Version control software and tools
Информатика, кибернетика и программирование
Version control softwre nd tools1 Version control lso clled subversion control or revision control helps lrge projects from spinning out of control by letting individul progrmmers writers or project mngers tckle project from different ngles without getting in ech others wy nd without doing dmge tht cnt be undone. Version Control lets you trck your files over time. Youve probbly cooked up your own version control system got ny files like this: Lb1_1. dd version number or dte: Document_V1.
Lecture 14. Version control software and tools1
Version control, also called subversion control, or revision control, helps large projects from spinning out of control by letting individual programmers, writers, or project managers tackle a project from different angles without getting in each others way and without doing damage that cant be undone. Version Control lets you track your files over time.
Youve probably cooked up your own version control system, got any files like this:
Its why we use “Save As”. You want the new file without obliterating the old one. Its a common problem, and solutions are usually like this:
Our shared folder/naming system is fine for class projects or one-time papers. But not for software projects.
Large, fast-changing projects with many authors need a Version Control System to track changes and avoid general chaos. A good VCS does the following:
14.1 Version control concepts.
Most version control systems involve the following concepts.
A typical scenario goes like this:
Alice adds a file (list.txt) to the repository. She checks it out, makes a change (puts “milk” on the list), and checks it back in with a checkin message (“Added required item.”). The next morning, Bob updates his local working set and sees the latest revision of list.txt, which contains “milk”. He can browse the changelog or diff to see that Alice put “milk” the day before.
14.2 Version control solutions
Version control software, including the well known SVN and Git, was designed from the ground up to allow teams of programmers to work on a project together without wasting man-hours on paperwork. Instead of manually scanning branches of code and associated notes, version control allows for a central repository that is org
There are a lot of opinions regarding which version control framework is the best, and can force programmers and project management teams into fierce debate. When choosing the right version control for your project, you should consider that some of pros of one package you will come across are subjective, meaning the opinion of the programmer, and other factors, such as speed and IDE plug-in capabilities, overshadow the raw numbers.
The main difference between version control systems is whether they are server based or peer-to-peer. Either they have a centralized repository where code is checked out and back in with changes, or a setup where the code is frequently updated from peer sources, a more decentralized network, to keep code current.
Beyond that, you will also want to consider speed, functionality, and the learning curve associated with the system. Lets take a look at some of the major systems available and the reasons why some programmers prefer one over the other.
14.3 Concurrent Versions System
Concurrent Versions System (CVS) has been around since the 80s, and has been very popular with both commercial and open source developers. It is released under the GNU license, and uses a system to let users “check out” the code they are going to work on and “check in” their changes.
Originally, CVS handled conflicts between two programmers by only allowing for the latest version of the code to be worked on and updated. As such, it was a first come, first serve system where the user must publish changes quickly to ensure that other users havent beat them to the punch. Now, CVS can handle branching projects so the developed software can diverge into different products with unique features and will be reconciled at a later time.
The CVS server runs on Unix-like systems with client software that runs on multiple operating systems. It is considered the most mature version control system because it has been developed for such a long time and does not receive many requests for new features at this 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.
14.4 Apache Subversion
Apache Subversion (SVN) was created as an alternative to CVS that would fix some bugs in the CVS system while maintaining high compatibility with it. Like CVS, SVN is free and open source with the difference of being distributed under the Apache license as opposed to GNU.
To prevent corruption in the database from being corrupted, SVN employs a concept called atomic operations. Either all of the changes made to the source are applied or none are applied, meaning that no partial changes will break the original source.
Many developers have switched to SVN as it is a newer technology that takes the best features of CVS and improves upon them.
While CVSs branch operations are expensive and do not really lend themselves to long-term forks in the project, SVN is designed to allow for it, lending itself better to large, forked projects with many directions.
Criticism of SVN includes slower comparative speed and the lack of distributed revision control. Distributed revision control uses a peer-to-peer model rather than using a centralized server to store code updates. While a peer-to-peer model would work better for world-wide, open source projects, it may not be ideal in other situations. The downside to a dedicated server approach is that when the server is down, no clients are able to access the code.
First developed by Linus Torvalds of Linux fame, Git takes a radical approach that differs greatly from CVS and SVN. The original concepts for Git were to make a faster, distributed revision control system that would openly defy 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 native ports of Git are available for Windows as msysgit.
As there is no centralized server, Git does not lend itself to single developer projects or small teams as the code may not necessarily be available when using a non-repository computer. Workarounds exist for this problem, and some see Gits improved speed as a decent tradeoff for the hassle.
Git also comes equipped with a wide variety of tools to help users navigate the history system. Each instance of the source contains the entire history tree, which can be useful when developing without an internet connection.
Mercurial began close to the same time as Git and is also a distributed revision control tool. It was originally made to compete with Git for Linux kernel development, and as Git was selected, Mercurial has seen less success in that area. However, that is not to say that it is not used as many major developments use it, including OpenOffice.org.
Its different from other revision control systems in that Mercurial is primarily implemented in Python as opposed to C, but there are some instances where C is used.
Due to its distributed nature and its creation in Python, the Python language developers are considering a switch to Mercurial as it would allow non-core developers to have easier access to creating new trees and reverting changes.
Users have noted that Mercurial shares some features with SVN as well as being a distributed system, and because of the similarities, the learning curve for those already familiar with SVN will be less steep. The documentation for Mercurial also is more complete and will facilitate learning the differences faster.
Some of the major drawbacks to Mercurial include that it doesnt allow for two parents to be merged and unlike Git, it uses an extension system rather than being scriptable. That may be ideal for some programmers, but many find the power of Git to be a feature they dont want to trade off.
1 http://biz30.timedoctor.com/git-mecurial-and-cvs-comparison-of-svn-software/, http://betterexplained.com/articles/a-visual-guide-to-version-control/ and
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