CSC/ECE 517 Fall 2012/ch1 1w1 rk

Introduction

A version control system (VCS) is a process for managing software codes,files and directory structures and corresponding updates made to them during software development, web development etc. project or during subsequent project maintenances. Version control is also known as revision control, source control or software configuration management (SCM)[6]. Any software development project is a dynamic and fast paced environment. In a typical setting, development for a project is performed simultaneously by many developers. Thus, incorporating all the changes made, either simultaneously or at different times, poses a new type of challenge. This wikibook chapter focuses mainly on the development history of different version control systems.

Why version control systems are needed?

The major issues in a dynamic development environment can be described as:

• Obviously, merging of a lot of files. If we consider somebody to do merging manually, The sheer volume of communication required is overwhelming. This communication can be among the developers themselves in a small team or between the team leader in charge of merging and the rest of the group.
• Accountability: if one of the developers breaks the code flow during development, It is tough to detect and naturally nobody steps forward to take the blame. A version control system can precisely indicate who caused the break in the code.
• If a certain code implementation doesn't work as expected, Reverting to last satisfactory state may be required.
• Development on stale code may result in Code loss. for example, two developers downloaded a certain file at 10AM. One of the developers modified foo_1() method and uploaded modified file at 10:30AM.The other developer modified foo_2() method and uploaded the same file at 10:45AM. Since the functions are in the same file, the second upload will overwrite the file uploaded by first developer at 10:30AM causing loss of his part of work.
• Even a lone coder might need to review why he or she made a certain change in code. Comments during code check-in can be a useful resource in such situation.
• Version control provides a form of documentation which makes tracking easy. i.e.-tagging is a kind of snapshot of all files and documents at a particular stage of development, usually during a stable release. This allows the developers to work with the exact files that were included in that release for bug-fixing purpose.
• Sandboxing / Branching: Version control makes it possible to perform temporary code changes in an isolated area, usually in branch folder, for testing purpose. If the outcome is satisfactory then the code can be merged with the existing code, otherwise it can be discarded without having any impact on the main code.
• Now a days, software development teams are spread across different countries and work at different time zones. Use of a version control system for project aggregation becomes unavoidable.

Evolution of version control systems

VCS' can be broadly categorized into three groups based on the repository model. The sequence below follows the evolution of version control systems as well:

Local repository model

Local version control systems had one storage location of the files. It was called local because it didn't support networked commands or access like the other later softwares. So, all the developers had to use same computer system to access or modify files.

1. Source Code Control System (SCCS) is one of the pioneer source code revision softwares.[5] SCCS was developed at Bell Labs in 1972 by Marc J. Rochkind. Although it was originally developed for OS/MVT, SCCS was included in some UNIX distributions. SCCS ruled as the dominant VCS until Revision Control System was released.
2. Revision Control System(RCS) automated storing, retrieval, logging, identification, and merging frequent revisions of texts. RCS was first released in 1982 by Walter F. Tichy as an alternative to SCCS. It quickly gained popularity and almost replaced SCCS.[9]

RCS performed better by storing the most recent copy of file and then storing only reverse differences called "deltas".One of the shortcomings of RCS was that only one person at a time could edit the file. RCS could manage only single files, not a whole project or directory.

Ref.[3] is a comprehensive book on CSSC and RCS.

Client-server model

The client-server model utilizes a single repository for the version control which is accessible to the developers through network or internet. Concurrent Versions System (CVS) was one of the early client-server model based scheme. Dick Grune started developing CVS from 1984-1985 to allow his students to collaborate in a project according to their schedule. CVS was publicly released at 1986.The main improvement for CVS over RCS was that CVS could manage the whole project while RCS could only work on a file at a time.At the beginning, CVS was script based and called RCS at background. Later it was developed into a full fledged program. A lot of the IDE's (Emacs, Eclipse/aptana,Netbeans, Komodo, PHPEdit etc.) support CVS. CVSNT is a cross platform port of CVS with some modification. OpenCVS puts more emphasis on security.CVS check-in was not an atomic operation.So, if check-in process was disturbed, it could result in damage of existing project at repository. CollabNet started developing Subversion (SVN) as a successor of CVS with added functionality[2][7]. Different open source projects (i.e.-Apache Software Foundation,Ruby,SourceForge,Tigris.org, PHP, Python and MediaWiki) use Subversion. Subversion is an open source software due to it's Apache license[1].