Change Version Management

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Change / Version Management

• A look at
• CVS, Subversion and cfengine

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What is Version Management?

• Management of multiple revisions of the same unit
  of information
• Application Source Code, Art Resources such as
  blueprints or electronic models
• SCM (Software Configuration Management) -
  Revision control for software projects

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Why is it important?

• Teamwork
• Allows for information to be worked on by a team
  of people concurrently
• copy-modify-merge model
• Prevent overwriting of someone elses change
• Development
• Develop two versions of the software concurrently
• Fix minor bugs in a stable version while adding
  bleeding edge features to another
• Debugging
• Retrieve and run different versions of the
  software to determine in which version(s) the
  problem occurs
• Provides accountability, who made what change when

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SCM Packages

• Server Based
• CVS
• Subversion
• Rational Clear Case (proprietary)
• Distributed
• GNU Arch
• Git Used for Linux kernel development
• BitKeeper (proprietary)
• Many Others

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CVS

• Concurrent Versions System
• CVS developed from an earlier versioning system
  called Revision Control System (RCS)
• The code that eventually evolved into the current
  version of CVS started in April 1989
• CVS is extremely popular, and it does the job.

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CVS Terminology

• Module
• A single project managed by CVS
• Repository
• Where a CVS server stores the modules it manages
• Checking out
• Acquiring a copy of a module
• Working copy
• The checked out files
• Committing
• Updating the repository with the changes you made
  on your working copy
• Update
• Acquire the latest changes from the repository in
  the working copy

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CVS Problems

• Number of awkward limitations
• Changes are tracked per-file instead of
  per-change
• Commits aren't atomic
• Renaming files and directories is awkward
• Must be deleted and re-added
• Branching limitations
• Some of the maintainers of the original CVS have
  declared that the CVS code has become too crusty
  to effectively maintain.

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Subversion Background

• Designed specifically to be a modern replacement
  for CVS
• Dont need to break new ground in version control
  methodology
• Just fix CVS
• Match CVS's features, and preserve the same
  development model, but not duplicate CVS's most
  obvious flaws
• So even though low level implementation is vastly
  different, the higher level view and use of both
  systems is almost identical

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Advantages over CVS

• Atomic commits. Interrupted commit operations do
  not cause repository inconsistency or corruption
• Renamed/copied/removed files retain full revision
  history
• Native support for binary files, with
  space-efficient binary-diff storage
• Directories are versioned. Entire directory trees
  can be moved around and/or copied very quickly,
  and retain full revision history
• Optimized repository accesses. This reduces
  unnecessary network traffic to the repository
  host

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Subversions Architecture
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Typical use as client

• Help
• svn help subcommand
• Update your working copy
• svn update
• Make changes
• svn add
• svn delete
• svn copy
• svn move
• Examine your changes
• svn status
• svn diff
• svn revert
• Merge others' changes
• svn merge
• svn resolved
• Commit your changes
• svn commit

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Repository Setup

• svnadmin is the main tool used to maintain and
  administer the repository
• Repository creation
• svnadmin create /path/to/repos
• Cannot create repository on a network share
  (NFS/AFS etc.)
• Recommended directory structure
• /
• calc/
• trunk/
• tags/
• branches/

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Branching and Merging

• Branch
• Line of development that exists independently of
  another line, yet still shares a common history
  if you look far enough back in time.
• svn copy from trunk directory to branches
  directory
• Merge
• Copy the changes from the trunk into your branch
  or vice versa

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Tags

• Tag is just a snapshot of a project in time
• Official Releases, version 1.0, 1.1 etc.
• svn copy ltpathgt/trunk ltpathgt/tags/release-1.0 \
  -m "Tagging the 1.0 release of the project."
• Same method as creating a branch

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Server Configuration

• svnserve lightweight custom TCP/IP server
• Makes every repository on your system available
  to the network
• svnserve.conf controls authentication and
  authorization
• httpd use Apache WebDAV module
• Get httpd 2.0 up and running with the mod_dav
  module
• Install the mod_dav_svn plugin to mod_dav, which
  uses Subversion's libraries to access the
  repository
• Configure your httpd.conf file to export the
  repository

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Sys Admin uses for Subversion

• Create a repository to store all configuration
  files for the system
• Provides a traceable history of all changes made
  to the system
• No easy way to do this
• Must have 2 copies
• Real in use files
• Copy of them to checkout/commit to repository
• Up to the sys admin to make sure the repository
  is always updated when a config file is changed

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What is cfengine

• Environment for turning system policy into
  automated action
• Defining the way you want all hosts on your
  network to be configured
• Writing this in a single program which is read
  by every host on the network
• Running this program on every host in order to
  check and possibly fix the setup of the host
• Ability to specify general rules for large groups
  of hosts and special rules for exceptional hosts

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Why cfengine?

• Lots of repetitive and menial jobs involved in
  system maintenance
• cfengine allows these tasks to be fully automated
  which saves time and makes the admin available
  for more interesting work

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What about shell scripts?

• Heterogeneous environment
• Shell-scripts work very poorly
• Shell commands have differing syntax across
  different operating systems
• Locations and names of key files differ
• Scripts become unreadable because they contain so
  many tests to determine what kind of OS it was
  being run on
• Scripts distributed, placed only on the systems
  where they were relevant, out of sight and out of
  mind

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What can cfengine do?

• Check and configure the network interface on
  network hosts
• Edit text files for the system or for all users
• Make and maintain symbolic links
• Check and set the permissions and ownership of
  files
• Delete junk files which clutter the system
• Systematic, automated mounting of NFS filesystems
• Checking for the presence or absence of important
  files and filesystems
• Controlled execution of user scripts and shell
  commands
• Process management

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What is a cfengine program?

• Not an imperative language like Perl, but a
  declarative language that resembles Prolog
• Actions not listed in order but listed in bulk
• action-sequence defines the order
• cfagent.conf text file which contains cfengine
  program

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cfengine program cont.

• action-type
• classes
• list of actions

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Sample program

• control
• domain ( mydomain )
• actionsequence ( shellcommands )
• shellcommands
• All GNU/Linux machines
• linux
• /usr/bin/updatedb
• Just one host
• myhost
• /bin/echo Hi there

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Classes

• A class can be one of several things
• OS architecture, ultrix, sun4 etc.
• Unqualified name of a particular host
• Name of user-defined group of hosts
• Day of the week
• Hour of the day
• Etc.
• Compound Class
• myclass.sun4.Monday
• sun4ultrixosf
• mygroup.!myhost

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Cfengine services

• cfagent
• This component does the configuring of the system
  based on rules specified in the file cfagent.conf
• cfservd
• A daemon which acts as both a file server and a
  remote-cfengine executor. This daemon
  authenticates requests from the network and
  processes them according to rules specified in
  cfservd.conf. It works as a file server and as a
  mechanism for starting cfengine on a local host
  and piping its output back to the network
  connection
• cfrun
• This is a simple initiation program which can be
  used to run cfengine on a number of remote hosts.
  It can only ask cfengine on the remote host to
  run the configuration file it already has

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cfengine .conf files

• cfagent.conf Contains the program to be run
  on the local machine
• update.conf Where does cfagent get the master
  program?
• cfservd.conf Grant access to hosts who need
  master program. Only needed on the master
  system that is running cfservd

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Running cfengine

• Use cron as a front end to cfengine to run it
  regularly (every 10 mins, hourly etc.)
• Only need one entry in cron for every system
• Replace anything that was scheduled by cron in
  cfengine

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Combine cfengine and Subversion?

• Use Subversion to manage your cfengine
  programs. A mistake in a cfengine program can
  have extreme effects
• Use cfengine to manage all of the configuration
  files as described
• Use cfengine to also ensure that every change is
  committed to the Subversion repository

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More Info on cfengine

• http//www.gnu.org/software/cfengine/docs/cfengine
  -Tutorial.html