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发信人: jjksam (VOIP engineer), 信区: Linux_Unix
标 题: Subversion Introduction
发信站: 荔园晨风BBS站 (2005年12月12日14:09:56 星期一), 站内信件
许多的开源项目的源码管理都从CVS转向Subversion， 即SVN，这是为什么呢？下面来看看
Subversion的介绍和历史吧。相信看完就会明白了。
下面内容来自：http://opensource.oreilly.com/news/subversion_ch01.html
Version Control with Subversion: Introduction
by Ben Collins-Sussman, Brian W. Fitzpatrick, C. Michael Pilato
[ Editor's Note: This is a preview of Chapter 1 from O'Reilly's upcoming
Version Control with Subversion. ]
Version control is the art of managing changes to information. It has long
been a critical tool for programmers, who typically spend their time making
small changes to software and then undoing those changes the next day. But
the usefulness of version control software extends far beyond the bounds of
the software development world. Anywhere you can find people using computers
to manage information that changes often, there is room for version control.
And that's where Subversion comes into play.
This chapter contains a high-level introduction to Subversion: what it is;
what it does; how to get it.
What is Subversion?
Subversion is a free/open-source version control system. That is, Subversion
manages files and directories over time. Atree of files is placed into a
central repository. The repository is much like an ordinary file server,
except that it remembers every change ever made to the files and directories.
This lets you recover older versions of data, or examine the history of how
your data changed. In this regard, many people think of a version control
system as a sort of time machine.
Subversion can access its repository across networks, which allows it to be
used by people on different computers. At some level, the ability for various
people to modify and manage the same set of data 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 data, just undo that
change.
Some version control systems are also software configuration management (SCM)
systems. These systems are specifically tailored to manage trees of source
code, and have many features that are specific to software development—such
as nativelyunderstanding programming languages, or supplying tools for
building software. Subversion, however, is not one of these systems. It is a
general system that can be used to manage any collection of files. For you,
those files might be source code—for others, anything from grocery shopping
lists to digital video mixdowns and beyond.
Subversion's History
In early 2000, CollabNet, Inc. began seeking developers to write a
replacement for CVS. CollabNet offers a collaboration software suite called
SourceCast, of which one component is version control. Although SourceCast
used CVS as its initial version control system, CVS's limitations were
obvious from the beginning, and CollabNet knew it would eventually have to
find something better. Unfortunately, CVS had become the de facto standard in
the open source world largely because there wasn't anything better, at least
not under a free license. So CollabNet determined to write a new version
control system from scratch, retaining the basic ideas of CVS, but without
the bugs and misfeatures.
In February 2000, they contacted Karl Fogel, the author of Open Source
Development with CVS (Coriolis, 1999), and asked if he'd like to work on this
new project. Coincidentally, at the time Karl was already discussing a design
for a new version control system with his friend Jim Blandy. In 1995, the two
had started Cyclic Software, a company providing CVS support contracts, and
although they later sold the business, they still used CVS every day at their
jobs. Their frustration with CVS had led Jim to think carefully about better
ways to manage versioned data, and he'd already come up with not only the
name Subversion, but also with the basic design of the Subversion repository.
When CollabNet called, Karl immediately agreed to work on the project, and
Jim got his employer, RedHat Software, to essentially donate him to the
project for an indefinite period of time. CollabNet hired Karl and Ben
Collins-Sussman, and detailed design work began in May. With the help of some
well-placed prods from Brian Behlendorf and Jason Robbins of CollabNet, and
Greg Stein (at the time an independent developer active in the WebDAV/DeltaV
specification process), Subversion quickly attracted a community of active
developers. It turned out that many people had had the same frustrating
experiences with CVS, and welcomed the chance to finally do something about
it.
The original design team settled on some simple goals. They didn't want to
break new ground in version control methodology, they just wanted to fix CVS.
They decided that Subversion would match CVS's features, and preserve the
same development model, but not duplicate CVS's most obvious flaws. And
although it did not need to be a drop-in replacement for CVS, it should be
similar enough that any CVS user could make the switch with little effort.
After fourteen months of coding, Subversion became self-hosting on August 31,
2001. That is, Subversion developers stopped using CVS to manage Subversion's
own source code, and started using Subversion instead.
While CollabNet started the project, and still funds a large chunk of the
work (it pays the salaries of a few full-time Subversion developers),
Subversion is run like most open-source projects, governed by a loose,
transparent set of rules that encourage meritocracy. CollabNet's copyright
license is fully compliant with the Debian Free Software Guidelines. In other
words, anyone is free to download, modify, and redistribute Subversion as he
pleases; no permission from CollabNet or anyone else is required.
Subversion's Features
When discussing the features that Subversion brings to the version control
table, it is often helpful to speak of them in terms of how they improve upon
CVS's design. If you're not familiar with CVS, you may not understand all of
these features. And if you're not familiar with version control at all, your
eyes may glaze over unless you first read Chapter 2, in which we provide a
gentle introduction to version control in general.
Subversion provides:
Directory versioning
CVS only tracks the history of individual files, but Subversion implements a
virtual versioned filesystem that tracks changes to whole directory trees
over time. Files and directories are versioned.
True version history
Since CVS is limited to file versioning, operations such as copies and renames
— which might happen to files, but which are really changes to the contents
of some containing directory—aren't supported in CVS. Additionally, in CVS
you cannot replace a versioned file with some new thing of the same name
without the new item inheriting the history of the old—perhaps completely
unrelated— file. With Subversion, you can add, delete, copy, and rename both
files and directories. And every newly added file begins a with a fresh clean
history all its own.
Atomic commits
Acollection of modifications either goes into the repository completely, or
not at all. This allows developers to construct and commit changes as logical
chunks, and prevents problems that can occur when only a portion of a set of
changes is successfully sent to the repository.
Versioned metadata
Each file and directory has a set of properties—keys and their values—
associated with it. You can create and store any arbitrary key/value pairs
you wish. Properties are versioned over time, just like file contents.
Choice of network layers
Subversion has an abstracted notion of repository access, making it easy for
people to implement new network mechanisms. Subversion can plug into the
Apache HTTP Server as an extension module. This gives Subversion a big
advantage in stability and interoperability, and instant access to existing
features provided by that server—authentication, authorization, wire
compression, and so on. Amore lightweight, standalone Subversion server
process is also available. This server speaks a custom protocol which can be
easily tunneled over SSH.
Consistent data handling
Subversion expresses file differences using a binary differencing algorithm,
which works identically on both text (human-readable) and binary
(humanunreadable) files. Both types of files are stored equally compressed in
the repository, and differences are transmitted in both directions across the
network.
Efficient branching and tagging
The cost of branching and tagging need not be proportional to the project
size. Subversion creates branches and tags by simply copying the project,
using a mechanism similar to a hard-link. Thus these operations take only a
very small, constant amount of time.
Hackability
Subversion has no historical baggage; it is implemented as a collection of
shared C libraries with well-defined APIs. This makes Subversion extremely
maintainable and usable by other applications and languages.
Subversion's Architecture
Figure 1-1 illustrates what one might call a mile-high view of Subversion's
design.
http://opensource.oreilly.com/images/linux/svc_0101.gif
Figure 1-1. Subversion's architecture
On one end is a Subversion repository that holds all of your versioned data.
On the other end is your Subversion client program, which manages local
reflections of portions of that versioned data (called working copies).
Between these extremes are multiple routes through various Repository Access
(RA) layers. Some of these routes go across computer networks and through
network servers which then access the repository. Others bypass the network
altogether and access the repository directly.
Installing Subversion
Subversion is built on a portability layer called APR (the Apache Portable
Runtime library). This means Subversion should work on any operating system
that the Apache httpd server runs on: Windows, Linux, all flavors of BSD, Mac
OS X, Netware, and others.
The easiest way to get Subversion is to download a binary package built for
your operating system. Subversion's website often has these packages
available for download, posted by volunteers. The site usually contains
graphical installer packages for users of Microsoft operating systems. If you
run a Unix-like operating system, you can use your system's native package
distribution system (RPMs, DEBs, the ports tree, etc.) to get Subversion.
Alternately, you can build Subversion directly from source code. From the
Subversion website, download the latest source-code release. After unpacking
it, follow the instructions in the INSTALL file to build it. Note that a
released source package contains everything you need to build a command-line
client capable of talking to a remote repository (in particular, the apr,
apr-util, and neon libraries). But optional portions of Subversion have many
other dependencies, such as Berkeley DB and possibly Apache httpd. If you
want to do a complete build, make sure you have all of the packages
documented in the INSTALL file. If you plan to work on Subversion itself, you
can use your client program to grab the latest, bleeding-edge source code.
This is documented in "Get the Source Code."
Subversion's Components
Subversion, once installed, has a number of different pieces. The following
is a quick overview of what you get. Don't be alarmed if the brief
descriptions leave you scratching your head—there are plenty more pages in
this book devoted to alleviating that confusion.
svn
The command-line client program
svnversion
Aprogram for reporting the state (in terms of revisions of the items present)
of a working copy
svnlook
A tool for inspecting a Subversion repository
svnadmin
A tool for creating, tweaking or repairing a Subversion repository
svndumpfilter
A program for filtering Subversion repository dumpfile format streams
mod_dav_svn
Aplug-in module for the Apache HTTP Server, used to make your repository
available to others over a network
svnserve
Acustom standalone server program, runnable as a daemon process or invokable
by SSH; another way to make your repository available to others over a network
Assuming you have Subversion installed correctly, you should be ready to
start. The next two chapters will walk you through the use of svn,
Subversion's command-line client program.
--
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※ 修改:·jjksam 於 12月12日14:14:33 修改本文·[FROM: 211.96.110.173]
※ 来源:·荔园晨风BBS站 bbs.szu.edu.cn·[FROM: 211.96.110.173]
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