Version Control

Overview

Most software has a typical lifecycle: requirements analysis, design, code, test, release, repeat. There are two issues with this process. First, developers need to keep writing new features while quality-assurance teams take time to test supposedly-stable versions of the software. New work cannot halt while the software is tested. Second, the team almost always needs to support older, released versions of software; if a bug is discovered in the latest code, it most likely exists in released versions as well, and customers will want to get that bugfix without having to wait for a major new release.

Here's where version control can help. The core mission of a version control system is to enable collaborative editing and sharing of data (i.e. code files). Version control systems manage files and directories, and the changes made to them, over time. This allows you (and perhaps others in your team) to recover older versions of your files, or examine the history of how your files changed. In this regard, many people think of a version control system as a sort of “time machine”.

Subversion is one popular free/open-source version control system that is used widely in industry. Subversion can operate standalone on a single computer, or across networks, which allows people on different computers to use it to manage common files. At some level, the ability for various people to modify and manage the same files from their respective locations fosters collaboration. Progress can occur more quickly without a single conduit through which all modifications must occur. And because the work is versioned, you need not fear that quality is the trade-off for losing that conduit—if some incorrect change is made to the files, just undo that change.

Subversion is a centralized system for sharing information. At its core is a repository, which is a central store of files, usually maintained on a server computer. The repository stores information in the form of a filesystem tree —a typical hierarchy of files and directories. For SE2030, the Subversion repository resides on emerald.msoe.edu (which in case you are wondering, is an HP Proliant DL360 server running Debian Linux managed by CCSD). Using a Subversion client application on your PC, you can access the repository from your laptop. Any number of clients connect (perhaps simultaneously) to the repository, and then read or write to these files. There are various Subversion client applications available. The one you'll use in SE2030 is called TortoiseSVN, which you can download and install by following the link.

By writing data, a client makes the information available to others; by reading data, the client receives information from others. The figure below illustrates this.

So why is this interesting? So far, this sounds like the definition of a typical file server. And indeed, the repository is a kind of file server, but it's not your usual breed. What makes the Subversion repository special is that it remembers every change ever written to it: every change to every file, and even changes to the directory tree itself, such as the addition, deletion, and rearrangement of files and directories.

When a client reads files from the repository, it normally sees only the latest version of the filesystem tree. But the client also has the ability to view previous states of the filesystem. For example, a client can ask historical questions like, “What did this directory contain last Wednesday?” or “Who was the last person to change this file, and what changes did he make?” These are the sorts of questions that are at the heart of any version control system: systems that are designed to record and track changes to files over time.

Wait! You're probably saying to yourself: "Hey, this system seems to allow more than one person to work on the same files at the same time. What happens if two people are changing the same file? Won't one person's file save overwrite the other persons' changes?" That's a critical point, and to contend with this issue, version control systems implement something called Versioning Models. There are various models, but Subversion uses something called the Copy-Modify-Merge model. You need to understand the Subversion Versioning Model in order to use it effectively.

Links to sub-topics

An explanation of Versioning Models

Installing TortoiseSVN - the version control client software

Using TortoiseSVN with Subversion - the version control server software