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1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
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2
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3 <chapter id="chap:intro">
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4 <title>Introduction</title>
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5
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6 <sect1>
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7 <title>About revision control</title>
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8
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9 <para>Revision control is the process of managing multiple
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10 versions of a piece of information. In its simplest form, this
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11 is something that many people do by hand: every time you modify
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12 a file, save it under a new name that contains a number, each
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13 one higher than the number of the preceding version.</para>
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14
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15 <para>Manually managing multiple versions of even a single file is
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16 an error-prone task, though, so software tools to help automate
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17 this process have long been available. The earliest automated
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18 revision control tools were intended to help a single user to
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19 manage revisions of a single file. Over the past few decades,
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20 the scope of revision control tools has expanded greatly; they
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21 now manage multiple files, and help multiple people to work
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22 together. The best modern revision control tools have no
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23 problem coping with thousands of people working together on
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24 projects that consist of hundreds of thousands of files.</para>
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25
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26 <sect2>
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27 <title>Why use revision control?</title>
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28
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29 <para>There are a number of reasons why you or your team might
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30 want to use an automated revision control tool for a
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31 project.</para>
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32 <itemizedlist>
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33 <listitem><para>It will track the history and evolution of
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34 your project, so you don't have to. For every change,
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35 you'll have a log of <emphasis>who</emphasis> made it;
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36 <emphasis>why</emphasis> they made it;
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37 <emphasis>when</emphasis> they made it; and
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38 <emphasis>what</emphasis> the change
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39 was.</para></listitem>
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40 <listitem><para>When you're working with other people,
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41 revision control software makes it easier for you to
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42 collaborate. For example, when people more or less
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43 simultaneously make potentially incompatible changes, the
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44 software will help you to identify and resolve those
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45 conflicts.</para></listitem>
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46 <listitem><para>It can help you to recover from mistakes. If
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47 you make a change that later turns out to be in error, you
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48 can revert to an earlier version of one or more files. In
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49 fact, a <emphasis>really</emphasis> good revision control
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50 tool will even help you to efficiently figure out exactly
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51 when a problem was introduced (see section <xref
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52 linkend="sec:undo:bisect"/> for details).</para></listitem>
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53 <listitem><para>It will help you to work simultaneously on,
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54 and manage the drift between, multiple versions of your
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55 project.</para></listitem></itemizedlist>
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56 <para>Most of these reasons are equally valid---at least in
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57 theory---whether you're working on a project by yourself, or
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58 with a hundred other people.</para>
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59
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60 <para>A key question about the practicality of revision control
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61 at these two different scales (<quote>lone hacker</quote> and
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62 <quote>huge team</quote>) is how its
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63 <emphasis>benefits</emphasis> compare to its
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64 <emphasis>costs</emphasis>. A revision control tool that's
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65 difficult to understand or use is going to impose a high
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66 cost.</para>
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67
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68 <para>A five-hundred-person project is likely to collapse under
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69 its own weight almost immediately without a revision control
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70 tool and process. In this case, the cost of using revision
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71 control might hardly seem worth considering, since
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72 <emphasis>without</emphasis> it, failure is almost
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73 guaranteed.</para>
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74
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75 <para>On the other hand, a one-person <quote>quick hack</quote>
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76 might seem like a poor place to use a revision control tool,
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77 because surely the cost of using one must be close to the
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78 overall cost of the project. Right?</para>
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79
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80 <para>Mercurial uniquely supports <emphasis>both</emphasis> of
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81 these scales of development. You can learn the basics in just
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82 a few minutes, and due to its low overhead, you can apply
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83 revision control to the smallest of projects with ease. Its
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84 simplicity means you won't have a lot of abstruse concepts or
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85 command sequences competing for mental space with whatever
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86 you're <emphasis>really</emphasis> trying to do. At the same
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87 time, Mercurial's high performance and peer-to-peer nature let
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88 you scale painlessly to handle large projects.</para>
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89
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90 <para>No revision control tool can rescue a poorly run project,
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91 but a good choice of tools can make a huge difference to the
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92 fluidity with which you can work on a project.</para>
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93
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94 </sect2>
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95 <sect2>
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96 <title>The many names of revision control</title>
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97
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98 <para>Revision control is a diverse field, so much so that it
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99 doesn't actually have a single name or acronym. Here are a
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100 few of the more common names and acronyms you'll
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101 encounter:</para>
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102 <itemizedlist>
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103 <listitem><para>Revision control (RCS)</para></listitem>
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104 <listitem><para>Software configuration management (SCM), or
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105 configuration management</para></listitem>
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106 <listitem><para>Source code management</para></listitem>
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107 <listitem><para>Source code control, or source
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108 control</para></listitem>
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109 <listitem><para>Version control
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110 (VCS)</para></listitem></itemizedlist>
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111 <para>Some people claim that these terms actually have different
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112 meanings, but in practice they overlap so much that there's no
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113 agreed or even useful way to tease them apart.</para>
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114
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115 </sect2>
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116 </sect1>
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117 <sect1>
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118 <title>A short history of revision control</title>
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119
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120 <para>The best known of the old-time revision control tools is
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121 SCCS (Source Code Control System), which Marc Rochkind wrote at
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122 Bell Labs, in the early 1970s. SCCS operated on individual
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123 files, and required every person working on a project to have
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124 access to a shared workspace on a single system. Only one
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125 person could modify a file at any time; arbitration for access
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126 to files was via locks. It was common for people to lock files,
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127 and later forget to unlock them, preventing anyone else from
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128 modifying those files without the help of an
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129 administrator.</para>
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130
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131 <para>Walter Tichy developed a free alternative to SCCS in the
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132 early 1980s; he called his program RCS (Revison Control System).
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133 Like SCCS, RCS required developers to work in a single shared
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134 workspace, and to lock files to prevent multiple people from
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135 modifying them simultaneously.</para>
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136
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137 <para>Later in the 1980s, Dick Grune used RCS as a building block
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138 for a set of shell scripts he initially called cmt, but then
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139 renamed to CVS (Concurrent Versions System). The big innovation
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140 of CVS was that it let developers work simultaneously and
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141 somewhat independently in their own personal workspaces. The
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142 personal workspaces prevented developers from stepping on each
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143 other's toes all the time, as was common with SCCS and RCS. Each
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144 developer had a copy of every project file, and could modify
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145 their copies independently. They had to merge their edits prior
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146 to committing changes to the central repository.</para>
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147
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148 <para>Brian Berliner took Grune's original scripts and rewrote
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149 them in C, releasing in 1989 the code that has since developed
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150 into the modern version of CVS. CVS subsequently acquired the
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151 ability to operate over a network connection, giving it a
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152 client/server architecture. CVS's architecture is centralised;
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153 only the server has a copy of the history of the project. Client
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154 workspaces just contain copies of recent versions of the
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155 project's files, and a little metadata to tell them where the
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156 server is. CVS has been enormously successful; it is probably
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157 the world's most widely used revision control system.</para>
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158
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159 <para>In the early 1990s, Sun Microsystems developed an early
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160 distributed revision control system, called TeamWare. A
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161 TeamWare workspace contains a complete copy of the project's
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162 history. TeamWare has no notion of a central repository. (CVS
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163 relied upon RCS for its history storage; TeamWare used
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164 SCCS.)</para>
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165
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166 <para>As the 1990s progressed, awareness grew of a number of
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167 problems with CVS. It records simultaneous changes to multiple
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168 files individually, instead of grouping them together as a
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169 single logically atomic operation. It does not manage its file
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170 hierarchy well; it is easy to make a mess of a repository by
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171 renaming files and directories. Worse, its source code is
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172 difficult to read and maintain, which made the <quote>pain
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173 level</quote> of fixing these architectural problems
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174 prohibitive.</para>
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175
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176 <para>In 2001, Jim Blandy and Karl Fogel, two developers who had
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177 worked on CVS, started a project to replace it with a tool that
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178 would have a better architecture and cleaner code. The result,
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179 Subversion, does not stray from CVS's centralised client/server
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180 model, but it adds multi-file atomic commits, better namespace
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181 management, and a number of other features that make it a
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182 generally better tool than CVS. Since its initial release, it
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183 has rapidly grown in popularity.</para>
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184
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185 <para>More or less simultaneously, Graydon Hoare began working on
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186 an ambitious distributed revision control system that he named
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187 Monotone. While Monotone addresses many of CVS's design flaws
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188 and has a peer-to-peer architecture, it goes beyond earlier (and
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189 subsequent) revision control tools in a number of innovative
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190 ways. It uses cryptographic hashes as identifiers, and has an
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191 integral notion of <quote>trust</quote> for code from different
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192 sources.</para>
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193
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194 <para>Mercurial began life in 2005. While a few aspects of its
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195 design are influenced by Monotone, Mercurial focuses on ease of
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196 use, high performance, and scalability to very large
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197 projects.</para>
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198
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199 </sect1>
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200 <sect1>
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201 <title>Trends in revision control</title>
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202
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203 <para>There has been an unmistakable trend in the development and
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204 use of revision control tools over the past four decades, as
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205 people have become familiar with the capabilities of their tools
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206 and constrained by their limitations.</para>
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207
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208 <para>The first generation began by managing single files on
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209 individual computers. Although these tools represented a huge
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210 advance over ad-hoc manual revision control, their locking model
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211 and reliance on a single computer limited them to small,
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212 tightly-knit teams.</para>
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213
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214 <para>The second generation loosened these constraints by moving
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215 to network-centered architectures, and managing entire projects
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216 at a time. As projects grew larger, they ran into new problems.
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217 With clients needing to talk to servers very frequently, server
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218 scaling became an issue for large projects. An unreliable
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219 network connection could prevent remote users from being able to
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220 talk to the server at all. As open source projects started
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221 making read-only access available anonymously to anyone, people
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222 without commit privileges found that they could not use the
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223 tools to interact with a project in a natural way, as they could
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224 not record their changes.</para>
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225
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226 <para>The current generation of revision control tools is
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227 peer-to-peer in nature. All of these systems have dropped the
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228 dependency on a single central server, and allow people to
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229 distribute their revision control data to where it's actually
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230 needed. Collaboration over the Internet has moved from
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231 constrained by technology to a matter of choice and consensus.
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232 Modern tools can operate offline indefinitely and autonomously,
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233 with a network connection only needed when syncing changes with
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234 another repository.</para>
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235
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236 </sect1>
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237 <sect1>
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238 <title>A few of the advantages of distributed revision
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239 control</title>
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240
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241 <para>Even though distributed revision control tools have for
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242 several years been as robust and usable as their
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243 previous-generation counterparts, people using older tools have
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244 not yet necessarily woken up to their advantages. There are a
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245 number of ways in which distributed tools shine relative to
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246 centralised ones.</para>
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247
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248 <para>For an individual developer, distributed tools are almost
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249 always much faster than centralised tools. This is for a simple
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250 reason: a centralised tool needs to talk over the network for
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251 many common operations, because most metadata is stored in a
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252 single copy on the central server. A distributed tool stores
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253 all of its metadata locally. All else being equal, talking over
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254 the network adds overhead to a centralised tool. Don't
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255 underestimate the value of a snappy, responsive tool: you're
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256 going to spend a lot of time interacting with your revision
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257 control software.</para>
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258
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259 <para>Distributed tools are indifferent to the vagaries of your
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260 server infrastructure, again because they replicate metadata to
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261 so many locations. If you use a centralised system and your
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262 server catches fire, you'd better hope that your backup media
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263 are reliable, and that your last backup was recent and actually
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264 worked. With a distributed tool, you have many backups
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265 available on every contributor's computer.</para>
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266
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267 <para>The reliability of your network will affect distributed
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268 tools far less than it will centralised tools. You can't even
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269 use a centralised tool without a network connection, except for
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270 a few highly constrained commands. With a distributed tool, if
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271 your network connection goes down while you're working, you may
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272 not even notice. The only thing you won't be able to do is talk
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273 to repositories on other computers, something that is relatively
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274 rare compared with local operations. If you have a far-flung
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275 team of collaborators, this may be significant.</para>
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276
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277 <sect2>
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278 <title>Advantages for open source projects</title>
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279
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280 <para>If you take a shine to an open source project and decide
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281 that you would like to start hacking on it, and that project
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282 uses a distributed revision control tool, you are at once a
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283 peer with the people who consider themselves the
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284 <quote>core</quote> of that project. If they publish their
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285 repositories, you can immediately copy their project history,
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286 start making changes, and record your work, using the same
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287 tools in the same ways as insiders. By contrast, with a
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288 centralised tool, you must use the software in a <quote>read
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289 only</quote> mode unless someone grants you permission to
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290 commit changes to their central server. Until then, you won't
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291 be able to record changes, and your local modifications will
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292 be at risk of corruption any time you try to update your
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293 client's view of the repository.</para>
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294
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295 <sect3>
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296 <title>The forking non-problem</title>
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297
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298 <para>It has been suggested that distributed revision control
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299 tools pose some sort of risk to open source projects because
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300 they make it easy to <quote>fork</quote> the development of
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301 a project. A fork happens when there are differences in
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302 opinion or attitude between groups of developers that cause
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303 them to decide that they can't work together any longer.
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bos@553
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304 Each side takes a more or less complete copy of the
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305 project's source code, and goes off in its own
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306 direction.</para>
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307
|
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bos@553
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308 <para>Sometimes the camps in a fork decide to reconcile their
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309 differences. With a centralised revision control system, the
|
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bos@553
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310 <emphasis>technical</emphasis> process of reconciliation is
|
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311 painful, and has to be performed largely by hand. You have
|
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312 to decide whose revision history is going to
|
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313 <quote>win</quote>, and graft the other team's changes into
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314 the tree somehow. This usually loses some or all of one
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315 side's revision history.</para>
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316
|
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bos@553
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317 <para>What distributed tools do with respect to forking is
|
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318 they make forking the <emphasis>only</emphasis> way to
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319 develop a project. Every single change that you make is
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320 potentially a fork point. The great strength of this
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321 approach is that a distributed revision control tool has to
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322 be really good at <emphasis>merging</emphasis> forks,
|
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323 because forks are absolutely fundamental: they happen all
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324 the time.</para>
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325
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bos@553
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326 <para>If every piece of work that everybody does, all the
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327 time, is framed in terms of forking and merging, then what
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328 the open source world refers to as a <quote>fork</quote>
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329 becomes <emphasis>purely</emphasis> a social issue. If
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330 anything, distributed tools <emphasis>lower</emphasis> the
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331 likelihood of a fork:</para>
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332 <itemizedlist>
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bos@553
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333 <listitem><para>They eliminate the social distinction that
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334 centralised tools impose: that between insiders (people
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335 with commit access) and outsiders (people
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336 without).</para></listitem>
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bos@553
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337 <listitem><para>They make it easier to reconcile after a
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338 social fork, because all that's involved from the
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339 perspective of the revision control software is just
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340 another merge.</para></listitem></itemizedlist>
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bos@553
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341
|
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bos@553
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342 <para>Some people resist distributed tools because they want
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343 to retain tight control over their projects, and they
|
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344 believe that centralised tools give them this control.
|
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bos@553
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345 However, if you're of this belief, and you publish your CVS
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346 or Subversion repositories publically, there are plenty of
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347 tools available that can pull out your entire project's
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348 history (albeit slowly) and recreate it somewhere that you
|
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|
349 don't control. So while your control in this case is
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350 illusory, you are forgoing the ability to fluidly
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351 collaborate with whatever people feel compelled to mirror
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|
352 and fork your history.</para>
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bos@553
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353
|
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bos@553
|
354 </sect3>
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bos@553
|
355 </sect2>
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bos@553
|
356 <sect2>
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bos@553
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357 <title>Advantages for commercial projects</title>
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358
|
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bos@553
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359 <para>Many commercial projects are undertaken by teams that are
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360 scattered across the globe. Contributors who are far from a
|
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|
361 central server will see slower command execution and perhaps
|
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bos@553
|
362 less reliability. Commercial revision control systems attempt
|
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bos@553
|
363 to ameliorate these problems with remote-site replication
|
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|
364 add-ons that are typically expensive to buy and cantankerous
|
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|
365 to administer. A distributed system doesn't suffer from these
|
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|
366 problems in the first place. Better yet, you can easily set
|
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|
367 up multiple authoritative servers, say one per site, so that
|
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|
368 there's no redundant communication between repositories over
|
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bos@553
|
369 expensive long-haul network links.</para>
|
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|
370
|
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bos@553
|
371 <para>Centralised revision control systems tend to have
|
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bos@553
|
372 relatively low scalability. It's not unusual for an expensive
|
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bos@553
|
373 centralised system to fall over under the combined load of
|
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bos@553
|
374 just a few dozen concurrent users. Once again, the typical
|
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bos@553
|
375 response tends to be an expensive and clunky replication
|
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bos@553
|
376 facility. Since the load on a central server---if you have
|
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bos@553
|
377 one at all---is many times lower with a distributed tool
|
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bos@553
|
378 (because all of the data is replicated everywhere), a single
|
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bos@553
|
379 cheap server can handle the needs of a much larger team, and
|
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bos@553
|
380 replication to balance load becomes a simple matter of
|
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bos@553
|
381 scripting.</para>
|
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bos@553
|
382
|
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bos@553
|
383 <para>If you have an employee in the field, troubleshooting a
|
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bos@553
|
384 problem at a customer's site, they'll benefit from distributed
|
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bos@553
|
385 revision control. The tool will let them generate custom
|
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bos@553
|
386 builds, try different fixes in isolation from each other, and
|
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bos@553
|
387 search efficiently through history for the sources of bugs and
|
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bos@553
|
388 regressions in the customer's environment, all without needing
|
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bos@553
|
389 to connect to your company's network.</para>
|
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bos@553
|
390
|
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bos@553
|
391 </sect2>
|
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bos@553
|
392 </sect1>
|
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bos@553
|
393 <sect1>
|
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bos@553
|
394 <title>Why choose Mercurial?</title>
|
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bos@553
|
395
|
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bos@553
|
396 <para>Mercurial has a unique set of properties that make it a
|
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bos@553
|
397 particularly good choice as a revision control system.</para>
|
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bos@553
|
398 <itemizedlist>
|
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bos@553
|
399 <listitem><para>It is easy to learn and use.</para></listitem>
|
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bos@553
|
400 <listitem><para>It is lightweight.</para></listitem>
|
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bos@553
|
401 <listitem><para>It scales excellently.</para></listitem>
|
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bos@553
|
402 <listitem><para>It is easy to
|
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bos@553
|
403 customise.</para></listitem></itemizedlist>
|
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bos@553
|
404
|
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bos@553
|
405 <para>If you are at all familiar with revision control systems,
|
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bos@553
|
406 you should be able to get up and running with Mercurial in less
|
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bos@553
|
407 than five minutes. Even if not, it will take no more than a few
|
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bos@553
|
408 minutes longer. Mercurial's command and feature sets are
|
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bos@553
|
409 generally uniform and consistent, so you can keep track of a few
|
|
bos@553
|
410 general rules instead of a host of exceptions.</para>
|
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bos@553
|
411
|
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bos@553
|
412 <para>On a small project, you can start working with Mercurial in
|
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bos@553
|
413 moments. Creating new changes and branches; transferring changes
|
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bos@553
|
414 around (whether locally or over a network); and history and
|
|
bos@553
|
415 status operations are all fast. Mercurial attempts to stay
|
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bos@553
|
416 nimble and largely out of your way by combining low cognitive
|
|
bos@553
|
417 overhead with blazingly fast operations.</para>
|
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bos@553
|
418
|
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bos@553
|
419 <para>The usefulness of Mercurial is not limited to small
|
|
bos@553
|
420 projects: it is used by projects with hundreds to thousands of
|
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bos@553
|
421 contributors, each containing tens of thousands of files and
|
|
bos@553
|
422 hundreds of megabytes of source code.</para>
|
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bos@553
|
423
|
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bos@553
|
424 <para>If the core functionality of Mercurial is not enough for
|
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bos@553
|
425 you, it's easy to build on. Mercurial is well suited to
|
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bos@553
|
426 scripting tasks, and its clean internals and implementation in
|
|
bos@553
|
427 Python make it easy to add features in the form of extensions.
|
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bos@553
|
428 There are a number of popular and useful extensions already
|
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bos@553
|
429 available, ranging from helping to identify bugs to improving
|
|
bos@553
|
430 performance.</para>
|
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bos@553
|
431
|
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bos@553
|
432 </sect1>
|
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bos@553
|
433 <sect1>
|
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bos@553
|
434 <title>Mercurial compared with other tools</title>
|
|
bos@553
|
435
|
|
bos@553
|
436 <para>Before you read on, please understand that this section
|
|
bos@553
|
437 necessarily reflects my own experiences, interests, and (dare I
|
|
bos@553
|
438 say it) biases. I have used every one of the revision control
|
|
bos@553
|
439 tools listed below, in most cases for several years at a
|
|
bos@553
|
440 time.</para>
|
|
bos@553
|
441
|
|
bos@553
|
442
|
|
bos@553
|
443 <sect2>
|
|
bos@553
|
444 <title>Subversion</title>
|
|
bos@553
|
445
|
|
bos@553
|
446 <para>Subversion is a popular revision control tool, developed
|
|
bos@553
|
447 to replace CVS. It has a centralised client/server
|
|
bos@553
|
448 architecture.</para>
|
|
bos@553
|
449
|
|
bos@553
|
450 <para>Subversion and Mercurial have similarly named commands for
|
|
bos@553
|
451 performing the same operations, so if you're familiar with
|
|
bos@553
|
452 one, it is easy to learn to use the other. Both tools are
|
|
bos@553
|
453 portable to all popular operating systems.</para>
|
|
bos@553
|
454
|
|
bos@553
|
455 <para>Prior to version 1.5, Subversion had no useful support for
|
|
bos@553
|
456 merges. At the time of writing, its merge tracking capability
|
|
bos@553
|
457 is new, and known to be <ulink
|
|
bos@553
|
458 url="http://svnbook.red-bean.com/nightly/en/svn.branchmerge.advanced.html#svn.branchmerge.advanced.finalword">complicated
|
|
bos@553
|
459 and buggy</ulink>.</para>
|
|
bos@553
|
460
|
|
bos@553
|
461 <para>Mercurial has a substantial performance advantage over
|
|
bos@553
|
462 Subversion on every revision control operation I have
|
|
bos@553
|
463 benchmarked. I have measured its advantage as ranging from a
|
|
bos@553
|
464 factor of two to a factor of six when compared with Subversion
|
|
bos@553
|
465 1.4.3's <emphasis>ra_local</emphasis> file store, which is the
|
|
bos@553
|
466 fastest access method available. In more realistic
|
|
bos@553
|
467 deployments involving a network-based store, Subversion will
|
|
bos@553
|
468 be at a substantially larger disadvantage. Because many
|
|
bos@553
|
469 Subversion commands must talk to the server and Subversion
|
|
bos@553
|
470 does not have useful replication facilities, server capacity
|
|
bos@553
|
471 and network bandwidth become bottlenecks for modestly large
|
|
bos@553
|
472 projects.</para>
|
|
bos@553
|
473
|
|
bos@553
|
474 <para>Additionally, Subversion incurs substantial storage
|
|
bos@553
|
475 overhead to avoid network transactions for a few common
|
|
bos@553
|
476 operations, such as finding modified files
|
|
bos@553
|
477 (<literal>status</literal>) and displaying modifications
|
|
bos@553
|
478 against the current revision (<literal>diff</literal>). As a
|
|
bos@553
|
479 result, a Subversion working copy is often the same size as,
|
|
bos@553
|
480 or larger than, a Mercurial repository and working directory,
|
|
bos@553
|
481 even though the Mercurial repository contains a complete
|
|
bos@553
|
482 history of the project.</para>
|
|
bos@553
|
483
|
|
bos@553
|
484 <para>Subversion is widely supported by third party tools.
|
|
bos@553
|
485 Mercurial currently lags considerably in this area. This gap
|
|
bos@553
|
486 is closing, however, and indeed some of Mercurial's GUI tools
|
|
bos@553
|
487 now outshine their Subversion equivalents. Like Mercurial,
|
|
bos@553
|
488 Subversion has an excellent user manual.</para>
|
|
bos@553
|
489
|
|
bos@553
|
490 <para>Because Subversion doesn't store revision history on the
|
|
bos@553
|
491 client, it is well suited to managing projects that deal with
|
|
bos@553
|
492 lots of large, opaque binary files. If you check in fifty
|
|
bos@553
|
493 revisions to an incompressible 10MB file, Subversion's
|
|
bos@553
|
494 client-side space usage stays constant The space used by any
|
|
bos@553
|
495 distributed SCM will grow rapidly in proportion to the number
|
|
bos@553
|
496 of revisions, because the differences between each revision
|
|
bos@553
|
497 are large.</para>
|
|
bos@553
|
498
|
|
bos@553
|
499 <para>In addition, it's often difficult or, more usually,
|
|
bos@553
|
500 impossible to merge different versions of a binary file.
|
|
bos@553
|
501 Subversion's ability to let a user lock a file, so that they
|
|
bos@553
|
502 temporarily have the exclusive right to commit changes to it,
|
|
bos@553
|
503 can be a significant advantage to a project where binary files
|
|
bos@553
|
504 are widely used.</para>
|
|
bos@553
|
505
|
|
bos@553
|
506 <para>Mercurial can import revision history from a Subversion
|
|
bos@553
|
507 repository. It can also export revision history to a
|
|
bos@553
|
508 Subversion repository. This makes it easy to <quote>test the
|
|
bos@553
|
509 waters</quote> and use Mercurial and Subversion in parallel
|
|
bos@553
|
510 before deciding to switch. History conversion is incremental,
|
|
bos@553
|
511 so you can perform an initial conversion, then small
|
|
bos@553
|
512 additional conversions afterwards to bring in new
|
|
bos@553
|
513 changes.</para>
|
|
bos@553
|
514
|
|
bos@553
|
515
|
|
bos@553
|
516 </sect2>
|
|
bos@553
|
517 <sect2>
|
|
bos@553
|
518 <title>Git</title>
|
|
bos@553
|
519
|
|
bos@553
|
520 <para>Git is a distributed revision control tool that was
|
|
bos@553
|
521 developed for managing the Linux kernel source tree. Like
|
|
bos@553
|
522 Mercurial, its early design was somewhat influenced by
|
|
bos@553
|
523 Monotone.</para>
|
|
bos@553
|
524
|
|
bos@553
|
525 <para>Git has a very large command set, with version 1.5.0
|
|
bos@553
|
526 providing 139 individual commands. It has something of a
|
|
bos@553
|
527 reputation for being difficult to learn. Compared to Git,
|
|
bos@553
|
528 Mercurial has a strong focus on simplicity.</para>
|
|
bos@553
|
529
|
|
bos@553
|
530 <para>In terms of performance, Git is extremely fast. In
|
|
bos@553
|
531 several cases, it is faster than Mercurial, at least on Linux,
|
|
bos@553
|
532 while Mercurial performs better on other operations. However,
|
|
bos@553
|
533 on Windows, the performance and general level of support that
|
|
bos@553
|
534 Git provides is, at the time of writing, far behind that of
|
|
bos@553
|
535 Mercurial.</para>
|
|
bos@553
|
536
|
|
bos@553
|
537 <para>While a Mercurial repository needs no maintenance, a Git
|
|
bos@553
|
538 repository requires frequent manual <quote>repacks</quote> of
|
|
bos@553
|
539 its metadata. Without these, performance degrades, while
|
|
bos@553
|
540 space usage grows rapidly. A server that contains many Git
|
|
bos@553
|
541 repositories that are not rigorously and frequently repacked
|
|
bos@553
|
542 will become heavily disk-bound during backups, and there have
|
|
bos@553
|
543 been instances of daily backups taking far longer than 24
|
|
bos@553
|
544 hours as a result. A freshly packed Git repository is
|
|
bos@553
|
545 slightly smaller than a Mercurial repository, but an unpacked
|
|
bos@553
|
546 repository is several orders of magnitude larger.</para>
|
|
bos@553
|
547
|
|
bos@553
|
548 <para>The core of Git is written in C. Many Git commands are
|
|
bos@553
|
549 implemented as shell or Perl scripts, and the quality of these
|
|
bos@553
|
550 scripts varies widely. I have encountered several instances
|
|
bos@553
|
551 where scripts charged along blindly in the presence of errors
|
|
bos@553
|
552 that should have been fatal.</para>
|
|
bos@553
|
553
|
|
bos@553
|
554 <para>Mercurial can import revision history from a Git
|
|
bos@553
|
555 repository.</para>
|
|
bos@553
|
556
|
|
bos@553
|
557
|
|
bos@553
|
558 </sect2>
|
|
bos@553
|
559 <sect2>
|
|
bos@553
|
560 <title>CVS</title>
|
|
bos@553
|
561
|
|
bos@553
|
562 <para>CVS is probably the most widely used revision control tool
|
|
bos@553
|
563 in the world. Due to its age and internal untidiness, it has
|
|
bos@553
|
564 been only lightly maintained for many years.</para>
|
|
bos@553
|
565
|
|
bos@553
|
566 <para>It has a centralised client/server architecture. It does
|
|
bos@553
|
567 not group related file changes into atomic commits, making it
|
|
bos@553
|
568 easy for people to <quote>break the build</quote>: one person
|
|
bos@553
|
569 can successfully commit part of a change and then be blocked
|
|
bos@553
|
570 by the need for a merge, causing other people to see only a
|
|
bos@553
|
571 portion of the work they intended to do. This also affects
|
|
bos@553
|
572 how you work with project history. If you want to see all of
|
|
bos@553
|
573 the modifications someone made as part of a task, you will
|
|
bos@553
|
574 need to manually inspect the descriptions and timestamps of
|
|
bos@553
|
575 the changes made to each file involved (if you even know what
|
|
bos@553
|
576 those files were).</para>
|
|
bos@553
|
577
|
|
bos@553
|
578 <para>CVS has a muddled notion of tags and branches that I will
|
|
bos@553
|
579 not attempt to even describe. It does not support renaming of
|
|
bos@553
|
580 files or directories well, making it easy to corrupt a
|
|
bos@553
|
581 repository. It has almost no internal consistency checking
|
|
bos@553
|
582 capabilities, so it is usually not even possible to tell
|
|
bos@553
|
583 whether or how a repository is corrupt. I would not recommend
|
|
bos@553
|
584 CVS for any project, existing or new.</para>
|
|
bos@553
|
585
|
|
bos@553
|
586 <para>Mercurial can import CVS revision history. However, there
|
|
bos@553
|
587 are a few caveats that apply; these are true of every other
|
|
bos@553
|
588 revision control tool's CVS importer, too. Due to CVS's lack
|
|
bos@553
|
589 of atomic changes and unversioned filesystem hierarchy, it is
|
|
bos@553
|
590 not possible to reconstruct CVS history completely accurately;
|
|
bos@553
|
591 some guesswork is involved, and renames will usually not show
|
|
bos@553
|
592 up. Because a lot of advanced CVS administration has to be
|
|
bos@553
|
593 done by hand and is hence error-prone, it's common for CVS
|
|
bos@553
|
594 importers to run into multiple problems with corrupted
|
|
bos@553
|
595 repositories (completely bogus revision timestamps and files
|
|
bos@553
|
596 that have remained locked for over a decade are just two of
|
|
bos@553
|
597 the less interesting problems I can recall from personal
|
|
bos@553
|
598 experience).</para>
|
|
bos@553
|
599
|
|
bos@553
|
600 <para>Mercurial can import revision history from a CVS
|
|
bos@553
|
601 repository.</para>
|
|
bos@553
|
602
|
|
bos@553
|
603
|
|
bos@553
|
604 </sect2>
|
|
bos@553
|
605 <sect2>
|
|
bos@553
|
606 <title>Commercial tools</title>
|
|
bos@553
|
607
|
|
bos@553
|
608 <para>Perforce has a centralised client/server architecture,
|
|
bos@553
|
609 with no client-side caching of any data. Unlike modern
|
|
bos@553
|
610 revision control tools, Perforce requires that a user run a
|
|
bos@553
|
611 command to inform the server about every file they intend to
|
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bos@553
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612 edit.</para>
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613
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614 <para>The performance of Perforce is quite good for small teams,
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615 but it falls off rapidly as the number of users grows beyond a
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616 few dozen. Modestly large Perforce installations require the
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617 deployment of proxies to cope with the load their users
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618 generate.</para>
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619
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620
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621 </sect2>
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622 <sect2>
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623 <title>Choosing a revision control tool</title>
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624
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625 <para>With the exception of CVS, all of the tools listed above
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626 have unique strengths that suit them to particular styles of
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627 work. There is no single revision control tool that is best
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628 in all situations.</para>
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629
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630 <para>As an example, Subversion is a good choice for working
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631 with frequently edited binary files, due to its centralised
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632 nature and support for file locking.</para>
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633
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634 <para>I personally find Mercurial's properties of simplicity,
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635 performance, and good merge support to be a compelling
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636 combination that has served me well for several years.</para>
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637
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638
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639 </sect2>
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640 </sect1>
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641 <sect1>
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642 <title>Switching from another tool to Mercurial</title>
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643
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644 <para>Mercurial is bundled with an extension named <literal
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645 role="hg-ext">convert</literal>, which can incrementally
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646 import revision history from several other revision control
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647 tools. By <quote>incremental</quote>, I mean that you can
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648 convert all of a project's history to date in one go, then rerun
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649 the conversion later to obtain new changes that happened after
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650 the initial conversion.</para>
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651
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652 <para>The revision control tools supported by <literal
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653 role="hg-ext">convert</literal> are as follows:</para>
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654 <itemizedlist>
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655 <listitem><para>Subversion</para></listitem>
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656 <listitem><para>CVS</para></listitem>
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657 <listitem><para>Git</para></listitem>
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658 <listitem><para>Darcs</para></listitem></itemizedlist>
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659
|
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bos@553
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660 <para>In addition, <literal role="hg-ext">convert</literal> can
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661 export changes from Mercurial to Subversion. This makes it
|
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662 possible to try Subversion and Mercurial in parallel before
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663 committing to a switchover, without risking the loss of any
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664 work.</para>
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665
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bos@553
|
666 <para>The <command role="hg-ext-conver">convert</command> command
|
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667 is easy to use. Simply point it at the path or URL of the
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668 source repository, optionally give it the name of the
|
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669 destination repository, and it will start working. After the
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670 initial conversion, just run the same command again to import
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671 new changes.</para>
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672 </sect1>
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673 </chapter>
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674
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675 <!--
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676 local variables:
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677 sgml-parent-document: ("00book.xml" "book" "chapter")
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678 end:
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679 -->
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