5 Version Control & Collaboration

BBS Course: Good Software Engineering Practice for R Packages

Kevin

February 10, 2023

Disclaimer

Any opinions expressed in this presentation and on the following slides are solely those of the presenter and not necessarily those of their respective employer or company.

Overview, demo, practical
Can only scratch surface
More resources on website

Trade-offs in code development

Working alone

no coordination overhead
no review
lack of diversity
can slack on documentation
fragile long-term maintenance

Working in a team

coordination overhead
mutual review of code
different approaches
forced to document
more robust long-term maintenance

Key issue:
Manage complexity over time or between people

Version control systems (VCS)

Manage different versions of a piece of work
Compare and merge diverged versions effectively¹

flowchart LR
  A[<font size=4> Kevin v1] --> B[<font size=4> Kevin v2]
  B --> C[<font size=4> Kevin v3]
  B --> D[<font size=4> Daniel v1]
  D --> E[<font size=4> Kevin + Daniel v4]
  C --> E

Code is complex system \(\leadsto\) ideal application of VCS
Compounded by multiple people ‘fiddling’ with it!

git basics

Enter git the ‘Latin of data science’

Author Linus Torvalds, for work on Linux kernel
Essentially a database with snapshots of a monitored ‘repository’ (directory)
Optimized to compute line-based changes
Integrated in RStudio IDE, Visual Studio Code
De facto standard not just in the R world
Alternatives: mercurial, SCM, …

Stage & commit

gitGraph
   commit
   commit
   commit
   commit
   commit

‘Stage’ changes for inspection
- allows to inspect propose changes before locking them in
Permanently ‘commit’ changes to git

\(\leadsto\) Chain of versions with incremental changes

Line-based differences - the ‘diff’

Changes in git are line-based
Additions (green) & deletions (red) between commits

Going back in time

Every commit has unique hash value
Can ‘checkout’ old commit (browse history)

git checkout [commit hash to browse]

Can ‘reset’ changes

git reset --hard [commit hash to reset to]

Removes need for my-file_final_v2_2019.R
Time travelling has its dangers…¹

Branching

gitGraph
   commit
   commit
   branch feature
   checkout feature
   commit
   commit
   checkout main
   commit

Variations of repository: ‘branches’

git checkout -b [my new branch]

Quick switching between branches

git checkout [branch name]

‘Merging’ two branches

gitGraph
   commit
   commit
   branch feature
   checkout feature
   commit
   commit
   checkout main
   commit
   merge feature

Consolidate diverged ‘branches’
Usually merged automergically
Conflicting changes
Line edited in source/target branch - keep which?
Resolving merge conflicts beyond today’s scope

Example of ‘gitflow’

gitGraph
   commit tag: "v0.0.1"
   commit
   branch feature-1
   checkout feature-1
   commit
   commit
   checkout main
   branch feature-2
   checkout feature-2
   commit
   checkout feature-1
   commit
   checkout main
   commit tag: "bugfix"
   merge feature-1 tag: "v0.1.0"
   checkout feature-2
   commit

‘gitflow’: specific workflow for git repositories
features developed on branches, then merged into ‘main’

Version Control & Collaboration

git itself is command line tool for version control
git platforms add UI for collaboration¹
- GitHub.com de facto standard in R (alternative: GitLab.com)
git + GitHub
- VCS (git)
- Web hosting of code (GitHub)
- Organisation with issues, discussions (GitHub)
- Automation of checks/test (GitHub)

git platforms

GitHub.com

Huge number of R packages developed there:
100 million developers on GitHub.com (Jan ’23)
372 million repositories, 28 million public (Jan ’23)
‘Facebook’ of developers / social coding
Discuss problems / propose changes

Branches & pull requests

Branches are a git concept
Git platforms add concept of ‘pull request’ (PR)¹
- PR is ‘suggested merge’ from branch A to B
- Usually from ‘feature A’ to ‘main’
Allow to preview problems before merge and discuss changes
Once everyone is happy, a pull request² can be merged
Every PR has an associated branch, but not every branch has a PR
More in the demo!

Automating things with GitHub

GitHub provides
- cloud compute
- scripting framework (GitHub actions)
Allows task automation, e.g.
- run unittests
- build & host documentation
- static code analysis (linting)
Most important actions for R: github.com/r-lib/actions
Extremely useful to enforce best-practices & quality

A typical GitHub workflow

sequenceDiagram
    participant K as Kevin
    participant GH as GitHub server
    participant D as Daniel
    K->>K: make change locally & commit to <feature>
    K->>GH: push commit
    K->>GH: open pull request
    GH->>GH: run automated checks
    K->>D: request review
    D->>D: review code
    D->>K: request changes
    K->>K: implement changes locally & commit
    K->>GH: push commit
    GH->>GH: run automated checks
    K->>D: request review
    D->>D: review code
    D->>GH: approve changes, unblocking merge
    K->>GH: merge <feature> into <main>
    GH->>GH: run automated checks on <main>
    D->>GH: pull newest version of <main>

Looks awefully complicated, why?

Efficient collaboration with novice/untrusted contributors
- Maintainer: automated checks reduce review burden
- Contributor: no need to check manually
Branching promotes asynchronous work on features
Full history - can always go back

\(\leadsto\) making code-collaboration scalable

Demo

How does a pull request work?
First steps for setting up the next practical
Don’t panic - steps are all documented at
https://github.com/kkmann/simulatr

Practical - collaboration on GitHub

Work in teams of ~3
Go to https://github.com/kkmann/simulatr and read through the instructions in the README.md file
The repository is a template to practice collaboration on GitHub
Only one member per team needs to use the template and invite the others as collaborators!
Take some time to checkout the README.md file and set up your environment in posit cloud
Can you fix the errors with some pull requests?
The purpose of this exercise is to explore the collaboration functionality of GitHub - not to produce a perfect package ;)

License information

Creator (initial author): Kevin Kunzmann
This work is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License.
The source files are hosted at github.com/kkmann/workshop-r-swe
Important: To use this work you must provide the name of the creator (initial author), a link to the material, a link to the license, and indicate if changes were made