bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: Introduction to Distributed Version Control with Mercurial bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13: bgneal@13:
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Introduction to Distributed Version Control with Mercurial

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Presenter Notes

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Agenda

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  • Brief history of version control tools
    • bgneal@16:
  • Features of distributed version control
    • bgneal@16:
  • Brief introduction to Mercurial
    • bgneal@16:
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Presenter Notes

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# whoami

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Brian Neal <bgneal1@rockwellcollins.com>

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Started at Rockwell Collins in July 1999

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You might remember me from the following programs:

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  • UMS (July 1999 - Spring 2000)
    • bgneal@13:
  • Surgical Strike - (Spring - Fall 2000)
    • bgneal@13:
  • JTRS 2B - (Fall 2000 - Summer 2001)
    • bgneal@13:
  • SCAMP SEP - (Summer 2001 - October 2004)
    • bgneal@13:
  • TTNT (January 2005 - Present)
    • bgneal@13:
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Presenter Notes

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Disclaimers

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  • This is not a sales pitch
    • bgneal@16:
  • I actually like respect Subversion bgneal@16:
    • bgneal@16:
  • I am not an expert
      bgneal@16:
    • I'm probably making this look too easy
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Presenter Notes

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Why learn about DVCS?

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  • DVCS have exploded in the open source world
      bgneal@16:
    • Interestingly, Subversion is growing in corporate environments
      • bgneal@16:
    bgneal@16:
    • bgneal@16:
  • Frustration with current tools
    • bgneal@16:
  • Interesting to see how the tools have evolved
      bgneal@16:
    • As a software practitioner, you should know what your options are
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Presenter Notes

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Brief History of Version Control Tools

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First Generation

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  • No networking!
    • bgneal@13:
  • Concurrency method: locks
    • bgneal@13:
  • Examples:
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    • SCCS - 1972
      • bgneal@13:
    • RCS - 1982
      • bgneal@13:
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Presenter Notes

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Brief History of Version Control Tools

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Second Generation

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  • Client/Server networking; CVCS (Centralized Version Control)
    • bgneal@13:
  • Concurrency method: merge before commit
    • bgneal@13:
  • Examples:
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    • CVS - 1990
      • bgneal@13:
    • IBM Rational ClearCase - 1992
      • bgneal@13:
    • Visual SourceSafe - 1994
      • bgneal@13:
    • Perforce - 1995
      • bgneal@13:
    • SVN - 2000
      • bgneal@13:
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Presenter Notes

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Brief History of Version Control Tools

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Third Generation

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  • Distributed networking; DVCS (Distributed Version Control)
    • bgneal@13:
  • Concurrency method: commit then merge
    • bgneal@13:
  • Examples:
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    • Bitkeeper - 2000
      • bgneal@13:
    • Darcs - 2003
      • bgneal@13:
    • Monotone - 2003
      • bgneal@13:
    • Git - 2005
      • bgneal@13:
    • Mercurial - 2005
      • bgneal@13:
    • Fossil - 2006
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    • Bazaar - 2007
      • bgneal@13:
    • Veracity - 2011
      • bgneal@13:
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Presenter Notes

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What is a CVCS Again?

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images/cvcs.png
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Presenter Notes

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So what's a DVCS look like?

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images/dvcs.png bgneal@13:

All repositories are peers. By convention only, one repository is designated the master. bgneal@13: It is possible for all peers to exchange changes (via push and pull operations).

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Presenter Notes

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What does a DVCS buy you?

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  • Private workspaces
    • bgneal@13:
  • Offline mode
    • bgneal@13:
  • Speed
    • bgneal@13:
  • Scalability
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  • Split Geography
    • bgneal@13:
  • Flexible workflows
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  • Distributed backups
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  • Easier merging
    • bgneal@13:
  • Commit before merge
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Presenter Notes

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Private Workspaces

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  • You can safely experiment in your local repository
    • bgneal@13:
  • Commit as often as you like without affecting the team
      bgneal@13:
    • Encourages "micro-commits"
      • bgneal@13:
    • With SVN, people often hold back until an entire feature is complete
      • bgneal@13:
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  • Encourages experimentation
      bgneal@13:
    • Make cheap repository clones to try things out
      • bgneal@13:
    • Not visible to coworkers
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Presenter Notes

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Offline Mode

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  • Work when you have spotty or no network access
    • bgneal@13:
  • Full access to the repo when offline
    • bgneal@13:
  • Can work on multiple tasks offline
    • bgneal@13:
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  • Work on bug report #1
    • bgneal@13:
  • Commit
    • bgneal@13:
  • Work on bug report #2
    • bgneal@13:
  • With a CVCS, you'd have both fixes in the same pending commit
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Presenter Notes

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Speed

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  • Local operations are fast
      bgneal@13:
    • No, really, you will notice a big difference
      • bgneal@13:
    • Switching between branches will blow your mind
      • bgneal@13:
    bgneal@13:
    • bgneal@13:
  • Initial pull down of an entire repository might be slower than a CVCS
      bgneal@13:
    • Then again, you might be surprised
      • bgneal@16:
    • Mercurial, for example, stores an entire repository in less space than bgneal@13: a SVN working copy in many cases
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Presenter Notes

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Scalability

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  • Some CVCS systems require heavy weight hardware to support the server
      bgneal@13:
    • E.g. ClearCase with a thousand users
      • bgneal@13:
    • With a DVCS, only pushes & pulls contact a central server
      • bgneal@13:
    • Most of the work is done locally
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Presenter Notes

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Split Geography

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  • Imagine a team split between Cedar Rapids & Richardson...
    • bgneal@13:
  • With a CVCS, you have to pick where to put the server
    • bgneal@13:
  • The remote location is stuck with network latency & associated problems
    • bgneal@13:
  • With a DVCS, each site can have a central repository
      bgneal@16:
    • The two central repos can be synch'ed when convenient or even scripted
      • bgneal@13:
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    • bgneal@13:
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Presenter Notes

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Flexible Workflows

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  • There is no internal concept of a central repository
    • bgneal@13:
  • A central repository exits only by convention
    • bgneal@13:
  • More elaborate topologies and workflows can be created:
      bgneal@13:
    • A SW team may push to a central SW repo
      • bgneal@13:
    • Periodically changes from the SW repo are pushed to a QA repo
      • bgneal@13:
    • Changes from the QA repo can be pushed into a release repo
      • bgneal@13:
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    • bgneal@13:
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Presenter Notes

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Distributed Backups

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  • With a DVCS, multiple copies of the repository exist on multiple machines
    • bgneal@13:
  • Of course, this is no substitute for a real backup strategy!
    • bgneal@13:
  • But usually, and with short notice, you can easily designate a repository as bgneal@13: the central repository in an emergency
    • bgneal@13:
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Presenter Notes

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Easier Merging

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  • Subversion has a bad reputation for merging
      bgneal@13:
    • Some of this is not warranted...
        bgneal@13:
      • Merge tracking was added in SVN v1.5
        • bgneal@16:
      • Steadily improved in v1.6 - v1.7
        • bgneal@13:
      bgneal@13:
      • bgneal@13:
    • SVN does not handle file renames and tree conflicts very well
      • bgneal@16:
    • A lot of teams simply avoid merging out of fear or superstition
      • bgneal@13:
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Presenter Notes

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Easier Merging (cont.)

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    bgneal@13:
  • Why is merging better in a DVCS?
      bgneal@13:
    • Merging simply has to work correctly and be easy in a DVCS
        bgneal@13:
      • More attention was paid to this aspect by DVCS's
        • bgneal@13:
      bgneal@13:
      • bgneal@16:
    • DVCS's use directed acyclic graphs internally to represent change sets
        bgneal@13:
      • More information is available to make merge decisions
        • bgneal@13:
      • Easier to find common ancestors of code
        • bgneal@13:
      bgneal@13:
      • bgneal@13:
    • Developer changes and merge changes are separate
        bgneal@13:
      • "Commit before merge"
        • bgneal@13:
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Presenter Notes

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Commit Before Merge

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    bgneal@13:
  • With a CVCS:
      bgneal@13:
    • You make changes in your working copy
      • bgneal@16:
    • Before you can commit, you often must perform an update
        bgneal@13:
      • This may trigger a merge
        • bgneal@13:
      • Your changes are now mixed up with your friendly coworkers' changes
        • bgneal@13:
      • Sometimes this can be a problem...
        • bgneal@13:
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      • bgneal@13:
    bgneal@13:
    • bgneal@13:
  • With a DVCS:
      bgneal@13:
    • You make changes in your working copy
      • bgneal@13:
    • You commit locally!
      • bgneal@13:
    • You can then choose to pull changes from others and merge
        bgneal@13:
      • Your changes are already safely tucked away and can be retrieved later if things go wrong
        • bgneal@13:
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      • bgneal@13:
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Presenter Notes

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Okay, what's the catch?

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Potential drawbacks to a DVCS

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  • Practically no support for locks
      bgneal@13:
    • This makes working with binary files difficult in a team environment
      • bgneal@13:
    bgneal@13:
    • bgneal@13:
  • Huge repositories are not practical
    • bgneal@16:
  • Heterogeneous repositories are not practical
      bgneal@13:
    • Not a good idea to mix, say, software, systems, and firmware in the same repo
      • bgneal@13:
    • Not a good idea to mix multiple products in the same repo
      • bgneal@16:
    • Arguably, this applies to a CVCS as well
      • bgneal@13:
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    • bgneal@13:
  • No support for path-based access control
      bgneal@13:
    • With current tools, you either can access the full repo, or nothing
      • bgneal@13:
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Presenter Notes

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Introduction to Mercurial

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  • Overview
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  • Repositories & working directories
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  • Changesets
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  • Branches & Tags
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  • Example workflow
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  • Command overview
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Presenter Notes

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Mercurial Overview

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  • Distributed Version Control System
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  • Free, open source software licensed under GPL Version 2
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  • Available for Microsoft Windows, GNU/Linux, Mac OS X, Solaris 11 Express
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  • Written in Python with a small amount of C
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  • Extensible with official and 3rd party extensions
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  • TortoiseHg is a popular GUI for Windows
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  • Reputation for being fast & easy to get started with
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  • Can be served via Apache Web server
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  • Repository hooks
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  • Integrates with the Trac issue tracker
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  • Can import history from other tools, including Subversion
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Presenter Notes

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Repositories & Working Directories

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A repository consists of two things:

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  • Your working directory (similar to a working copy in SVN)
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  • The repository itself (also known as "the store")
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    • A .hg directory at the top of your working copy
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Example:

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  • Repositories communicate via the push and pull commands
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    • Push & Pull do not affect your working directory
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      • An update or merge must be performed to receive remote changes into your working directory
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Presenter Notes

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What's in a Repository?

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A repository consists of a directed, acyclic graph of changesets

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  • Each changeset can have 0, 1, or 2 parents (and infinite children)
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  • A changeset with 0 parents is the root
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  • A changeset with 2 parents is the result of a merge
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  • The newest changeset is called the tip, a special tag name
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Presenter Notes

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What's a Changeset?

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A changeset is an atomic collection of changes and some meta information. bgneal@13: The meta information includes:

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  • Who made the changes
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  • Why - the commit message
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  • The name of the branch the changes were made on ("default" is the default)
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  • A local revision number
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  • A changeset ID; a 40 digit hex number (SHA-1 hash of the changeset & parents)
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A changeset can be named by:

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  • Revision number (within a repository)
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  • Changeset ID (globally)
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  • Tag name
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Presenter Notes

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Branches & Tags

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  • In the simple case, each changeset appear in a line
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  • When a changeset develops 2 or more children, a branch occurs
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    • The latest revision of a branch is called a head
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    • A merge is when two branches join back together
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    • Branches can be given names; the default branch name is "default"
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  • Changesets can be given human readable names, or tags
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    • Local tags are only visible within a repository
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    • Regular tags are revision controlled and propagate to other repos
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    • The newest head in a repository is a tag called tip
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Presenter Notes

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A note on branching...

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  • Branching happens all the time when multiple developers are involved
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  • It is so natural you may not notice it at first
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  • Branching often has two meanings:
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    • The "normal" day-to-day branching when developers combine changes
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    • The conceptual branching that occurs when:
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      • You need a feature branch
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      • etc.
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Presenter Notes

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Ways to Branch in Hg

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  • Just make another clone of the repository!
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    • Slowest, safest, easiest to understand
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    • Hard links are used (where available) to minimize disk space
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  • Bookmarks
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    • Lightweight tags can be applied to changesets
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    • Similar to how Git does branches
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    • Bookmarks can be deleted; they are not permanent
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  • Named branches
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    • Actually recording the branch name in the changeset
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  • Anonymous branches
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    • Update to any old changeset, make changes, commit
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    • "Good" for emergency fixes, bad for remembering what you did
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Presenter Notes

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Example Workflow

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images/workflow1.png
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Example Workflow (cont.)

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images/workflow2.png
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Example Workflow (cont.)

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  • Notice that after Alice's pull:
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    • Her working directory is unaffected
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    • Her repository only has one head => no merging required
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Presenter Notes

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SVN Commands for Review

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Basic SVN commands:

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  • add, remove, copy, move, mkdir
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  • checkout, commit, update, revert
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  • merge, resolved, diff
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  • status, log
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  • lock, unlock
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Presenter Notes

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Basic Mercurial Commands

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Mercurial (hg) vs SVN commands:

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  • add, remove, copy, move, mkdir
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  • checkout, commit, update, revert
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  • merge, resolve, diff, heads
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  • status, log
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  • lock, unlock
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Additional "distributed commands":

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  • clone, push, pull
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  • incoming, outgoing
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Not a whole lot to learn above SVN

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Presenter Notes

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References

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This presentation is available at:

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Presenter Notes

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Questions?

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Presenter Notes

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