bos@559: <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
bos@26: 
bos@559: <preface id="chap:preface">
bos@587:   <?dbhtml filename="preface.html"?>
bos@559:   <title>Preface</title>
bos@26: 
bos@583:   <sect1>
bos@583:     <title>Why revision control? Why Mercurial?</title>
bos@583: 
bos@584:     <para id="x_6d">Revision control is the process of managing multiple
bos@583:       versions of a piece of information.  In its simplest form, this
bos@583:       is something that many people do by hand: every time you modify
bos@583:       a file, save it under a new name that contains a number, each
bos@583:       one higher than the number of the preceding version.</para>
bos@583: 
bos@584:     <para id="x_6e">Manually managing multiple versions of even a single file is
bos@583:       an error-prone task, though, so software tools to help automate
bos@583:       this process have long been available.  The earliest automated
bos@583:       revision control tools were intended to help a single user to
bos@583:       manage revisions of a single file.  Over the past few decades,
bos@583:       the scope of revision control tools has expanded greatly; they
bos@583:       now manage multiple files, and help multiple people to work
bos@583:       together.  The best modern revision control tools have no
bos@583:       problem coping with thousands of people working together on
bos@583:       projects that consist of hundreds of thousands of files.</para>
bos@583: 
bos@584:     <para id="x_6f">The arrival of distributed revision control is relatively
bos@583:       recent, and so far this new field has grown due to people's
bos@583:       willingness to explore ill-charted territory.</para>
bos@583: 
bos@584:     <para id="x_70">I am writing a book about distributed revision control
bos@583:       because I believe that it is an important subject that deserves
bos@583:       a field guide. I chose to write about Mercurial because it is
bos@583:       the easiest tool to learn the terrain with, and yet it scales to
bos@583:       the demands of real, challenging environments where many other
bos@583:       revision control tools buckle.</para>
bos@583: 
bos@583:     <sect2>
bos@583:       <title>Why use revision control?</title>
bos@583: 
bos@584:       <para id="x_71">There are a number of reasons why you or your team might
bos@583: 	want to use an automated revision control tool for a
bos@583: 	project.</para>
bos@583: 
bos@583:       <itemizedlist>
bos@584: 	<listitem><para id="x_72">It will track the history and evolution of
bos@583: 	    your project, so you don't have to.  For every change,
bos@583: 	    you'll have a log of <emphasis>who</emphasis> made it;
bos@583: 	    <emphasis>why</emphasis> they made it;
bos@583: 	    <emphasis>when</emphasis> they made it; and
bos@583: 	    <emphasis>what</emphasis> the change
bos@583: 	    was.</para></listitem>
bos@584: 	<listitem><para id="x_73">When you're working with other people,
bos@583: 	    revision control software makes it easier for you to
bos@583: 	    collaborate.  For example, when people more or less
bos@583: 	    simultaneously make potentially incompatible changes, the
bos@583: 	    software will help you to identify and resolve those
bos@583: 	    conflicts.</para></listitem>
bos@584: 	<listitem><para id="x_74">It can help you to recover from mistakes.  If
bos@583: 	    you make a change that later turns out to be in error, you
bos@583: 	    can revert to an earlier version of one or more files.  In
bos@583: 	    fact, a <emphasis>really</emphasis> good revision control
bos@583: 	    tool will even help you to efficiently figure out exactly
bos@592: 	    when a problem was introduced (see <xref
bos@583: 	      linkend="sec:undo:bisect"/> for details).</para></listitem>
bos@584: 	<listitem><para id="x_75">It will help you to work simultaneously on,
bos@583: 	    and manage the drift between, multiple versions of your
bos@583: 	    project.</para></listitem>
bos@583:       </itemizedlist>
bos@583: 
bos@584:       <para id="x_76">Most of these reasons are equally valid---at least in
bos@583: 	theory---whether you're working on a project by yourself, or
bos@583: 	with a hundred other people.</para>
bos@583: 
bos@584:       <para id="x_77">A key question about the practicality of revision control
bos@583: 	at these two different scales (<quote>lone hacker</quote> and
bos@583: 	<quote>huge team</quote>) is how its
bos@583: 	<emphasis>benefits</emphasis> compare to its
bos@583: 	<emphasis>costs</emphasis>.  A revision control tool that's
bos@583: 	difficult to understand or use is going to impose a high
bos@583: 	cost.</para>
bos@583: 
bos@584:       <para id="x_78">A five-hundred-person project is likely to collapse under
bos@583: 	its own weight almost immediately without a revision control
bos@583: 	tool and process. In this case, the cost of using revision
bos@583: 	control might hardly seem worth considering, since
bos@583: 	<emphasis>without</emphasis> it, failure is almost
bos@583: 	guaranteed.</para>
bos@583: 
bos@584:       <para id="x_79">On the other hand, a one-person <quote>quick hack</quote>
bos@583: 	might seem like a poor place to use a revision control tool,
bos@583: 	because surely the cost of using one must be close to the
bos@583: 	overall cost of the project.  Right?</para>
bos@583: 
bos@584:       <para id="x_7a">Mercurial uniquely supports <emphasis>both</emphasis> of
bos@583: 	these scales of development.  You can learn the basics in just
bos@583: 	a few minutes, and due to its low overhead, you can apply
bos@583: 	revision control to the smallest of projects with ease.  Its
bos@583: 	simplicity means you won't have a lot of abstruse concepts or
bos@583: 	command sequences competing for mental space with whatever
bos@583: 	you're <emphasis>really</emphasis> trying to do.  At the same
bos@583: 	time, Mercurial's high performance and peer-to-peer nature let
bos@583: 	you scale painlessly to handle large projects.</para>
bos@583: 
bos@584:       <para id="x_7b">No revision control tool can rescue a poorly run project,
bos@583: 	but a good choice of tools can make a huge difference to the
bos@583: 	fluidity with which you can work on a project.</para>
bos@583: 
bos@583:     </sect2>
bos@583: 
bos@583:     <sect2>
bos@583:       <title>The many names of revision control</title>
bos@583: 
bos@584:       <para id="x_7c">Revision control is a diverse field, so much so that it is
bos@583: 	referred to by many names and acronyms.  Here are a few of the
bos@583: 	more common variations you'll encounter:</para>
bos@583:       <itemizedlist>
bos@584: 	<listitem><para id="x_7d">Revision control (RCS)</para></listitem>
bos@584: 	<listitem><para id="x_7e">Software configuration management (SCM), or
bos@583: 	    configuration management</para></listitem>
bos@584: 	<listitem><para id="x_7f">Source code management</para></listitem>
bos@584: 	<listitem><para id="x_80">Source code control, or source
bos@583: 	    control</para></listitem>
bos@584: 	<listitem><para id="x_81">Version control
bos@583: 	    (VCS)</para></listitem></itemizedlist>
bos@584:       <para id="x_82">Some people claim that these terms actually have different
bos@583: 	meanings, but in practice they overlap so much that there's no
bos@583: 	agreed or even useful way to tease them apart.</para>
bos@583: 
bos@583:     </sect2>
bos@583:   </sect1>
bos@26: 
bos@559:   <sect1>
bos@559:     <title>This book is a work in progress</title>
bos@26: 
bos@584:     <para id="x_83">I am releasing this book while I am still writing it, in the
bos@583:       hope that it will prove useful to others.  I am writing under an
bos@583:       open license in the hope that you, my readers, will contribute
bos@583:       feedback and perhaps content of your own.</para>
bos@200: 
bos@559:   </sect1>
bos@559:   <sect1>
bos@559:     <title>About the examples in this book</title>
bos@200: 
bos@584:     <para id="x_84">This book takes an unusual approach to code samples.  Every
bos@559:       example is <quote>live</quote>---each one is actually the result
bos@559:       of a shell script that executes the Mercurial commands you see.
bos@559:       Every time an image of the book is built from its sources, all
bos@559:       the example scripts are automatically run, and their current
bos@559:       results compared against their expected results.</para>
bos@200: 
bos@584:     <para id="x_85">The advantage of this approach is that the examples are
bos@559:       always accurate; they describe <emphasis>exactly</emphasis> the
bos@559:       behaviour of the version of Mercurial that's mentioned at the
bos@559:       front of the book.  If I update the version of Mercurial that
bos@559:       I'm documenting, and the output of some command changes, the
bos@559:       build fails.</para>
bos@200: 
bos@584:     <para id="x_86">There is a small disadvantage to this approach, which is
bos@559:       that the dates and times you'll see in examples tend to be
bos@559:       <quote>squashed</quote> together in a way that they wouldn't be
bos@559:       if the same commands were being typed by a human.  Where a human
bos@559:       can issue no more than one command every few seconds, with any
bos@559:       resulting timestamps correspondingly spread out, my automated
bos@559:       example scripts run many commands in one second.</para>
bos@200: 
bos@584:     <para id="x_87">As an instance of this, several consecutive commits in an
bos@559:       example can show up as having occurred during the same second.
bos@559:       You can see this occur in the <literal
bos@592: 	role="hg-ext">bisect</literal> example in <xref
bos@589: 	linkend="sec:undo:bisect"/>, for instance.</para>
bos@200: 
bos@584:     <para id="x_88">So when you're reading examples, don't place too much weight
bos@559:       on the dates or times you see in the output of commands.  But
bos@559:       <emphasis>do</emphasis> be confident that the behaviour you're
bos@559:       seeing is consistent and reproducible.</para>
bos@26: 
bos@559:   </sect1>
bos@583: 
bos@583:   <sect1>
bos@583:     <title>Trends in the field</title>
bos@583: 
bos@584:     <para id="x_89">There has been an unmistakable trend in the development and
bos@583:       use of revision control tools over the past four decades, as
bos@583:       people have become familiar with the capabilities of their tools
bos@583:       and constrained by their limitations.</para>
bos@583: 
bos@584:     <para id="x_8a">The first generation began by managing single files on
bos@583:       individual computers.  Although these tools represented a huge
bos@583:       advance over ad-hoc manual revision control, their locking model
bos@583:       and reliance on a single computer limited them to small,
bos@583:       tightly-knit teams.</para>
bos@583: 
bos@584:     <para id="x_8b">The second generation loosened these constraints by moving
bos@583:       to network-centered architectures, and managing entire projects
bos@583:       at a time.  As projects grew larger, they ran into new problems.
bos@583:       With clients needing to talk to servers very frequently, server
bos@583:       scaling became an issue for large projects.  An unreliable
bos@583:       network connection could prevent remote users from being able to
bos@583:       talk to the server at all.  As open source projects started
bos@583:       making read-only access available anonymously to anyone, people
bos@583:       without commit privileges found that they could not use the
bos@583:       tools to interact with a project in a natural way, as they could
bos@583:       not record their changes.</para>
bos@583: 
bos@584:     <para id="x_8c">The current generation of revision control tools is
bos@583:       peer-to-peer in nature.  All of these systems have dropped the
bos@583:       dependency on a single central server, and allow people to
bos@583:       distribute their revision control data to where it's actually
bos@583:       needed.  Collaboration over the Internet has moved from
bos@583:       constrained by technology to a matter of choice and consensus.
bos@583:       Modern tools can operate offline indefinitely and autonomously,
bos@583:       with a network connection only needed when syncing changes with
bos@583:       another repository.</para>
bos@583: 
bos@583:   </sect1>
bos@583:   <sect1>
bos@583:     <title>A few of the advantages of distributed revision
bos@583:       control</title>
bos@583: 
bos@584:     <para id="x_8d">Even though distributed revision control tools have for
bos@583:       several years been as robust and usable as their
bos@583:       previous-generation counterparts, people using older tools have
bos@583:       not yet necessarily woken up to their advantages.  There are a
bos@583:       number of ways in which distributed tools shine relative to
bos@583:       centralised ones.</para>
bos@583: 
bos@584:     <para id="x_8e">For an individual developer, distributed tools are almost
bos@583:       always much faster than centralised tools.  This is for a simple
bos@583:       reason: a centralised tool needs to talk over the network for
bos@583:       many common operations, because most metadata is stored in a
bos@583:       single copy on the central server.  A distributed tool stores
bos@583:       all of its metadata locally.  All else being equal, talking over
bos@583:       the network adds overhead to a centralised tool.  Don't
bos@583:       underestimate the value of a snappy, responsive tool: you're
bos@583:       going to spend a lot of time interacting with your revision
bos@583:       control software.</para>
bos@583: 
bos@584:     <para id="x_8f">Distributed tools are indifferent to the vagaries of your
bos@583:       server infrastructure, again because they replicate metadata to
bos@583:       so many locations.  If you use a centralised system and your
bos@583:       server catches fire, you'd better hope that your backup media
bos@583:       are reliable, and that your last backup was recent and actually
bos@583:       worked.  With a distributed tool, you have many backups
bos@583:       available on every contributor's computer.</para>
bos@583: 
bos@584:     <para id="x_90">The reliability of your network will affect distributed
bos@583:       tools far less than it will centralised tools.  You can't even
bos@583:       use a centralised tool without a network connection, except for
bos@583:       a few highly constrained commands.  With a distributed tool, if
bos@583:       your network connection goes down while you're working, you may
bos@583:       not even notice.  The only thing you won't be able to do is talk
bos@583:       to repositories on other computers, something that is relatively
bos@583:       rare compared with local operations.  If you have a far-flung
bos@583:       team of collaborators, this may be significant.</para>
bos@583: 
bos@583:     <sect2>
bos@583:       <title>Advantages for open source projects</title>
bos@583: 
bos@584:       <para id="x_91">If you take a shine to an open source project and decide
bos@583: 	that you would like to start hacking on it, and that project
bos@583: 	uses a distributed revision control tool, you are at once a
bos@583: 	peer with the people who consider themselves the
bos@583: 	<quote>core</quote> of that project.  If they publish their
bos@583: 	repositories, you can immediately copy their project history,
bos@583: 	start making changes, and record your work, using the same
bos@583: 	tools in the same ways as insiders.  By contrast, with a
bos@583: 	centralised tool, you must use the software in a <quote>read
bos@583: 	  only</quote> mode unless someone grants you permission to
bos@583: 	commit changes to their central server.  Until then, you won't
bos@583: 	be able to record changes, and your local modifications will
bos@583: 	be at risk of corruption any time you try to update your
bos@583: 	client's view of the repository.</para>
bos@583: 
bos@583:       <sect3>
bos@583: 	<title>The forking non-problem</title>
bos@583: 
bos@584: 	<para id="x_92">It has been suggested that distributed revision control
bos@583: 	  tools pose some sort of risk to open source projects because
bos@583: 	  they make it easy to <quote>fork</quote> the development of
bos@583: 	  a project.  A fork happens when there are differences in
bos@583: 	  opinion or attitude between groups of developers that cause
bos@583: 	  them to decide that they can't work together any longer.
bos@583: 	  Each side takes a more or less complete copy of the
bos@583: 	  project's source code, and goes off in its own
bos@583: 	  direction.</para>
bos@583: 
bos@584: 	<para id="x_93">Sometimes the camps in a fork decide to reconcile their
bos@583: 	  differences. With a centralised revision control system, the
bos@583: 	  <emphasis>technical</emphasis> process of reconciliation is
bos@583: 	  painful, and has to be performed largely by hand.  You have
bos@583: 	  to decide whose revision history is going to
bos@583: 	  <quote>win</quote>, and graft the other team's changes into
bos@583: 	  the tree somehow. This usually loses some or all of one
bos@583: 	  side's revision history.</para>
bos@583: 
bos@584: 	<para id="x_94">What distributed tools do with respect to forking is
bos@583: 	  they make forking the <emphasis>only</emphasis> way to
bos@583: 	  develop a project.  Every single change that you make is
bos@583: 	  potentially a fork point.  The great strength of this
bos@583: 	  approach is that a distributed revision control tool has to
bos@583: 	  be really good at <emphasis>merging</emphasis> forks,
bos@583: 	  because forks are absolutely fundamental: they happen all
bos@583: 	  the time.</para>
bos@583: 
bos@584: 	<para id="x_95">If every piece of work that everybody does, all the
bos@583: 	  time, is framed in terms of forking and merging, then what
bos@583: 	  the open source world refers to as a <quote>fork</quote>
bos@583: 	  becomes <emphasis>purely</emphasis> a social issue.  If
bos@583: 	  anything, distributed tools <emphasis>lower</emphasis> the
bos@583: 	  likelihood of a fork:</para>
bos@583: 	<itemizedlist>
bos@584: 	  <listitem><para id="x_96">They eliminate the social distinction that
bos@583: 	      centralised tools impose: that between insiders (people
bos@583: 	      with commit access) and outsiders (people
bos@583: 	      without).</para></listitem>
bos@584: 	  <listitem><para id="x_97">They make it easier to reconcile after a
bos@583: 	      social fork, because all that's involved from the
bos@583: 	      perspective of the revision control software is just
bos@583: 	      another merge.</para></listitem></itemizedlist>
bos@583: 
bos@584: 	<para id="x_98">Some people resist distributed tools because they want
bos@583: 	  to retain tight control over their projects, and they
bos@583: 	  believe that centralised tools give them this control.
bos@583: 	  However, if you're of this belief, and you publish your CVS
bos@583: 	  or Subversion repositories publicly, there are plenty of
bos@583: 	  tools available that can pull out your entire project's
bos@583: 	  history (albeit slowly) and recreate it somewhere that you
bos@583: 	  don't control.  So while your control in this case is
bos@583: 	  illusory, you are forgoing the ability to fluidly
bos@583: 	  collaborate with whatever people feel compelled to mirror
bos@583: 	  and fork your history.</para>
bos@583: 
bos@583:       </sect3>
bos@583:     </sect2>
bos@583:     <sect2>
bos@583:       <title>Advantages for commercial projects</title>
bos@583: 
bos@584:       <para id="x_99">Many commercial projects are undertaken by teams that are
bos@583: 	scattered across the globe.  Contributors who are far from a
bos@583: 	central server will see slower command execution and perhaps
bos@583: 	less reliability.  Commercial revision control systems attempt
bos@583: 	to ameliorate these problems with remote-site replication
bos@583: 	add-ons that are typically expensive to buy and cantankerous
bos@583: 	to administer.  A distributed system doesn't suffer from these
bos@583: 	problems in the first place.  Better yet, you can easily set
bos@583: 	up multiple authoritative servers, say one per site, so that
bos@583: 	there's no redundant communication between repositories over
bos@583: 	expensive long-haul network links.</para>
bos@583: 
bos@584:       <para id="x_9a">Centralised revision control systems tend to have
bos@583: 	relatively low scalability.  It's not unusual for an expensive
bos@583: 	centralised system to fall over under the combined load of
bos@583: 	just a few dozen concurrent users.  Once again, the typical
bos@583: 	response tends to be an expensive and clunky replication
bos@583: 	facility.  Since the load on a central server---if you have
bos@583: 	one at all---is many times lower with a distributed tool
bos@583: 	(because all of the data is replicated everywhere), a single
bos@583: 	cheap server can handle the needs of a much larger team, and
bos@583: 	replication to balance load becomes a simple matter of
bos@583: 	scripting.</para>
bos@583: 
bos@584:       <para id="x_9b">If you have an employee in the field, troubleshooting a
bos@583: 	problem at a customer's site, they'll benefit from distributed
bos@583: 	revision control. The tool will let them generate custom
bos@583: 	builds, try different fixes in isolation from each other, and
bos@583: 	search efficiently through history for the sources of bugs and
bos@583: 	regressions in the customer's environment, all without needing
bos@583: 	to connect to your company's network.</para>
bos@583: 
bos@583:     </sect2>
bos@583:   </sect1>
bos@583:   <sect1>
bos@583:     <title>Why choose Mercurial?</title>
bos@583: 
bos@584:     <para id="x_9c">Mercurial has a unique set of properties that make it a
bos@583:       particularly good choice as a revision control system.</para>
bos@583:     <itemizedlist>
bos@584:       <listitem><para id="x_9d">It is easy to learn and use.</para></listitem>
bos@584:       <listitem><para id="x_9e">It is lightweight.</para></listitem>
bos@584:       <listitem><para id="x_9f">It scales excellently.</para></listitem>
bos@584:       <listitem><para id="x_a0">It is easy to
bos@583: 	  customise.</para></listitem></itemizedlist>
bos@583: 
bos@584:     <para id="x_a1">If you are at all familiar with revision control systems,
bos@583:       you should be able to get up and running with Mercurial in less
bos@583:       than five minutes.  Even if not, it will take no more than a few
bos@583:       minutes longer.  Mercurial's command and feature sets are
bos@583:       generally uniform and consistent, so you can keep track of a few
bos@583:       general rules instead of a host of exceptions.</para>
bos@583: 
bos@584:     <para id="x_a2">On a small project, you can start working with Mercurial in
bos@583:       moments. Creating new changes and branches; transferring changes
bos@583:       around (whether locally or over a network); and history and
bos@583:       status operations are all fast.  Mercurial attempts to stay
bos@583:       nimble and largely out of your way by combining low cognitive
bos@583:       overhead with blazingly fast operations.</para>
bos@583: 
bos@584:     <para id="x_a3">The usefulness of Mercurial is not limited to small
bos@583:       projects: it is used by projects with hundreds to thousands of
bos@583:       contributors, each containing tens of thousands of files and
bos@583:       hundreds of megabytes of source code.</para>
bos@583: 
bos@584:     <para id="x_a4">If the core functionality of Mercurial is not enough for
bos@583:       you, it's easy to build on.  Mercurial is well suited to
bos@583:       scripting tasks, and its clean internals and implementation in
bos@583:       Python make it easy to add features in the form of extensions.
bos@583:       There are a number of popular and useful extensions already
bos@583:       available, ranging from helping to identify bugs to improving
bos@583:       performance.</para>
bos@583: 
bos@583:   </sect1>
bos@583:   <sect1>
bos@583:     <title>Mercurial compared with other tools</title>
bos@583: 
bos@584:     <para id="x_a5">Before you read on, please understand that this section
bos@583:       necessarily reflects my own experiences, interests, and (dare I
bos@583:       say it) biases.  I have used every one of the revision control
bos@583:       tools listed below, in most cases for several years at a
bos@583:       time.</para>
bos@583: 
bos@583: 
bos@583:     <sect2>
bos@583:       <title>Subversion</title>
bos@583: 
bos@584:       <para id="x_a6">Subversion is a popular revision control tool, developed
bos@583: 	to replace CVS.  It has a centralised client/server
bos@583: 	architecture.</para>
bos@583: 
bos@584:       <para id="x_a7">Subversion and Mercurial have similarly named commands for
bos@583: 	performing the same operations, so if you're familiar with
bos@583: 	one, it is easy to learn to use the other.  Both tools are
bos@583: 	portable to all popular operating systems.</para>
bos@583: 
bos@584:       <para id="x_a8">Prior to version 1.5, Subversion had no useful support for
bos@583: 	merges. At the time of writing, its merge tracking capability
bos@583: 	is new, and known to be <ulink
bos@583: 	  url="http://svnbook.red-bean.com/nightly/en/svn.branchmerge.advanced.html#svn.branchmerge.advanced.finalword">complicated 
bos@583: 	  and buggy</ulink>.</para>
bos@583: 
bos@584:       <para id="x_a9">Mercurial has a substantial performance advantage over
bos@583: 	Subversion on every revision control operation I have
bos@583: 	benchmarked.  I have measured its advantage as ranging from a
bos@583: 	factor of two to a factor of six when compared with Subversion
bos@583: 	1.4.3's <emphasis>ra_local</emphasis> file store, which is the
bos@583: 	fastest access method available.  In more realistic
bos@583: 	deployments involving a network-based store, Subversion will
bos@583: 	be at a substantially larger disadvantage.  Because many
bos@583: 	Subversion commands must talk to the server and Subversion
bos@583: 	does not have useful replication facilities, server capacity
bos@583: 	and network bandwidth become bottlenecks for modestly large
bos@583: 	projects.</para>
bos@583: 
bos@584:       <para id="x_aa">Additionally, Subversion incurs substantial storage
bos@583: 	overhead to avoid network transactions for a few common
bos@583: 	operations, such as finding modified files
bos@583: 	(<literal>status</literal>) and displaying modifications
bos@583: 	against the current revision (<literal>diff</literal>).  As a
bos@583: 	result, a Subversion working copy is often the same size as,
bos@583: 	or larger than, a Mercurial repository and working directory,
bos@583: 	even though the Mercurial repository contains a complete
bos@583: 	history of the project.</para>
bos@583: 
bos@584:       <para id="x_ab">Subversion is widely supported by third party tools.
bos@583: 	Mercurial currently lags considerably in this area.  This gap
bos@583: 	is closing, however, and indeed some of Mercurial's GUI tools
bos@583: 	now outshine their Subversion equivalents.  Like Mercurial,
bos@583: 	Subversion has an excellent user manual.</para>
bos@583: 
bos@584:       <para id="x_ac">Because Subversion doesn't store revision history on the
bos@583: 	client, it is well suited to managing projects that deal with
bos@583: 	lots of large, opaque binary files.  If you check in fifty
bos@583: 	revisions to an incompressible 10MB file, Subversion's
bos@583: 	client-side space usage stays constant The space used by any
bos@583: 	distributed SCM will grow rapidly in proportion to the number
bos@583: 	of revisions, because the differences between each revision
bos@583: 	are large.</para>
bos@583: 
bos@584:       <para id="x_ad">In addition, it's often difficult or, more usually,
bos@583: 	impossible to merge different versions of a binary file.
bos@583: 	Subversion's ability to let a user lock a file, so that they
bos@583: 	temporarily have the exclusive right to commit changes to it,
bos@583: 	can be a significant advantage to a project where binary files
bos@583: 	are widely used.</para>
bos@583: 
bos@584:       <para id="x_ae">Mercurial can import revision history from a Subversion
bos@583: 	repository. It can also export revision history to a
bos@583: 	Subversion repository.  This makes it easy to <quote>test the
bos@583: 	  waters</quote> and use Mercurial and Subversion in parallel
bos@583: 	before deciding to switch.  History conversion is incremental,
bos@583: 	so you can perform an initial conversion, then small
bos@583: 	additional conversions afterwards to bring in new
bos@583: 	changes.</para>
bos@583: 
bos@583: 
bos@583:     </sect2>
bos@583:     <sect2>
bos@583:       <title>Git</title>
bos@583: 
bos@584:       <para id="x_af">Git is a distributed revision control tool that was
bos@583: 	developed for managing the Linux kernel source tree.  Like
bos@583: 	Mercurial, its early design was somewhat influenced by
bos@583: 	Monotone.</para>
bos@583: 
bos@584:       <para id="x_b0">Git has a very large command set, with version 1.5.0
bos@583: 	providing 139 individual commands.  It has something of a
bos@583: 	reputation for being difficult to learn.  Compared to Git,
bos@583: 	Mercurial has a strong focus on simplicity.</para>
bos@583: 
bos@584:       <para id="x_b1">In terms of performance, Git is extremely fast.  In
bos@583: 	several cases, it is faster than Mercurial, at least on Linux,
bos@583: 	while Mercurial performs better on other operations.  However,
bos@583: 	on Windows, the performance and general level of support that
bos@583: 	Git provides is, at the time of writing, far behind that of
bos@583: 	Mercurial.</para>
bos@583: 
bos@584:       <para id="x_b2">While a Mercurial repository needs no maintenance, a Git
bos@583: 	repository requires frequent manual <quote>repacks</quote> of
bos@583: 	its metadata.  Without these, performance degrades, while
bos@583: 	space usage grows rapidly.  A server that contains many Git
bos@583: 	repositories that are not rigorously and frequently repacked
bos@583: 	will become heavily disk-bound during backups, and there have
bos@583: 	been instances of daily backups taking far longer than 24
bos@583: 	hours as a result.  A freshly packed Git repository is
bos@583: 	slightly smaller than a Mercurial repository, but an unpacked
bos@583: 	repository is several orders of magnitude larger.</para>
bos@583: 
bos@584:       <para id="x_b3">The core of Git is written in C.  Many Git commands are
bos@583: 	implemented as shell or Perl scripts, and the quality of these
bos@583: 	scripts varies widely. I have encountered several instances
bos@583: 	where scripts charged along blindly in the presence of errors
bos@583: 	that should have been fatal.</para>
bos@583: 
bos@584:       <para id="x_b4">Mercurial can import revision history from a Git
bos@583: 	repository.</para>
bos@583: 
bos@583: 
bos@583:     </sect2>
bos@583:     <sect2>
bos@583:       <title>CVS</title>
bos@583: 
bos@584:       <para id="x_b5">CVS is probably the most widely used revision control tool
bos@583: 	in the world.  Due to its age and internal untidiness, it has
bos@583: 	been only lightly maintained for many years.</para>
bos@583: 
bos@584:       <para id="x_b6">It has a centralised client/server architecture.  It does
bos@583: 	not group related file changes into atomic commits, making it
bos@583: 	easy for people to <quote>break the build</quote>: one person
bos@583: 	can successfully commit part of a change and then be blocked
bos@583: 	by the need for a merge, causing other people to see only a
bos@583: 	portion of the work they intended to do.  This also affects
bos@583: 	how you work with project history.  If you want to see all of
bos@583: 	the modifications someone made as part of a task, you will
bos@583: 	need to manually inspect the descriptions and timestamps of
bos@583: 	the changes made to each file involved (if you even know what
bos@583: 	those files were).</para>
bos@583: 
bos@584:       <para id="x_b7">CVS has a muddled notion of tags and branches that I will
bos@583: 	not attempt to even describe.  It does not support renaming of
bos@583: 	files or directories well, making it easy to corrupt a
bos@583: 	repository.  It has almost no internal consistency checking
bos@583: 	capabilities, so it is usually not even possible to tell
bos@583: 	whether or how a repository is corrupt.  I would not recommend
bos@583: 	CVS for any project, existing or new.</para>
bos@583: 
bos@584:       <para id="x_b8">Mercurial can import CVS revision history.  However, there
bos@583: 	are a few caveats that apply; these are true of every other
bos@583: 	revision control tool's CVS importer, too.  Due to CVS's lack
bos@583: 	of atomic changes and unversioned filesystem hierarchy, it is
bos@583: 	not possible to reconstruct CVS history completely accurately;
bos@583: 	some guesswork is involved, and renames will usually not show
bos@583: 	up.  Because a lot of advanced CVS administration has to be
bos@583: 	done by hand and is hence error-prone, it's common for CVS
bos@583: 	importers to run into multiple problems with corrupted
bos@583: 	repositories (completely bogus revision timestamps and files
bos@583: 	that have remained locked for over a decade are just two of
bos@583: 	the less interesting problems I can recall from personal
bos@583: 	experience).</para>
bos@583: 
bos@584:       <para id="x_b9">Mercurial can import revision history from a CVS
bos@583: 	repository.</para>
bos@583: 
bos@583: 
bos@583:     </sect2>
bos@583:     <sect2>
bos@583:       <title>Commercial tools</title>
bos@583: 
bos@584:       <para id="x_ba">Perforce has a centralised client/server architecture,
bos@583: 	with no client-side caching of any data.  Unlike modern
bos@583: 	revision control tools, Perforce requires that a user run a
bos@583: 	command to inform the server about every file they intend to
bos@583: 	edit.</para>
bos@583: 
bos@584:       <para id="x_bb">The performance of Perforce is quite good for small teams,
bos@583: 	but it falls off rapidly as the number of users grows beyond a
bos@583: 	few dozen. Modestly large Perforce installations require the
bos@583: 	deployment of proxies to cope with the load their users
bos@583: 	generate.</para>
bos@583: 
bos@583: 
bos@583:     </sect2>
bos@583:     <sect2>
bos@583:       <title>Choosing a revision control tool</title>
bos@583: 
bos@584:       <para id="x_bc">With the exception of CVS, all of the tools listed above
bos@583: 	have unique strengths that suit them to particular styles of
bos@583: 	work.  There is no single revision control tool that is best
bos@583: 	in all situations.</para>
bos@583: 
bos@584:       <para id="x_bd">As an example, Subversion is a good choice for working
bos@583: 	with frequently edited binary files, due to its centralised
bos@583: 	nature and support for file locking.</para>
bos@583: 
bos@584:       <para id="x_be">I personally find Mercurial's properties of simplicity,
bos@583: 	performance, and good merge support to be a compelling
bos@583: 	combination that has served me well for several years.</para>
bos@583: 
bos@583: 
bos@583:     </sect2>
bos@583:   </sect1>
bos@583:   <sect1>
bos@583:     <title>Switching from another tool to Mercurial</title>
bos@583: 
bos@584:     <para id="x_bf">Mercurial is bundled with an extension named <literal
bos@583: 	role="hg-ext">convert</literal>, which can incrementally
bos@583:       import revision history from several other revision control
bos@583:       tools.  By <quote>incremental</quote>, I mean that you can
bos@583:       convert all of a project's history to date in one go, then rerun
bos@583:       the conversion later to obtain new changes that happened after
bos@583:       the initial conversion.</para>
bos@583: 
bos@584:     <para id="x_c0">The revision control tools supported by <literal
bos@583: 	role="hg-ext">convert</literal> are as follows:</para>
bos@583:     <itemizedlist>
bos@584:       <listitem><para id="x_c1">Subversion</para></listitem>
bos@584:       <listitem><para id="x_c2">CVS</para></listitem>
bos@584:       <listitem><para id="x_c3">Git</para></listitem>
bos@584:       <listitem><para id="x_c4">Darcs</para></listitem></itemizedlist>
bos@584: 
bos@584:     <para id="x_c5">In addition, <literal role="hg-ext">convert</literal> can
bos@583:       export changes from Mercurial to Subversion.  This makes it
bos@583:       possible to try Subversion and Mercurial in parallel before
bos@583:       committing to a switchover, without risking the loss of any
bos@583:       work.</para>
bos@583: 
bos@584:     <para id="x_c6">The <command role="hg-ext-convert">convert</command> command
bos@583:       is easy to use.  Simply point it at the path or URL of the
bos@583:       source repository, optionally give it the name of the
bos@583:       destination repository, and it will start working.  After the
bos@583:       initial conversion, just run the same command again to import
bos@583:       new changes.</para>
bos@583:   </sect1>
bos@583: 
bos@583:   <sect1>
bos@583:     <title>A short history of revision control</title>
bos@583: 
bos@584:     <para id="x_c7">The best known of the old-time revision control tools is
bos@583:       SCCS (Source Code Control System), which Marc Rochkind wrote at
bos@583:       Bell Labs, in the early 1970s.  SCCS operated on individual
bos@583:       files, and required every person working on a project to have
bos@583:       access to a shared workspace on a single system.  Only one
bos@583:       person could modify a file at any time; arbitration for access
bos@583:       to files was via locks.  It was common for people to lock files,
bos@583:       and later forget to unlock them, preventing anyone else from
bos@583:       modifying those files without the help of an
bos@583:       administrator.</para>
bos@583: 
bos@584:     <para id="x_c8">Walter Tichy developed a free alternative to SCCS in the
bos@583:       early 1980s; he called his program RCS (Revision Control System).
bos@583:       Like SCCS, RCS required developers to work in a single shared
bos@583:       workspace, and to lock files to prevent multiple people from
bos@583:       modifying them simultaneously.</para>
bos@583: 
bos@584:     <para id="x_c9">Later in the 1980s, Dick Grune used RCS as a building block
bos@583:       for a set of shell scripts he initially called cmt, but then
bos@583:       renamed to CVS (Concurrent Versions System).  The big innovation
bos@583:       of CVS was that it let developers work simultaneously and
bos@583:       somewhat independently in their own personal workspaces.  The
bos@583:       personal workspaces prevented developers from stepping on each
bos@583:       other's toes all the time, as was common with SCCS and RCS. Each
bos@583:       developer had a copy of every project file, and could modify
bos@583:       their copies independently.  They had to merge their edits prior
bos@583:       to committing changes to the central repository.</para>
bos@583: 
bos@584:     <para id="x_ca">Brian Berliner took Grune's original scripts and rewrote
bos@583:       them in C, releasing in 1989 the code that has since developed
bos@583:       into the modern version of CVS.  CVS subsequently acquired the
bos@583:       ability to operate over a network connection, giving it a
bos@583:       client/server architecture.  CVS's architecture is centralised;
bos@583:       only the server has a copy of the history of the project. Client
bos@583:       workspaces just contain copies of recent versions of the
bos@583:       project's files, and a little metadata to tell them where the
bos@583:       server is.  CVS has been enormously successful; it is probably
bos@583:       the world's most widely used revision control system.</para>
bos@583: 
bos@584:     <para id="x_cb">In the early 1990s, Sun Microsystems developed an early
bos@583:       distributed revision control system, called TeamWare.  A
bos@583:       TeamWare workspace contains a complete copy of the project's
bos@583:       history.  TeamWare has no notion of a central repository.  (CVS
bos@583:       relied upon RCS for its history storage; TeamWare used
bos@583:       SCCS.)</para>
bos@583: 
bos@584:     <para id="x_cc">As the 1990s progressed, awareness grew of a number of
bos@583:       problems with CVS.  It records simultaneous changes to multiple
bos@583:       files individually, instead of grouping them together as a
bos@583:       single logically atomic operation.  It does not manage its file
bos@583:       hierarchy well; it is easy to make a mess of a repository by
bos@583:       renaming files and directories.  Worse, its source code is
bos@583:       difficult to read and maintain, which made the <quote>pain
bos@583: 	level</quote> of fixing these architectural problems
bos@583:       prohibitive.</para>
bos@583: 
bos@584:     <para id="x_cd">In 2001, Jim Blandy and Karl Fogel, two developers who had
bos@583:       worked on CVS, started a project to replace it with a tool that
bos@583:       would have a better architecture and cleaner code.  The result,
bos@583:       Subversion, does not stray from CVS's centralised client/server
bos@583:       model, but it adds multi-file atomic commits, better namespace
bos@583:       management, and a number of other features that make it a
bos@583:       generally better tool than CVS. Since its initial release, it
bos@583:       has rapidly grown in popularity.</para>
bos@583: 
bos@584:     <para id="x_ce">More or less simultaneously, Graydon Hoare began working on
bos@583:       an ambitious distributed revision control system that he named
bos@583:       Monotone. While Monotone addresses many of CVS's design flaws
bos@583:       and has a peer-to-peer architecture, it goes beyond earlier (and
bos@583:       subsequent) revision control tools in a number of innovative
bos@583:       ways.  It uses cryptographic hashes as identifiers, and has an
bos@583:       integral notion of <quote>trust</quote> for code from different
bos@583:       sources.</para>
bos@583: 
bos@584:     <para id="x_cf">Mercurial began life in 2005.  While a few aspects of its
bos@583:       design are influenced by Monotone, Mercurial focuses on ease of
bos@583:       use, high performance, and scalability to very large
bos@583:       projects.</para>
bos@583: 
bos@583:   </sect1>
bos@583: 
bos@583:   <sect1>
bos@583:     <title>Colophon&emdash;this book is Free</title>
bos@26: 
bos@584:     <para id="x_d0">This book is licensed under the Open Publication License,
bos@559:       and is produced entirely using Free Software tools.  It is
bos@580:       typeset with DocBook XML.  Illustrations are drawn and rendered with
bos@559:       <ulink url="http://www.inkscape.org/">Inkscape</ulink>.</para>
bos@26: 
bos@584:     <para id="x_d1">The complete source code for this book is published as a
bos@559:       Mercurial repository, at <ulink
bos@559: 	url="http://hg.serpentine.com/mercurial/book">http://hg.serpentine.com/mercurial/book</ulink>.</para>
bos@559: 
bos@559:   </sect1>
bos@559: </preface>
bos@559: <!--
bos@559: local variables: 
bos@559: sgml-parent-document: ("00book.xml" "book" "preface")
bos@559: end:
bos@559: -->