Introduction to Git --- Fall 2020 # Lecture 1: Why use version control? <!-- .slide: data-background="#ffffff" -->    <small>Slides: https://hackmd.io/@hpc2n-git-2020/L1-motivation</small> --- ## What is version control? --- 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 --- ### Version control systems (VCS) ... systems responsible for **managing changes** ... --- ## Why use version control? --- 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" --> --- #### 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" --> --- ### 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" --> --- ### 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" --> --- ## Practical use cases What are the practical use cases for VCS? --- ### 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" --> --- ### 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" --> --- ### 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 ---