EinführungVersionkontrolle ist die Kunst Änderungen zu Verwalten.
Es war lange Zeit ein kritisches Werkzeug für Programmierer
die üblicherweise ihre Zeit damit verbrachten kleine Änderungen
an Software vorzunehmen und diese am nächsten Tag wieder rückgängig
zu machen.
Aber die Nützlichkeit von Versionkontrollsoftware liegt weit
außerhalb der Grenzen der Softwareentwicklungswelt.
Überall wo Menschen Computer Verwenden, um Daten zu Verwalten
ändern sich diese häufig/ständig. Und das ist der Punkt wo
Subversion ins Spiel kommt.
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.Das Kapitel enthält eine Einführung in Subversion auf hoher Ebene.
Was es ist? Was es tut? Wie man es bekommt?
This chapter contains a high-level introduction to
Subversion—what it is; what it does; how to get it.Was ist Subversion?
What is Subversion?Subversion ist ein freies/Open-Soucrce Versionskontroll
System. Subversion verwaltet Dateien und Verzeichnisse
über die Zeit hinweg. Ein Baum der Dateien wird in ein Zentrales
repository abgelegt. Das Repository
ist wie ein gewöhlicher File Server, aussser das der sich
an jede jemals gemachte Änderung erinnert. Das erlaubt
es Ihnen jede ältere Version Ihrer Daten wieder her zustellen,
oder die Geschichte der Änderungen zu untersuchen.
In dieser Hinsicht, stellen sich viele Leute ein
Versionskontrollsystem als eine Art von
Zeitmaschine vor.
Subversion is a free/open-source version control system.
That is, Subversion manages files and directories over time. A
tree 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 your files and directories. This allows you to
recover older versions of your 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.Subversions erlaubt den Zugriff auf sein Repository durch
ein Netzwerk, womit der Zugriff von Leuten auf unterschiedlichen
Rechnern ermöglicht wird.
...At some Level die Möglichkeit verschiedener Leute Datensätze
an unterschiedlichen Orten zu Verwalten und zu modifizieren
fördert die Zusammenarbeit. Fortschritt kann früher eintreten
als ohne eine Kanaliserung durch die alle Änderung laufen müssen.
Und Aufgrund der Tatsache, dass die Arbeit Versioniert wird brauchen
Sie nicht zu befürchten, dass die Qualität der Ausgleich für den
Verlust der Kanalisierung ist wenn eine Fehlerhafte Änderung
an den Daten durchgeführt wurde. Es kann einfach rückgängig gemacht
werden.
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.
Einige Versionskontrollesystem sind auch Software Konfigurations
Management Systems (SKM). Diese Systeme sind speziell darauf
zugeschnitten, Quellcodebäume zu verwalten. Sie haben sehr
viele Funktionalitäten, die auf die Software Entwicklung
ausgerichtet sind. z.B. die Angebore Fähigkeit Programmiersprachen
zu verstehen oder liefern Spezielle Werkzeuge zum Erzeugen
(build) von Software. Subververion, aber ist keines von diesen Systemen.
Es ist ein allgemeines System welches zur Verwaltung
jeder Art von Dateien verwendet werden kann.
Für Sie diese Dateien können Quellcode sein, während es für andere
eine Einkaufsliste im Supermarkt ist bis zur Video Liste und
darüber hinaus.
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 natively
understanding 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
Die Geschichte von Subversion
Anfang 2000, CollabNet, Inc. (http://www.collab.net)
suchte Entwickler to eine Ersatz für CVS erstellen sollten.
CollabNet bot eine Collaboration Software Suite genannt
SourceCast an, von deren eine Komponente Versionskontrolle ist.
Obwohl SourceCast CVS als initiale Versionskontrolle verwendete,
die Grenzen von CVS waren von Anfang an offensichtlich, wusste
CollabNet dass es etwas besseres finden mußte.
Bedauerlicherweise war CVS ein de facto
Standard in der Welt der Open Source weil es nichts besseres gab,
wenigstens nicht unter einer freien Lizenz.
Somit entschied sich CollabNet ein neues Versionskontrollsystem
von Grund auf neu zu entwickeln, aber unter zurückhaltender
Übernahme der Grundideen von CVS, aber ohne deren Fehler und Misständen.
In early 2000, CollabNet, Inc. (http://www.collab.net) 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.Im Frebruar 2000, nahmen Sie kontakt zu Karl Fogel, dem Autor
von Open Source Development with CVS
(Coriolis, 1999) und fragten Ihn ob er an diesem neuen
Projekt arbeiten wollte. Zu dieser Zeit war Karl Fogel
zufälligerweise in Kontakt mit seinem Freund Jim Blandy und
diskutierte ein neues Design eines neuen Versionskontrollsystems.
1995 hatten die beiden Cyclic Software gegründet welches Support
Verträge für CVS bot, welche Sie später verkauften, wärend die
Verwendung von CVS in der täglichen Arbeit weiter geführt wurde.
Die Frustration mit CVS gab Jim den Anstoss genau über
das Konzept der Versionierten Daten
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 FeaturesWhen 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 , in which we provide a gentle introduction
to version control in general.Subversion provides:Directory versioningCVS 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 historySince 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 commitsA collection 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 metadataEach 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 layersSubversion 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. A more lightweight, standalone
Subversion server process is also available. This server
speaks a custom protocol which can be easily tunneled over
SSH.Consistent data handlingSubversion expresses file differences using a binary
differencing algorithm, which works identically on both
text (human-readable) and binary (human-unreadable) 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.
HackabilitySubversion 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 ArchitectureFigure 1.1 illustrates what one might call a
mile-high view of Subversion's design.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 SubversionSubversion 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
(http://subversion.tigris.org) 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 .Subversion's ComponentsSubversion, 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.svnThe command-line client program.svnversionA program for reporting the state (in terms of
revisions of the items present) of a working copy.svnlookA tool for inspecting a Subversion repository.svnadminA tool for creating, tweaking or repairing a Subversion
repository.svndumpfilterA program for filtering Subversion repository dumpfile
format streams.mod_dav_svnA plug-in module for the Apache HTTP Server, used to
make your repository available to others over a
network.svnserveA custom 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.