subversion (1)

8/12/2019 Subversion (1)

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Subversion

David Turner

Dec 24, 2007


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What is Subversion?

Subversion is a version control system.

A version control system allows users to

manage files, directories, and the changes made

to them. Subversion can manage any sort of file

collection (not only source code).


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Working copy

Working copy

Working copy

Repository

Internet


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Reasons to Use Subversion

You can work more easily with other developers

on software development projects.

You can undo changes to obtain earlier versions

of files. Subversion is well known and free.

Subversion has been under development since

2000.


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CVS versus Subversion

Subversion fixes problems with CVS.

(See subversion book for details.)

Subversion is being adopted as a

replacement for CVS.


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Subversion Architecture

http://svnbook.red-

bean.com/en/1.4/images/ch01dia1.png
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Repository versus Working Copy

Project code is stored in a server in a data store

referred to as a repository.

Developers check out copies of the project

code into their local environments. Thesecopies are referred to as working copies.

After making changes to a working copy, the

developer commits changes to the repository.

Other developers get these changes by

updating their working copies.


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Differentials

current

version

previous

version

previous

to theprevious

version


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Atomic commits

A collection of modifications either goes

into the repository completely, or not at all.

In other words, a commit will either

altogether succeed, or it will altogether fail.


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Properties

Each file and directory has a set of

propertieskeys and their values

associated with it.

You can keep certain files from being

written into the repository by setting the

svn:ignore property to a string that

matches the files you want to omit. Suchfiles are said to be unversioned.


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Binary versus Character Data

Subversion expresses file differences

using a binary differencing algorithm,

which works identically on both binary and

character-based files.

Both types of files are stored in

compressed format in the repository.

CVS treats these file types differently.


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The Fundamental Problem

User A gets a copy of file X from the datastore.

User B gets a copy of file X from the data

store. User A changes X and writes the new X

back into the data store.

User B changes his older version of X andwrites this into the data store, over-writing

As changes.


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Lock-Modify-Unlock

The lock-modify-unlock solution to thefundamental problem has several problems:

Two users may want to modify two separate

parts of the file, which means one user mustwait.

After locking a file, the user may forget to unlockit.

Locking does not solve incompatibility problemsbetween separate files. (See subversion bookfor details.)


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Copy-Modify-Merge

Subversion uses a copy-modify-merge approach instead of locking. User A gets a working copy of X.

User B gets a working copy of X.

User A changes his working copy of X.

User B changes her working copy of X.

A saves his copy of X into the repository. B tries to save his copy of X into the repository, but it fails, because

her changes were made to a now stale version of X

B performs an update, which results in As changes to X to bemerged into Bs version of X.

If As changes do not conflict with Bs changes, the update silently

completes. If As changes conflict with Bs changes, subversioninserts annotation into X describing the conflicts and then reports theproblem to B. B then manually resolves the conflict.

Whether B needed to manually resolve conflicts or not, the next stepis for B to commit her changes into the repository, which nowsucceeds.


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Copy-Modify-Merge in Practice

With copy-modify-merge, users do not waiton each other.

In practice, conflicts are rare and are

usually straightforward to resolve. Copy-modify-merge does not work well

with binary files, because changes can not

be merged. For this reason, subversionprovides a lock-modify-unlock processwhen needed.


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Revisions

Subversion transactions are atomic: they

either succeed entirely or fail entirely

After the repository is initially created, it is

an empty folder and has revision number

0.

After committing to a repository with

revision number n, the repository is

changed to version n+1.


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Revisions

This diagram and the following text were taken from the online

version of the Subversion book.

When Subversion users talk about revision 5 of foo.c,

they really mean foo.c as it appears in revision 5.


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Revisions and Working Copies

Working copies do not always correspond

to any single revision of the repository;

they may contain files from several

different revisions.


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Revisions and Working Copies

User A checks out repository repo.

repo/system.h 3repo/system.cpp 3

User A modifies system.h and commits this file.


User B commits changes to system.cpp, and A updates.



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States of a Working File

Unchanged, and current

Locally changed, and current

Unchanged, and out-of-date Locally changed, and out-of-date


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Update and Commit

update commit

Unchanged, and

current

does nothing does nothing

Locally changed,

and current

does nothing writes changes

into repo

Unchanged, and

out-of-date

replaces working

file with new one

does nothing

Locally changed,

and out-of-date

merges changes

into working file

operation fails

with out-of-date

error

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