

Generating from templates and importing

There are two more ways to create “copies” of repositories into your user space:

A repository can be marked as template and new repositories can be generated from it, like using a cookie-cutter. The newly created repository will start with a new history, only one commit, and not inherit the history of the template.

You can import a repository from another hosting service or web address. This will preserve the history of the imported project.

Discussion

Visit one of the repositories/projects that you have used recently and try to find out how many forks exist and where they are.
In which situations could it be useful to start from a “template” repository by generating?

Synchronizing changes between repositories

We need a mechanism to communicate changes between the repositories.
We will pull or fetch updates from remote repositories (we will soon discuss the difference between pull and fetch).
We will push updates to remote repositories.
We will learn how to suggest changes within repositories on GitHub and across repositories (pull request).
Repositories that are forked or cloned do not automatically synchronize themselves: We will learn how to update forks (by pulling from the “central” repository).
A main difference between cloning a repository and forking a repository is that the former is a general operation for generating copies of a repository to different computers, whereas forking is a particular operation implemented on GitHub/GitLab.