SCM

• Software Configuration Management
• Tracking and controlling changes to files used in software development
• Based on revision control (version control)
• Used for managing builds and releases
• Used for accounting and auditing of process and product

We use version control for …

• Coordinating source code for a particular release
• Collecting metrics on software productivity
• Studying the process of development
• Informing non-developers of the current state of the source code
• Bringing new and absent developers up to date

diff and patch

• Distribute changes efficiently
• Simplistic form of handling versions
• diff utility creates a patch file
• The patch utility applies the patch file to the starting code to create the updated file

Ex: Create a Patch

Ex: Apply a Patch

Revision Control (Version Control)

• After an editor and a compiler, the most crucial tool for software development
• Essential to the coordination of changes among collaborating developers
• Essential to a solitary developer working on anything non-trivial
• Maintains a history of changes
• Management of branches and product families
• Defines workflow
• All parts of development revolve around version control

Common Features

• Versioning down to file level
• Text Files: Only understands the lexical level (i.e., a source-code file is a file of characters)
  • No understanding of the syntactic structure of code
• Binary Files: Stays at the file level

Management Models

• File Locking
• Original
• Primarily not used for source code anymore
• Version Merging
• More recent
• What developers use for source code

Management Model: File Locking

• Only one developer at a time has access to a file/resource
• Lock-Modify-Unlock
• One developer at a time has the "token"; other developers have to wait
• Library model
• Advantage: No merging problems
• Disadvantage: Prevents other developers from working
• Disadvantage: Impractical in distributed development due to time/space differences

Management Model: Version Merging

• No restrictions on access
• Developers can work simultaneously
• Copy-Modify-Merge
• Advantage: No restrictions on working
• Disadvantage: Merge issues

Current Practice

• A large majority of the usage of version control is Version Merging
• File Locking is typically only used for binary files (e.g., MS Word files)
• May find old projects (and developers) that use File Locking

Centralized Version Control

• e.g., Subversion (SVN), ClearCase, Vault
• A single central repository, local working copies
• Access controlled by the server
• One sequence of version numbers
• Traditional approach

SVN View

• (remote) repository:
• The single, central SVN repository typically running on a remote machine
• working copy:
• Sometimes referred to as a local repository, but it is not
• The code you checked out into your filesystem
• Where you modify your files

SVN

• Versions identified by monotonically increasing numbers
• URLs identify both the location of a central repository and directories/files in the central repository
• Each commit has an author
• Support for per-directory permissions, with some limitations

SVN Workflow

• Update the working copy
• Perform changes in a local repository ("working copy")
• Commit changes

Common SVN Issues

• Need access to a server to create a shared repository
• No distinction between private and public changes
• Merging is difficult
• Branching creates problems

Distributed Version Control

• e.g., Git, Bazaar, Darcs, Mercurial, Monotone, SVK
• Peer-to-peer, no central repository; all are repository copies
• No one sequence of version "numbers" (Why?)
• Access controlled by the server

Git

• Distributed revision control and SCM (Source Code Management) system
• Created in 2005 by Linus Torvalds for Linux kernel development
• Used by major companies, e.g., Microsoft, Apple
• Built-in to many IDEs
• Fluency in Git is a requirement for anybody in software engineering

Git View

• repository
• Stored in the (hidden) directory .git

• What you clone from another repository

• working copy
• The code you checkout into your filesystem
• The files that you see
• Where you modify the file

Git Characteristics

• Each commit has a hash, currently a SHA1 id (160-bit numbers in hexadecimal), e.g.,
  • dd2cfbe16889f213f33d61b594a2088e97c99856
• URL only identifies the location of the repository. Always have branches and tags, where the default branch is the "main" (previously "master").
• Each commit has an author and a committer

Git Characteristics

• peer-to-peer
• Each copy is a full-fledged repository and can be worked on locally without access to a central server
• Each user clones the repository, makes changes, and pushes the changes

Git Benefits

• Records complete new version
• Handles local and remote repositories
• Tracks merged data
• Staging changes

Git Comparison

• Advantages: fast, flexible, powerful, multiuser
• Disadvantages: complex, challenging to learn, GUI tools less developed than SVN tools
• Despite disadvantages, Git is a standard tool for software engineering and software development in general
• Also used as a data format for applications

GitHub

• One issue with Git is that to collaborate with others, your repository must be public
• GitHub is a hosting service for software development projects using Git
• Web-based hosting site for Git repositories
• > 372 million repositories with ~28 million public repos, > 100 million users
• Founded in 2008, Microsoft purchased in 2018
• An account at GitHub is necessary for software development and software engineering (not just this class)

Git Influence

• Microsoft:
• Visual Studio Online 2015
• Visual Studio 2019: New Git Experience
• Azure Repos
• Apple:
• Current: XCode 13 and macOS Big Sur 11
• XCode - Git/SVN support through XCode 8
• XCode 9: Dropped SVN support
• XCode 11: SVN deprecated
• macOS Catalina 10.15: No longer installed on the command line
• This is not a criticism of using SVN for CS 1 and CSII

Git Tools

• Command-line git.
• GUI Tools
• Note: Apple XCode

Recommendation: command-line Git

• Command-line git is the proper Git; everything else is an approximation
• Most answers to Git questions show the command line answer
• As you use the command line, you start to remember commands (not the case in GUIs)
• Often, the GUI "easy" solutions are not the only way to fix a problem and are often not the best
• GUIs overlay proper git and try to make it simpler, but they fail
• Can script/automate command-line solutions

Platform

• Linux - Command-line git
• macOS - Command-line git
• Windows - Git Bash in Git for Windows

SVN/Git Command Comparison