Introduction to Git --- Fall 2020
# Lecture 1: Why use version control?
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<small>Slides: https://hackmd.io/@hpc2n-git-2020/L1-motivation</small>
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## What is version control?
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In software engineering, version control (also known as revision control, source control, or source code management) is a class of systems responsible for __managing changes__ to computer programs, documents, large web sites, or other collections of information.
--- Wikipedia
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### Version control systems (VCS)
... systems responsible for **managing changes** ...
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## Why use version control?
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In an ideal world, things develop linearly:
- Every new version is an improvement upon the previous version. <!-- .element: class="fragment" data-fragment-index="1" -->
- No need to backtrack. <!-- .element: class="fragment" -->
- Everyone known what everyone else is doing <!-- .element: class="fragment" -->
- In the end, things are simply finished. <!-- .element: class="fragment" -->
```graphviz
digraph {
rankdir=LR
Mon[label="Monday's\n improvements"] [fixedsize=circle]
Tue[label="Tuesday's\n improvements"] [fixedsize=circle]
Wed[label="Wednesday's\n improvements"] [fixedsize=circle]
Mon -> Tue
Tue -> Wed
}
```
<!-- .element: class="fragment" data-fragment-index="1" -->
---
In real world, things develop non-linearly:
- A new version can anything between <!-- .element: class="fragment" data-fragment-index="1" -->
- a complete catastrophe and
- a major breakthrough.
- People do not know what others are doing <!-- .element: class="fragment" data-fragment-index="2" -->
- Sometimes we are simply fixing earlier mistakes... <!-- .element: class="fragment" data-fragment-index="3" -->
```graphviz
digraph {
rankdir=LR
Mon[label="Monday's\n improvements"] [fixedsize=circle]
Tue[label="Tuesday's\n mistakes"] [fixedsize=circle]
Wed[label="Wednesday's\n corrections"] [fixedsize=circle]
Mon -> Tue
Tue -> Wed
}
```
<!-- .element: class="fragment" data-fragment-index="3" -->
---
### Going back to an earlier version
Sometimes, it is easier to simply backtrack to an *earlier version*...
```graphviz
digraph {
rankdir=LR
Mon[label="Monday's\n improvements"] [fixedsize=circle]
Tue[label="Tuesday's\n mistakes"] [fixedsize=circle]
Wed[label="Wednesday's\n improvements"] [fixedsize=circle]
Mon -> Tue
Mon -> Wed
}
```
---
#### Where is this *earlier version*?
- CTRL + Z <!-- .element: class="fragment" -->
- my_file.txt, my_file.txt.old, ... <!-- .element: class="fragment" -->
- My project/ <!-- .element: class="fragment" -->
- 2020-08-12/
- 2020-08-13/
- ...
- Daily home directory backup <!-- .element: class="fragment" -->
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#### Challenges and obstacles
- Prone to mistakes <!-- .element: class="fragment" -->
- CTRL + Z has limits, overwritten/deleted files, human/hardware error <!-- .element: class="fragment" -->
- How much to save? <!-- .element: class="fragment" -->
- Individual files? Everything? How much space is required? <!-- .element: class="fragment" -->
- How to organize versions? <!-- .element: class="fragment" -->
- What is the difference between different versions? <!-- .element: class="fragment" -->
*Overall, difficult to manage!* <!-- .element: class="fragment" -->
---
#### What about the granularity?
```graphviz
digraph {
rankdir=LR
subgraph cluster1 {
t1a [label="Component A\n improvement"] [fixedsize=circle]
t1b [label="Component B\n mistake"] [fixedsize=circle]
t1c [label="Component C\n improvement"] [fixedsize=circle]
label="Mondays's changes"
}
subgraph cluster2 {
t2a [label="Component A\n improvement"] [fixedsize=circle]
t2b [label="Component B\n correction"] [fixedsize=circle]
t2c [label="Component C\n mistake"] [fixedsize=circle]
label="Tuesday's changes"
}
subgraph cluster3 {
t3a [label="Component A\n mistake"] [fixedsize=circle]
t3b [label="Component B\n improvement"] [fixedsize=circle]
t3c [label="Component C\n correction"] [fixedsize=circle]
label="Wednesday's changes"
}
t1a -> t2a
t1b -> t2b
t1c -> t2c
t2a -> t3a
t2b -> t3b
t2c -> t3c
}
```
*This compounds the problems!*
---
#### How does VCS solve this?
- Stores the history using snapshots (commits) <!-- .element: class="fragment" -->
- Each snapshot represents the project in a given point of time <!-- .element: class="fragment" -->
- Manages snapshots and associated metadata <!-- .element: class="fragment" -->
- Naming (tags), comments, dates, authors, etc <!-- .element: class="fragment" -->
- Easy to move between different snapshots <!-- .element: class="fragment" -->
- Can handle different degrees of granularity <!-- .element: class="fragment" -->
- Can handle multiple development paths (branches) <!-- .element: class="fragment" -->
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### Comparing and joining
- VCS makes it easy to compare different snapshots <!-- .element: class="fragment" -->
- Named revisions, comments, time information, author information <!-- .element: class="fragment" -->
- Diff tools <!-- .element: class="fragment" -->
- Search tools <!-- .element: class="fragment" -->
- Bisection search <!-- .element: class="fragment" -->
- VCS also allows the joining (merging) of different snapshots <!-- .element: class="fragment" -->
- Easy to experiment with ideas <!-- .element: class="fragment" -->
---
### Collaboration
- One of the primary functions of VCS is to allow collaboration <!-- .element: class="fragment" -->
- Usual setup: server (remote) + multiple clients <!-- .element: class="fragment" -->
- People work locally and send (push) the changes to the server <!-- .element: class="fragment" -->
- VCS keeps track of what has been done and by whom <!-- .element: class="fragment" -->
- Safer since mistakes can be easily remedied <!-- .element: class="fragment" -->
- The contributions of several people can be merged <!-- .element: class="fragment" -->
---
### Backup
- VCS functions as an backup <!-- .element: class="fragment" -->
- Locally, the system maintains a copy of each file <!-- .element: class="fragment" -->
- Usually only the changes or the files that have changed are stored <!-- .element: class="fragment" -->
- Globally, lost files can be recovered from the server <!-- .element: class="fragment" -->
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### Integration
- VCSs such as Git have been integrated with several services <!-- .element: class="fragment" -->
- HackMD, Overleaf, ...
- Services such as GitHub can do almost everything for you <!-- .element: class="fragment" -->
- Store history, distribute, testing / continuous integration, bug reports, milestones, website, ... <!-- .element: class="fragment" -->
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## Practical use cases
What are the practical use cases for VCS?
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### Source code
- Many VCSs are designed for managing source code <!-- .element: class="fragment" -->
- Manage deployment (production, development, testing, etc) <!-- .element: class="fragment" -->
- Manage published versions (v0.1 etc) <!-- .element: class="fragment" -->
- Manage (experimental) features <!-- .element: class="fragment" -->
- Bug hunting <!-- .element: class="fragment" -->
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### Latex files
- Track which version of a manuscript has been <!-- .element: class="fragment" -->
- submitted,
- revised and/or
- accepted
- Collaboration between several authors <!-- .element: class="fragment" -->
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### HPC: batch files and data
- Track different version of your batch scripts <!-- .element: class="fragment" -->
- Easy to check the used configuration afterwords
- Track input and output files <!-- .element: class="fragment" -->
- Limited to smallish files
---
{"title":"Lecture 1: Motivation","tags":"Lecture, Mirko","description":"Why use version control?"}