Concepts of Version Control

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Concepts of Version Control

• A Technology-Independent View

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Topics

• Well discuss basic version control concepts
  including
• What version is and why we care
• Repositories and archives
• Checking in and check out archives
• Reporting on archives
• Branching and merging archives
• Security

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What is Version Control?

• Over time files evolve and change.
• Tracking these changes can be difficult, since
  the changes stem from different project teams.
• Version control allows users to maintain the
  integrity of files that are subject to change.

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Why Do We Care?

• Version control systems provide users the ability
  to serialize changes to a given file.
• Most such systems also allow users to revert to
  an earlier form of a file.
• Anything can be stored under a good version
  control system including source code, documents,
  images, and binaries.

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Common Version Control Software

• Some common software for version control
  includes
• PVCS
• Microsoft Visual SourceSafe
• CVS / RCS

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Version Control Basics

• Although there are many version control systems
  on the market, most provide the same basic
  features.
• This section describes some of those features.

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Repository

• All version control systems work based on a
  repository.
• The repository holds a copy of each file under
  control of the version control system.
• The repository generally maintains an initial
  copy of each file along with a log of all
  subsequent changes made to that file.

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Adding Files to the Repository

• Not all of the files on your system are
  automatically version controlled when a version
  control system is installed.
• To version a file, you must explicitly place
  that file into the repository.
• A file under control of the version control
  software is commonly called an archive.

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Checking Out Archives From the Repository

• Once a file has been placed under revision
  control, you must check it out of the repository
  in order to change it.
• This ensures that one user doesnt inadvertently
  overwrite the changes made by another user.
• This is called checking out an archive.

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Checking Out Archives From the Repository

• Every version control system is essentially a
  library with each archive a book.
• At any given time, one and only person can have
  possession of a book.
• In version control systems, only the user with
  possession of an archive can change that archive.
• It is also possible to check out read-only copies
  of archives.

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Checking In Files to the Repository

• Once you have an archive checked out, you can
  make changes to it.
• Once you have completed the changes, it must be
  checked back into the repository so that it is
  again available to the other users of the version
  control system.
• This is called checking in an archive.

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Unlocking a Revision

• Sometimes we check out an archive intending to
  make changes to it, but then discover we dont
  really have to.
• We dont want to check in an archive that hasnt
  changed.
• Most version control systems provide the ability
  to unlock a checked out archive without actually
  checking it in.

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Revisions

• When you save your changes using a version
  control system, the new version of the archive is
  given a new revision number.
• It isnt necessary to know how the revision
  numbers are generated (its a form of magic), but
  you should know that they represent the changes
  in an archive over time.

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Checking Out Archives by Revision

• Sometimes we want to check out an archive by a
  specific revision number.
• This is useful if we need to revert to an earlier
  code base in order to diagnose a bug.
• Be careful! If you make changes to earlier
  revision of an archive and check it back in, you
  lose all of the changes in the later revisions of
  that archive.

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Repository Utilities

• Most version control systems provide one or more
  of the following utilities
• Reports on an archives revision history
• Reports on the differences between two revisions
  of an archive
• Branching and merging
• Security

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Report on Revision History

• A report on revision history generally tells you,
  by revision, what changes were made to an
  archive.
• This depends on the person making the changes to
  add comments to the archive when it is checked
  in.
• Always do this!

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Report on Differences Between Two Revisions

• A report on differences is essentially a
  sophisticated comparison algorithm.
• It attempts to determine where lines have been
  added to, changed, or removed from one revision
  when compared to another.
• It is not always precise, but for source code it
  can be useful for determining what changed
  between the revisions.

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Branching and Merging

• Sometimes multiple development teams need access
  to the same archives for different purposes.
• One team might be responsible for maintenance and
  bug fixes while another is responsible for new
  development and enhancements.
• We need a way to allow these teams to co-exist
  without stepping on one anothers code base.

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Branching

• Branching allows us to create two different
  revisions from a single, common, root archive.
• Each team could thus make its changes
  independently to its own branch of the root.
• This is dangerous!

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Merging

• At some point, well need to combine the branched
  revisions of the archive.
• This is common with a new release of a product
  we need to merge the bug fixes with the new
  features.
• Modern version control systems allow us to merge
  branches back into a single revision.
• This often requires human intervention.

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Security

• We often define basic security on our repository
  including
• The users who can access the repository
• The abilities of that the users can exercise on
  the archives
• Different projects with their own repositories,
  might not allow developers from other projects
  access to their archives.

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Review

• Weve discussed basic version control concepts
  including
• What version is and why we care
• Repositories and archives
• Checking in and check out archives
• Reporting on archives
• Branching and merging archives
• Security