Copyright © 2015 – Curt Hill Version Control Systems Why use? What systems? What functions?

Copyright © 2015 – Curt Hill

Version Control Systems

Why use?What systems?What functions?

What are they?• Software that stores different

versions of documents– Documents are usually source code – May also include auxiliary items:

• Test cases• Documentation • Make files

• Allow developers to recreate the way the system was at a previous time– Somewhat like an editor undo but much

larger and more complicatedCopyright © 2015 – Curt Hill

Problems That are Solved• Program is updated but the fix broke

something else– Cannot find all the places changed to

restore to original state– Problem not detected until much later

• Two people make changes to a program– Both changes work alone– When put together it does not work


Versions• Often refers to a succession of the

same program– Windows 7 and 8 are different versions

• May also refer to different targets– Linux for Intel vs. Linux for ARM– Lots in common but differences in

various places


What do they do?• Keep multiple versions of any

relevant documents• Maintain information about changes

– Who made the change– Comments on what the change did– Dates of the versions

• Allows code to be checked out, changed and then checked back in

• Show differences between two files


What not to keep?• Anything that can be generated:• Object or machine language

– Generated by compilers

• Documentation that is machine generated by:– JavaDoc– Doxygen– Any other document generator

• Anything else that can never change


The Process• A repository keeps the documents

– Very specialized database

• Developer checks out a document• This is edited to improve it• It is checked back in (aka committed)

with the control information• If the repository supports:

– Automatic unit tests are run– Failure of these can retract the check in


Terminology• Version – updated copy of a document• Branch – new target of a document

– Versions are automatically created, but branches are not

– Allows multiple current documents

• Merge – making two branches into one• Project – a set of files pertaining to

one program or system• Conflict – a check in of two items that

do not agreeCopyright © 2015 – Curt Hill

First Branch Support• The C preprocessor was the first

attempt to handle multiple platforms• This uses the #if, #ifdef, #ifndef etc.• Code specific to a single hardware or

compiler could be incorporated without separating into two separate files– The separate files will defy proper

maintenance


Example Branch• Version 1.0 is released• Version 1.1 is an improvement• Before 1.1 is ready for release a

problem is found and a bug fix generated– Create 1.0.1

• We now have a branch of 1.1 and 1.0.1


A Branch


1.0 1.1

1.0.1

Example Merge• Two branches

– 1.1 is still under development– 1.0.1 is stable

• Version 1.1 has some improvements which we do not want to discard

• What is needed is to merge the fix of 1.0.1 into 1.1


A Merge


1.0 1.1

1.0.1

1.2

Example Again• It is possible that the changes of 1.1

did not need the bug fix of 1.0.1 because it tried another approach to the same code– If so the 1.0.1 becomes a dead end and

is not developed further

• The version numbers are likely to be maintained by the version control and more version numbers will be generated


Branching Again• The branch before produced a dead

end after the changes were merged with the developed new version

• Branches are often more permanent– One branch for Windows– One branch for Mac– Both will be continued indefinitely


Simultaneous Checkout


1.2

1.2a

1.2b

Suppose two or more developers check out the same code in a day.What will happen on check in?

Several Options• Some systems only allow the first

check in– This will force the second one to

reconcile the two sets of changes

• Some systems will accept both provided they do not touch any of the same lines

• Some will merge both and then run tests and reject them if they do not work


Check in


1.2

1.3

1.3

Top checked in first and retained. Bottom checks out 1.3 and then applies different changes.

1.4

Distributed• Many of the older version control

systems relied on a single repository– Easy to maintain, but only one disk error

from disaster

• Distributed version controls are more robust but much more complicated


Distributed Again• A distributed version control system

has multiple repositories • How are they kept synchronized?• A push is a repository telling others

about changes in a project• A pull is a repository asking for

updates from another• Push/pull usually involves multiple

files instead of single files


Systems• SCCS• RCS• PVCS• Subversion• GIT• Mercurial• Many others


SCCS• Source Code Control System• UNIX• May be first, no later that early

1970s• Stored originals and then the

differences between a version and the next version


RCS• Revision Control System • Update of SCCS, also on UNIX• Keep current and uses differences to

go back in time


CVS• Concurrent Version System• Update of RCS but operates on

several types of systems• Uses client / server approach


Subversion• An update of CVS• May apply versioning to directories

and metadata– Not just files

• Atomic check-in– No overwriting

• Client Server model– Clients may be on different machines


SVN Architecture


GIT• An open-source, distributed version

control system– GITHUB seems to be the current choice

for open source projects

• Developed by Linus Torvalds for the LINUX kernel

• Designed for speed and scalability• Each developer gets a local copy of

the repository


Mercurial• Another open-source, distributed

version control system with many similarities to GIT


Conclusions• Version control is a requirement for

software development• Allows easy recreation of the project

at a particular time– Which makes it easier to find when a bug

was introduced

• Also supports branches– Different platforms or different

functionality while still maintaining the common code base

• The check out and check in feature enables teams


Post Conclusions• Lets see if we can examine some of

these– Subversion– Git
