CSC/ECE 517 Fall 2011/ch1 1f rs

Introduction

Version control or Revision Control is indispensable in large projects. They make sure that projects are not spinning out of control by letting different programmers work on the project from a different perspective without getting in each other’s way and without doing any damage that cannot be undone.

The article compares all the Version Control Systems like RCS, CVS and Distributed Version Control Systems from a developer's point of view.It further extends the comparison to popular version control applications like RCS, SVN, CVS, Clearcase, Mercurial and Git.

Comparison of Version Control Systems

Local Version Control

This is a primitive approach of version control. The system operates on single files only.<ref>http://en.wikipedia.org/wiki/Revision_Control_System</ref> It has no way of working with an entire project. Changes to a file are stored as 'deltas' with the aid of a diff utility. Since the approach maintain many copies of files, it is generally cumbersome with reduced efficiency.

Centralized Version Control

CVS implements a Client-Server model to achieve version control. A central repository is maintained which is used by all the developers of a project. Programmers can only have a working copy of the project. The model allows a developer to checkout only the required files or branches. In this model all operations on the code (commit, exchange of code etc.) involve communication with the central server via a network. This means that these operations are not available when the user is offline. Also issues in communication with the server may disrupt the development activity. Any changes to the project on the server involves an authentication process. Hence only users with certain privileges can commit code on the central server . A developer can checkpoint or create branches on his work only by committing his changes to the central repository. Since all the developers commit changes on the same repository, it is generally challenging to resolve conflicts when merging or reconciling changes from other branches.

Distributed Version Control

This system implements a peer to peer model to achieve version control. No reference copy of the codebase exist by default. However in practice, an official reference copy is indeed maintained. The model requires the developers to possess a full blown backup of the repository , including the entire history. As a result, the developers can collaborate directly without the need of a central authority. Hence it allows developers to be productive even when offline. (e.g. when travelling). Also the overhead of communicating with a central repository is overcome in this approach. Branching and merging fit much better in this approach as each user has his own branch. The administrator's major work is to merge appropriate changes from branches of different users. This is relatively simpler compared to the centralized system. As a result, the distributed system scales much better with the number of people.

Summary of Centralized versus Distributed Approach

Centralized Version Control	Distributed Version Control
Client - server model	peer- to -peer model
Single central repository.	Multiple repositories
Changes can be committed only Online	Changes can be committed offline
Allows developer to checkout required files/branches.	Complete repository has to be downloaded by the programmer
Merging and Branching operations are complicated as multiple programmers work on the same repository at the same time.	Branching is simple as each programmer gets a branch
Repository maintenance becomes more challenging with number of people	Scales better with number of people

Comparison of Version Control Applications

Overview of Version Control Applications

RCS: The Revision Control System (RCS)

RCS manages multiple revisions of files. RCS operates on a set of files. RCS automates the process of storing, retrieval, logging, identification, and merging of multiple revisions. It is useful for text that is modified frequently which includes source code, programs, documentation, graphics, papers, and letters. This is the most basic and most primitive Version Control System known to programmers.

More information about RCS can be found at RCS

ClearCase:

ClearCase is a comprehensive software configuration management system. It is used to manage multiple versions of evolving software systems, tracks which versions were used in software builds, used to perform builds of individual programs or complete releases according to user-defined version and system specifications.

These capabilities enable ClearCase to address the issues and critical requirements of organizations that develop software:

Effective development — ClearCase enables users to work efficiently, allowing them to fine-tune the balance between sharing each other's work and isolating themselves from destabilizing changes done. ClearCase automatically manages the sharing of both source files and the files produced by software builds.

Effective management — ClearCase tracks the software build process, so that users can determine what was built, and how it was built. Further, ClearCase can instantly recreate the source base from which a software system was built, allowing it to be rebuilt, debugged, and updated — all without interfering with other programming work.

Enforcement of development policies — ClearCase enables project administrators to define development policies and procedures, and to automate their enforcement.

More information about ClearCase can be found at ClearCase

CVS: The Concurrent Versions System (CVS)

CVS is a Client-Server software revision control system in the field of software development. Version control software keeps track of all the changes in a set of files, and makes sure that several developers (potentially widely separated in space and/or time) can collaborate in real time. The CVS server runs on Unix-like systems with client software that runs on different operating systems on multiple client machines. It is regarded as the most mature version control system because it has been developed for such a long time and has stood the test of time. A fork project of CVS, CVSNT was created to run CVS on Windows servers, and it is currently being actively developed to increase functionality and make it more usable on Windows platform.

More information about CVS can be found at CVS

SVN: Sub Version (SVN)

SVN was created as an alternative to CVS that would be useful to fix some known issues in CVS and also maintaining high compatibility with it. SVN is free and open source with the difference of being distributed under the Apache license as opposed to GNU which distributes licenses for CVS. To prevent the database from being corrupted, SVN adopts a technique called 'Atomic Operations'. Either all of the modifications made to the source are committed or none are committed. Partial changes will not enter the original source. Many developers have switched to SVN as it is a newer technology that starts with the best features of CVS and improves upon them. While branch operations in CVS are expensive and do not really lend themselves to long-term forks in the project, SVN is designed to allow for it. Hence it lends itself better to large forked projects with many directions. Some known disadvantages of SVN includes slower operation speeds and the lack of distributed revision control. Distributed revision control uses a peer-to-peer model as compared to using a centralized server to store code modifications. Peer-to-peer model work better for open source projects which are spread over multiple far away physical locations. The downside to a dedicated server approach is the issue of not having redundancy in the case when the server is down leading to no access to clients.

More information about SVN can be found at SVN

Git:

Git takes a totally diverse approach that differs greatly in comparison to CVS and SVN. The original concepts for Git were to make a faster, distributed version control system that would openly invalidate conventions and practices used in CVS. It is primarily developed for Linux and has the highest speeds on there. It will also run on other Unix-like systems, and Windows agents are known as msysgit. As there is no centralized server, Git is not very suitable for single developer projects or small teams as the code may not necessarily be available when using a normal computer. Workarounds exist for this problem. Developers look out for Git’s improved speed as a decent trade off for the hassle.

More information about Git can be found at Git

Mercurial:

Mercurial is a distributed revision control tool which began close to the same time as Git. Mercurial was originally made to compete with Git for Linux kernel development. Mercurial is primarily implemented in Python as opposed to C, but there are some instances where C is used. This is a major difference compared to other version control systems in Developer's point of view. Users feel that Mercurial is similar to SVN with respect to certain features, as well as being a distributed system. This acts as an advantage for the tool as the learning curve for those already familiar with SVN will be less steep. The documentation for Mercurial is very well compiled and facilitates understanding the differences faster. One of the major drawback to Mercurial is that it does not allow for two parents to be merged. Unlike Git, it uses an extension system rather than using scripts. That may be ideal for some section of programmers, but many find the speed of operation of Git to be a feature they do not want to trade off for anything.

More information about Mercurial can be found at Mercurial

Basic Features

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Software	Repository model	Concurrency model	Programming Languages	Scope of Change	Atomic commits	File Renames	Interactive Commits	Partial Checkout/clone	Platforms supported
RCS	Set of files	Merge	C	File	No	No	No	Yes	Unix-like
CVS	Client-Server	Merge	C	File	No	No	No	Yes	Windows, Mac OS X
SVN	Client-Server	Merge or Lock	C	Tree	Yes	Yes	No	Yes	Windows, Mac OS X
Clear Case	Client-Server	Merge or Lock	C, Java, Perl	File	Partial	Yes	No	Yes	Linux, Windows, AIX, Solaris, HP UX, i5/OS, OS/390, z/OS
Git	Distributed	Merge	C,Perl	Tree	Yes	Partial	Yes	No	POSIX, Windows, Mac OS X
Mercurial	Distributed	Merge	Python, C	Tree	Yes	Yes	Yes	No	Windows, Mac OS X

Feature	Feature Description
Repository Model	Describes the relationship between various copies of the source code repository.
Concurrency Model	Describes how changes to the working copy are managed to prevent simultaneous edits from causing nonsensical data in the repository.
Programming Languages	The coding language in which the application is being developed
Scope of Change	Describes whether changes are recorded for individual files or for entire directory trees.
Atomic commits	Refers to a guarantee that all changes made are merged, or that no change at all will be made.
File Renames	Describes whether a system allows files to be renamed while retaining their version history.
Interactive Commits	Interactive commits allow the user to cherrypick the patch-hunks that become part of a commit, instead of having only a file-level granularity.
Partial Checkout/clone	Ability to check out or clone only a specified subdirectory from a repository.

Basic Commands

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Software	Clone	Pull	Push	Checkout	Add	Remove	Merge	Commit	Revert	Rebase
RCS	Not Supported	Not Supported	Not Supported	co	Not Supported	rcsclean	rcsmerge	ci	Not Supported	Not Supported
CVS	Not Supported	Not Supported	Not Supported	checkout	add	rm	update -j	commit	rm, update	Not Supported
SVN	svnadmin hotcopy	svnadmin load	svnadmin dump	checkout	add	rm	merge	commit	revert	Not Supported
Clearcase	Not Supported	Not Supported	Not Supported	checkout	mkelem	rmelem	merge	checkin	unco/rmver	findmerge
Git	Clone	Fetch	Push	Clone	add	rm	merge	commit	checkout	rebase
Mercurial	Clone	Pull	Push	Clone	add	rm	merge	commit	revert	rebase

Command	Command Description
Clone	Create an identical instance of a repository
Pull	Download revisions from a remote repository to a local repository
Push	Upload revisions from a local repository to a remote repository
Checkout	Create a local working copy from a (remote) repository
Add an element	Mark specified files to be added to repository at next commit
Remove an element	Mark specified files to be removed at next commit
Merge	Apply the differences between two sources to a working copy path
Commit	Record changes in the repository
Checkout	Restore working copy file from repository
Rebase	Forward-port local commits to the updated upstream head

Sample Workflow involving Clearcase

This section provides an example of a sample workflow used by a programmer to maintain the code in Clearcase.<ref>http://www.ibm.com/developerworks/rational/library/836.html</ref>

Command to create a new view:

cleartool mkview -tag atag  storage-location 

Command to create a new file:

cleartool mkelem <file_name>

Command to create a new branch:

cleartool mkbranch -nc <version_selector>

Command to checkout a file for editing:

cleartool co -nc <file_name>

Command to check in a file after editing:

cleartool ci -nc <file_name>

Command to check the current working view:

ct pwv

Command to view/edit the current config spec for the view:

ct catcs /edcs

Command to merge and deliver the code changes:

cleartool merge -to target-path -insert contributor-version-selector <name_version>

Command to delete a file:

cleartool rmelem <file_name>

Command to delete a view:

cleartool rmview <view_name>

Choice of Version Control

The choice of version control systems depends on the developers and the projects. Teams use CVS because they are already used to it and have found ways to overcome its limitation. The project would continue to deploy CVS if the prospect of migrating to another revision control is cumbersome.SVN is more commonly used in the corporate world which believes that a single server technology would ease the transitions related to the project.Also, since SVN is highly matured, any help is readily available for the users.Distributed Systems like Git is more preferred for a open source project where several programmers work at different times and provide several updates to the code.

Hence Version Control systems have to be chosen based on the requirements of the project and its team.

References

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See also

• Information about Version Control Systems

• A Video presentation on "Git, The Basics"

• Linus Torvals on Git

• If version control systems were airlines

External links

• Comparison between Git and Mercurial by Google (summer 2008)
• Comparison of CVS and Subversion
• Revision Control Systems Comparison
• IBM Software Information Center
• Git Tutorial
• Choosing a Version Control System