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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:concepts">
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4 <?dbhtml filename="behind-the-scenes.html"?>
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5 <title>Behind the scenes</title>
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6
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7 <para id="x_2e8">Unlike many revision control systems, the concepts upon which
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8 Mercurial is built are simple enough that it's easy to understand
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9 how the software really works. Knowing this certainly isn't
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10 necessary, but I find it useful to have a <quote>mental
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11 model</quote> of what's going on.</para>
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12
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13 <para id="x_2e9">This understanding gives me confidence that Mercurial has been
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14 carefully designed to be both <emphasis>safe</emphasis> and
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15 <emphasis>efficient</emphasis>. And just as importantly, if it's
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16 easy for me to retain a good idea of what the software is doing
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17 when I perform a revision control task, I'm less likely to be
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18 surprised by its behaviour.</para>
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19
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20 <para id="x_2ea">In this chapter, we'll initially cover the core concepts
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21 behind Mercurial's design, then continue to discuss some of the
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22 interesting details of its implementation.</para>
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23
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24 <sect1>
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25 <title>Mercurial's historical record</title>
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26
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27 <sect2>
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28 <title>Tracking the history of a single file</title>
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29
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30 <para id="x_2eb">When Mercurial tracks modifications to a file, it stores
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31 the history of that file in a metadata object called a
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32 <emphasis>filelog</emphasis>. Each entry in the filelog
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33 contains enough information to reconstruct one revision of the
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34 file that is being tracked. Filelogs are stored as files in
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35 the <filename role="special"
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36 class="directory">.hg/store/data</filename> directory. A
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37 filelog contains two kinds of information: revision data, and
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38 an index to help Mercurial to find a revision
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39 efficiently.</para>
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40
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41 <para id="x_2ec">A file that is large, or has a lot of history, has its
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42 filelog stored in separate data
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43 (<quote><literal>.d</literal></quote> suffix) and index
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44 (<quote><literal>.i</literal></quote> suffix) files. For
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45 small files without much history, the revision data and index
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46 are combined in a single <quote><literal>.i</literal></quote>
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47 file. The correspondence between a file in the working
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48 directory and the filelog that tracks its history in the
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49 repository is illustrated in figure <xref
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50 linkend="fig:concepts:filelog"/>.</para>
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51
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52 <figure id="fig:concepts:filelog">
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53 <title>Relationships between files in working directory and
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54 filelogs in repository</title>
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55 <mediaobject>
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56 <imageobject><imagedata fileref="filelog"/></imageobject>
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57 <textobject><phrase>XXX add text</phrase></textobject>
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58 </mediaobject>
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59 </figure>
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60
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61 </sect2>
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62 <sect2>
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63 <title>Managing tracked files</title>
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64
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65 <para id="x_2ee">Mercurial uses a structure called a
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66 <emphasis>manifest</emphasis> to collect together information
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67 about the files that it tracks. Each entry in the manifest
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68 contains information about the files present in a single
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69 changeset. An entry records which files are present in the
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70 changeset, the revision of each file, and a few other pieces
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71 of file metadata.</para>
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72
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73 </sect2>
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74 <sect2>
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75 <title>Recording changeset information</title>
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76
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77 <para id="x_2ef">The <emphasis>changelog</emphasis> contains information
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78 about each changeset. Each revision records who committed a
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79 change, the changeset comment, other pieces of
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80 changeset-related information, and the revision of the
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81 manifest to use.</para>
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82
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83 </sect2>
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84 <sect2>
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85 <title>Relationships between revisions</title>
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86
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87 <para id="x_2f0">Within a changelog, a manifest, or a filelog, each
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88 revision stores a pointer to its immediate parent (or to its
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89 two parents, if it's a merge revision). As I mentioned above,
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90 there are also relationships between revisions
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91 <emphasis>across</emphasis> these structures, and they are
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92 hierarchical in nature.</para>
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93
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94 <para id="x_2f1">For every changeset in a repository, there is exactly one
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95 revision stored in the changelog. Each revision of the
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96 changelog contains a pointer to a single revision of the
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97 manifest. A revision of the manifest stores a pointer to a
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98 single revision of each filelog tracked when that changeset
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99 was created. These relationships are illustrated in figure
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100 <xref linkend="fig:concepts:metadata"/>.</para>
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101
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102 <figure id="fig:concepts:metadata">
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103 <title>Metadata relationships</title>
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104 <mediaobject>
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105 <imageobject><imagedata fileref="metadata"/></imageobject>
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106 <textobject><phrase>XXX add text</phrase></textobject>
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107 </mediaobject>
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108 </figure>
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109
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110 <para id="x_2f3">As the illustration shows, there is
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111 <emphasis>not</emphasis> a <quote>one to one</quote>
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112 relationship between revisions in the changelog, manifest, or
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113 filelog. If the manifest hasn't changed between two
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114 changesets, the changelog entries for those changesets will
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115 point to the same revision of the manifest. If a file that
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116 Mercurial tracks hasn't changed between two changesets, the
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117 entry for that file in the two revisions of the manifest will
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118 point to the same revision of its filelog.</para>
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119
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120 </sect2>
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121 </sect1>
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122 <sect1>
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123 <title>Safe, efficient storage</title>
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124
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125 <para id="x_2f4">The underpinnings of changelogs, manifests, and filelogs are
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126 provided by a single structure called the
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127 <emphasis>revlog</emphasis>.</para>
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128
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129 <sect2>
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130 <title>Efficient storage</title>
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131
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132 <para id="x_2f5">The revlog provides efficient storage of revisions using a
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133 <emphasis>delta</emphasis> mechanism. Instead of storing a
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134 complete copy of a file for each revision, it stores the
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135 changes needed to transform an older revision into the new
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136 revision. For many kinds of file data, these deltas are
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137 typically a fraction of a percent of the size of a full copy
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138 of a file.</para>
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139
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140 <para id="x_2f6">Some obsolete revision control systems can only work with
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141 deltas of text files. They must either store binary files as
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142 complete snapshots or encoded into a text representation, both
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143 of which are wasteful approaches. Mercurial can efficiently
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144 handle deltas of files with arbitrary binary contents; it
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145 doesn't need to treat text as special.</para>
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146
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147 </sect2>
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148 <sect2 id="sec:concepts:txn">
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149 <title>Safe operation</title>
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150
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151 <para id="x_2f7">Mercurial only ever <emphasis>appends</emphasis> data to
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152 the end of a revlog file. It never modifies a section of a
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153 file after it has written it. This is both more robust and
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154 efficient than schemes that need to modify or rewrite
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155 data.</para>
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156
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157 <para id="x_2f8">In addition, Mercurial treats every write as part of a
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158 <emphasis>transaction</emphasis> that can span a number of
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159 files. A transaction is <emphasis>atomic</emphasis>: either
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160 the entire transaction succeeds and its effects are all
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161 visible to readers in one go, or the whole thing is undone.
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162 This guarantee of atomicity means that if you're running two
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163 copies of Mercurial, where one is reading data and one is
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164 writing it, the reader will never see a partially written
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165 result that might confuse it.</para>
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166
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167 <para id="x_2f9">The fact that Mercurial only appends to files makes it
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168 easier to provide this transactional guarantee. The easier it
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169 is to do stuff like this, the more confident you should be
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170 that it's done correctly.</para>
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171
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172 </sect2>
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173 <sect2>
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174 <title>Fast retrieval</title>
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175
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176 <para id="x_2fa">Mercurial cleverly avoids a pitfall common to all earlier
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177 revision control systems: the problem of <emphasis>inefficient
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178 retrieval</emphasis>. Most revision control systems store
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179 the contents of a revision as an incremental series of
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180 modifications against a <quote>snapshot</quote>. To
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181 reconstruct a specific revision, you must first read the
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182 snapshot, and then every one of the revisions between the
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183 snapshot and your target revision. The more history that a
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184 file accumulates, the more revisions you must read, hence the
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185 longer it takes to reconstruct a particular revision.</para>
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186
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187 <figure id="fig:concepts:snapshot">
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188 <title>Snapshot of a revlog, with incremental deltas</title>
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189 <mediaobject>
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190 <imageobject><imagedata fileref="snapshot"/></imageobject>
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191 <textobject><phrase>XXX add text</phrase></textobject>
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192 </mediaobject>
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193 </figure>
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194
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195 <para id="x_2fc">The innovation that Mercurial applies to this problem is
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196 simple but effective. Once the cumulative amount of delta
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197 information stored since the last snapshot exceeds a fixed
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198 threshold, it stores a new snapshot (compressed, of course),
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199 instead of another delta. This makes it possible to
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200 reconstruct <emphasis>any</emphasis> revision of a file
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201 quickly. This approach works so well that it has since been
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202 copied by several other revision control systems.</para>
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203
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204 <para id="x_2fd">Figure <xref linkend="fig:concepts:snapshot"/> illustrates
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205 the idea. In an entry in a revlog's index file, Mercurial
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206 stores the range of entries from the data file that it must
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207 read to reconstruct a particular revision.</para>
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208
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209 <sect3>
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210 <title>Aside: the influence of video compression</title>
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211
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212 <para id="x_2fe">If you're familiar with video compression or have ever
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213 watched a TV feed through a digital cable or satellite
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214 service, you may know that most video compression schemes
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215 store each frame of video as a delta against its predecessor
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216 frame. In addition, these schemes use <quote>lossy</quote>
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217 compression techniques to increase the compression ratio, so
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218 visual errors accumulate over the course of a number of
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219 inter-frame deltas.</para>
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220
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221 <para id="x_2ff">Because it's possible for a video stream to <quote>drop
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222 out</quote> occasionally due to signal glitches, and to
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223 limit the accumulation of artefacts introduced by the lossy
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224 compression process, video encoders periodically insert a
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225 complete frame (called a <quote>key frame</quote>) into the
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226 video stream; the next delta is generated against that
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227 frame. This means that if the video signal gets
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228 interrupted, it will resume once the next key frame is
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229 received. Also, the accumulation of encoding errors
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230 restarts anew with each key frame.</para>
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231
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232 </sect3>
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233 </sect2>
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234 <sect2>
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235 <title>Identification and strong integrity</title>
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236
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237 <para id="x_300">Along with delta or snapshot information, a revlog entry
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238 contains a cryptographic hash of the data that it represents.
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239 This makes it difficult to forge the contents of a revision,
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240 and easy to detect accidental corruption.</para>
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241
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242 <para id="x_301">Hashes provide more than a mere check against corruption;
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243 they are used as the identifiers for revisions. The changeset
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244 identification hashes that you see as an end user are from
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245 revisions of the changelog. Although filelogs and the
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246 manifest also use hashes, Mercurial only uses these behind the
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247 scenes.</para>
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248
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249 <para id="x_302">Mercurial verifies that hashes are correct when it
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250 retrieves file revisions and when it pulls changes from
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251 another repository. If it encounters an integrity problem, it
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252 will complain and stop whatever it's doing.</para>
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253
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254 <para id="x_303">In addition to the effect it has on retrieval efficiency,
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255 Mercurial's use of periodic snapshots makes it more robust
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256 against partial data corruption. If a revlog becomes partly
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257 corrupted due to a hardware error or system bug, it's often
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258 possible to reconstruct some or most revisions from the
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259 uncorrupted sections of the revlog, both before and after the
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260 corrupted section. This would not be possible with a
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261 delta-only storage model.</para>
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262
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263 </sect2>
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264 </sect1>
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265 <sect1>
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266 <title>Revision history, branching, and merging</title>
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267
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268 <para id="x_304">Every entry in a Mercurial revlog knows the identity of its
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269 immediate ancestor revision, usually referred to as its
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270 <emphasis>parent</emphasis>. In fact, a revision contains room
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271 for not one parent, but two. Mercurial uses a special hash,
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272 called the <quote>null ID</quote>, to represent the idea
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273 <quote>there is no parent here</quote>. This hash is simply a
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274 string of zeroes.</para>
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275
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276 <para id="x_305">In figure <xref linkend="fig:concepts:revlog"/>, you can see
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277 an example of the conceptual structure of a revlog. Filelogs,
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278 manifests, and changelogs all have this same structure; they
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279 differ only in the kind of data stored in each delta or
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280 snapshot.</para>
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281
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282 <para id="x_306">The first revision in a revlog (at the bottom of the image)
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283 has the null ID in both of its parent slots. For a
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284 <quote>normal</quote> revision, its first parent slot contains
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285 the ID of its parent revision, and its second contains the null
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286 ID, indicating that the revision has only one real parent. Any
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287 two revisions that have the same parent ID are branches. A
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288 revision that represents a merge between branches has two normal
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289 revision IDs in its parent slots.</para>
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290
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291 <figure id="fig:concepts:revlog">
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292 <title>The conceptual structure of a revlog</title>
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293 <mediaobject>
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294 <imageobject><imagedata fileref="revlog"/></imageobject>
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295 <textobject><phrase>XXX add text</phrase></textobject>
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296 </mediaobject>
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297 </figure>
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298
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299 </sect1>
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300 <sect1>
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301 <title>The working directory</title>
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302
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303 <para id="x_307">In the working directory, Mercurial stores a snapshot of the
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304 files from the repository as of a particular changeset.</para>
|
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bos@559
|
305
|
|
bos@584
|
306 <para id="x_308">The working directory <quote>knows</quote> which changeset
|
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bos@559
|
307 it contains. When you update the working directory to contain a
|
|
bos@559
|
308 particular changeset, Mercurial looks up the appropriate
|
|
bos@559
|
309 revision of the manifest to find out which files it was tracking
|
|
bos@559
|
310 at the time that changeset was committed, and which revision of
|
|
bos@559
|
311 each file was then current. It then recreates a copy of each of
|
|
bos@559
|
312 those files, with the same contents it had when the changeset
|
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bos@559
|
313 was committed.</para>
|
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bos@559
|
314
|
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bos@584
|
315 <para id="x_309">The <emphasis>dirstate</emphasis> contains Mercurial's
|
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bos@559
|
316 knowledge of the working directory. This details which
|
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bos@559
|
317 changeset the working directory is updated to, and all of the
|
|
bos@559
|
318 files that Mercurial is tracking in the working
|
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bos@559
|
319 directory.</para>
|
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bos@559
|
320
|
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bos@584
|
321 <para id="x_30a">Just as a revision of a revlog has room for two parents, so
|
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bos@559
|
322 that it can represent either a normal revision (with one parent)
|
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bos@559
|
323 or a merge of two earlier revisions, the dirstate has slots for
|
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bos@559
|
324 two parents. When you use the <command role="hg-cmd">hg
|
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bos@559
|
325 update</command> command, the changeset that you update to is
|
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bos@559
|
326 stored in the <quote>first parent</quote> slot, and the null ID
|
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bos@559
|
327 in the second. When you <command role="hg-cmd">hg
|
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bos@559
|
328 merge</command> with another changeset, the first parent
|
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bos@559
|
329 remains unchanged, and the second parent is filled in with the
|
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bos@559
|
330 changeset you're merging with. The <command role="hg-cmd">hg
|
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bos@559
|
331 parents</command> command tells you what the parents of the
|
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bos@559
|
332 dirstate are.</para>
|
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bos@559
|
333
|
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bos@559
|
334 <sect2>
|
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bos@559
|
335 <title>What happens when you commit</title>
|
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bos@559
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336
|
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bos@584
|
337 <para id="x_30b">The dirstate stores parent information for more than just
|
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bos@559
|
338 book-keeping purposes. Mercurial uses the parents of the
|
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bos@559
|
339 dirstate as <emphasis>the parents of a new
|
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bos@559
|
340 changeset</emphasis> when you perform a commit.</para>
|
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bos@559
|
341
|
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bos@591
|
342 <figure id="fig:concepts:wdir">
|
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bos@591
|
343 <title>The working directory can have two parents</title>
|
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bos@591
|
344 <mediaobject>
|
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bos@591
|
345 <imageobject><imagedata fileref="wdir"/></imageobject>
|
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bos@591
|
346 <textobject><phrase>XXX add text</phrase></textobject>
|
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bos@591
|
347 </mediaobject>
|
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bos@591
|
348 </figure>
|
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bos@559
|
349
|
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bos@584
|
350 <para id="x_30d">Figure <xref linkend="fig:concepts:wdir"/> shows the
|
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bos@559
|
351 normal state of the working directory, where it has a single
|
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bos@559
|
352 changeset as parent. That changeset is the
|
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bos@559
|
353 <emphasis>tip</emphasis>, the newest changeset in the
|
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bos@559
|
354 repository that has no children.</para>
|
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bos@559
|
355
|
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bos@591
|
356 <figure id="fig:concepts:wdir-after-commit">
|
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bos@591
|
357 <title>The working directory gains new parents after a
|
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bos@591
|
358 commit</title>
|
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bos@591
|
359 <mediaobject>
|
|
bos@591
|
360 <imageobject><imagedata fileref="wdir-after-commit"/></imageobject>
|
|
bos@591
|
361 <textobject><phrase>XXX add text</phrase></textobject>
|
|
bos@591
|
362 </mediaobject>
|
|
bos@591
|
363 </figure>
|
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bos@559
|
364
|
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bos@584
|
365 <para id="x_30f">It's useful to think of the working directory as
|
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bos@559
|
366 <quote>the changeset I'm about to commit</quote>. Any files
|
|
bos@559
|
367 that you tell Mercurial that you've added, removed, renamed,
|
|
bos@559
|
368 or copied will be reflected in that changeset, as will
|
|
bos@559
|
369 modifications to any files that Mercurial is already tracking;
|
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bos@559
|
370 the new changeset will have the parents of the working
|
|
bos@559
|
371 directory as its parents.</para>
|
|
bos@559
|
372
|
|
bos@584
|
373 <para id="x_310">After a commit, Mercurial will update the parents of the
|
|
bos@559
|
374 working directory, so that the first parent is the ID of the
|
|
bos@559
|
375 new changeset, and the second is the null ID. This is shown
|
|
bos@559
|
376 in figure <xref linkend="fig:concepts:wdir-after-commit"/>.
|
|
bos@559
|
377 Mercurial
|
|
bos@559
|
378 doesn't touch any of the files in the working directory when
|
|
bos@559
|
379 you commit; it just modifies the dirstate to note its new
|
|
bos@559
|
380 parents.</para>
|
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bos@559
|
381
|
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bos@559
|
382 </sect2>
|
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bos@559
|
383 <sect2>
|
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bos@559
|
384 <title>Creating a new head</title>
|
|
bos@559
|
385
|
|
bos@584
|
386 <para id="x_311">It's perfectly normal to update the working directory to a
|
|
bos@559
|
387 changeset other than the current tip. For example, you might
|
|
bos@559
|
388 want to know what your project looked like last Tuesday, or
|
|
bos@559
|
389 you could be looking through changesets to see which one
|
|
bos@559
|
390 introduced a bug. In cases like this, the natural thing to do
|
|
bos@559
|
391 is update the working directory to the changeset you're
|
|
bos@559
|
392 interested in, and then examine the files in the working
|
|
bos@559
|
393 directory directly to see their contents as they were when you
|
|
bos@559
|
394 committed that changeset. The effect of this is shown in
|
|
bos@559
|
395 figure <xref linkend="fig:concepts:wdir-pre-branch"/>.</para>
|
|
bos@559
|
396
|
|
bos@591
|
397 <figure id="fig:concepts:wdir-pre-branch">
|
|
bos@591
|
398 <title>The working directory, updated to an older
|
|
bos@591
|
399 changeset</title>
|
|
bos@591
|
400 <mediaobject>
|
|
bos@591
|
401 <imageobject><imagedata fileref="wdir-pre-branch"/></imageobject>
|
|
bos@591
|
402 <textobject><phrase>XXX add text</phrase></textobject>
|
|
bos@591
|
403 </mediaobject>
|
|
bos@591
|
404 </figure>
|
|
bos@559
|
405
|
|
bos@584
|
406 <para id="x_313">Having updated the working directory to an older
|
|
bos@559
|
407 changeset, what happens if you make some changes, and then
|
|
bos@559
|
408 commit? Mercurial behaves in the same way as I outlined
|
|
bos@559
|
409 above. The parents of the working directory become the
|
|
bos@559
|
410 parents of the new changeset. This new changeset has no
|
|
bos@559
|
411 children, so it becomes the new tip. And the repository now
|
|
bos@559
|
412 contains two changesets that have no children; we call these
|
|
bos@559
|
413 <emphasis>heads</emphasis>. You can see the structure that
|
|
bos@559
|
414 this creates in figure <xref
|
|
bos@559
|
415 linkend="fig:concepts:wdir-branch"/>.</para>
|
|
bos@559
|
416
|
|
bos@591
|
417 <figure id="fig:concepts:wdir-branch">
|
|
bos@591
|
418 <title>After a commit made while synced to an older
|
|
bos@591
|
419 changeset</title>
|
|
bos@591
|
420 <mediaobject>
|
|
bos@591
|
421 <imageobject><imagedata fileref="wdir-branch"/></imageobject>
|
|
bos@591
|
422 <textobject><phrase>XXX add text</phrase></textobject>
|
|
bos@591
|
423 </mediaobject>
|
|
bos@591
|
424 </figure>
|
|
bos@559
|
425
|
|
bos@559
|
426 <note>
|
|
bos@584
|
427 <para id="x_315"> If you're new to Mercurial, you should keep in mind a
|
|
bos@559
|
428 common <quote>error</quote>, which is to use the <command
|
|
bos@559
|
429 role="hg-cmd">hg pull</command> command without any
|
|
bos@559
|
430 options. By default, the <command role="hg-cmd">hg
|
|
bos@559
|
431 pull</command> command <emphasis>does not</emphasis>
|
|
bos@559
|
432 update the working directory, so you'll bring new changesets
|
|
bos@559
|
433 into your repository, but the working directory will stay
|
|
bos@559
|
434 synced at the same changeset as before the pull. If you
|
|
bos@559
|
435 make some changes and commit afterwards, you'll thus create
|
|
bos@559
|
436 a new head, because your working directory isn't synced to
|
|
bos@559
|
437 whatever the current tip is.</para>
|
|
bos@559
|
438
|
|
bos@584
|
439 <para id="x_316"> I put the word <quote>error</quote> in quotes because
|
|
bos@559
|
440 all that you need to do to rectify this situation is
|
|
bos@559
|
441 <command role="hg-cmd">hg merge</command>, then <command
|
|
bos@559
|
442 role="hg-cmd">hg commit</command>. In other words, this
|
|
bos@559
|
443 almost never has negative consequences; it just surprises
|
|
bos@559
|
444 people. I'll discuss other ways to avoid this behaviour,
|
|
bos@559
|
445 and why Mercurial behaves in this initially surprising way,
|
|
bos@559
|
446 later on.</para>
|
|
bos@559
|
447 </note>
|
|
bos@559
|
448
|
|
bos@559
|
449 </sect2>
|
|
bos@559
|
450 <sect2>
|
|
bos@559
|
451 <title>Merging heads</title>
|
|
bos@559
|
452
|
|
bos@584
|
453 <para id="x_317">When you run the <command role="hg-cmd">hg merge</command>
|
|
bos@559
|
454 command, Mercurial leaves the first parent of the working
|
|
bos@559
|
455 directory unchanged, and sets the second parent to the
|
|
bos@559
|
456 changeset you're merging with, as shown in figure <xref
|
|
bos@559
|
457 linkend="fig:concepts:wdir-merge"/>.</para>
|
|
bos@559
|
458
|
|
bos@591
|
459 <figure id="fig:concepts:wdir-merge">
|
|
bos@591
|
460 <title>Merging two heads</title>
|
|
bos@591
|
461 <mediaobject>
|
|
bos@591
|
462 <imageobject>
|
|
bos@591
|
463 <imagedata fileref="wdir-merge"/>
|
|
bos@591
|
464 </imageobject>
|
|
bos@591
|
465 <textobject><phrase>XXX add text</phrase></textobject>
|
|
bos@591
|
466 </mediaobject>
|
|
bos@591
|
467 </figure>
|
|
bos@559
|
468
|
|
bos@584
|
469 <para id="x_319">Mercurial also has to modify the working directory, to
|
|
bos@559
|
470 merge the files managed in the two changesets. Simplified a
|
|
bos@559
|
471 little, the merging process goes like this, for every file in
|
|
bos@559
|
472 the manifests of both changesets.</para>
|
|
bos@559
|
473 <itemizedlist>
|
|
bos@584
|
474 <listitem><para id="x_31a">If neither changeset has modified a file, do
|
|
bos@559
|
475 nothing with that file.</para>
|
|
bos@559
|
476 </listitem>
|
|
bos@584
|
477 <listitem><para id="x_31b">If one changeset has modified a file, and the
|
|
bos@559
|
478 other hasn't, create the modified copy of the file in the
|
|
bos@559
|
479 working directory.</para>
|
|
bos@559
|
480 </listitem>
|
|
bos@584
|
481 <listitem><para id="x_31c">If one changeset has removed a file, and the
|
|
bos@559
|
482 other hasn't (or has also deleted it), delete the file
|
|
bos@559
|
483 from the working directory.</para>
|
|
bos@559
|
484 </listitem>
|
|
bos@584
|
485 <listitem><para id="x_31d">If one changeset has removed a file, but the
|
|
bos@559
|
486 other has modified the file, ask the user what to do: keep
|
|
bos@559
|
487 the modified file, or remove it?</para>
|
|
bos@559
|
488 </listitem>
|
|
bos@584
|
489 <listitem><para id="x_31e">If both changesets have modified a file,
|
|
bos@559
|
490 invoke an external merge program to choose the new
|
|
bos@559
|
491 contents for the merged file. This may require input from
|
|
bos@559
|
492 the user.</para>
|
|
bos@559
|
493 </listitem>
|
|
bos@584
|
494 <listitem><para id="x_31f">If one changeset has modified a file, and the
|
|
bos@559
|
495 other has renamed or copied the file, make sure that the
|
|
bos@559
|
496 changes follow the new name of the file.</para>
|
|
bos@559
|
497 </listitem></itemizedlist>
|
|
bos@584
|
498 <para id="x_320">There are more details&emdash;merging has plenty of corner
|
|
bos@559
|
499 cases&emdash;but these are the most common choices that are
|
|
bos@559
|
500 involved in a merge. As you can see, most cases are
|
|
bos@559
|
501 completely automatic, and indeed most merges finish
|
|
bos@559
|
502 automatically, without requiring your input to resolve any
|
|
bos@559
|
503 conflicts.</para>
|
|
bos@559
|
504
|
|
bos@584
|
505 <para id="x_321">When you're thinking about what happens when you commit
|
|
bos@559
|
506 after a merge, once again the working directory is <quote>the
|
|
bos@559
|
507 changeset I'm about to commit</quote>. After the <command
|
|
bos@559
|
508 role="hg-cmd">hg merge</command> command completes, the
|
|
bos@559
|
509 working directory has two parents; these will become the
|
|
bos@559
|
510 parents of the new changeset.</para>
|
|
bos@559
|
511
|
|
bos@584
|
512 <para id="x_322">Mercurial lets you perform multiple merges, but you must
|
|
bos@559
|
513 commit the results of each individual merge as you go. This
|
|
bos@559
|
514 is necessary because Mercurial only tracks two parents for
|
|
bos@559
|
515 both revisions and the working directory. While it would be
|
|
bos@559
|
516 technically possible to merge multiple changesets at once, the
|
|
bos@559
|
517 prospect of user confusion and making a terrible mess of a
|
|
bos@559
|
518 merge immediately becomes overwhelming.</para>
|
|
bos@559
|
519
|
|
bos@559
|
520 </sect2>
|
|
bos@559
|
521 </sect1>
|
|
bos@559
|
522 <sect1>
|
|
bos@559
|
523 <title>Other interesting design features</title>
|
|
bos@559
|
524
|
|
bos@584
|
525 <para id="x_323">In the sections above, I've tried to highlight some of the
|
|
bos@559
|
526 most important aspects of Mercurial's design, to illustrate that
|
|
bos@559
|
527 it pays careful attention to reliability and performance.
|
|
bos@559
|
528 However, the attention to detail doesn't stop there. There are
|
|
bos@559
|
529 a number of other aspects of Mercurial's construction that I
|
|
bos@559
|
530 personally find interesting. I'll detail a few of them here,
|
|
bos@559
|
531 separate from the <quote>big ticket</quote> items above, so that
|
|
bos@559
|
532 if you're interested, you can gain a better idea of the amount
|
|
bos@559
|
533 of thinking that goes into a well-designed system.</para>
|
|
bos@559
|
534
|
|
bos@559
|
535 <sect2>
|
|
bos@559
|
536 <title>Clever compression</title>
|
|
bos@559
|
537
|
|
bos@584
|
538 <para id="x_324">When appropriate, Mercurial will store both snapshots and
|
|
bos@559
|
539 deltas in compressed form. It does this by always
|
|
bos@559
|
540 <emphasis>trying to</emphasis> compress a snapshot or delta,
|
|
bos@559
|
541 but only storing the compressed version if it's smaller than
|
|
bos@559
|
542 the uncompressed version.</para>
|
|
bos@559
|
543
|
|
bos@584
|
544 <para id="x_325">This means that Mercurial does <quote>the right
|
|
bos@559
|
545 thing</quote> when storing a file whose native form is
|
|
bos@559
|
546 compressed, such as a <literal>zip</literal> archive or a JPEG
|
|
bos@559
|
547 image. When these types of files are compressed a second
|
|
bos@559
|
548 time, the resulting file is usually bigger than the
|
|
bos@559
|
549 once-compressed form, and so Mercurial will store the plain
|
|
bos@559
|
550 <literal>zip</literal> or JPEG.</para>
|
|
bos@559
|
551
|
|
bos@584
|
552 <para id="x_326">Deltas between revisions of a compressed file are usually
|
|
bos@559
|
553 larger than snapshots of the file, and Mercurial again does
|
|
bos@559
|
554 <quote>the right thing</quote> in these cases. It finds that
|
|
bos@559
|
555 such a delta exceeds the threshold at which it should store a
|
|
bos@559
|
556 complete snapshot of the file, so it stores the snapshot,
|
|
bos@559
|
557 again saving space compared to a naive delta-only
|
|
bos@559
|
558 approach.</para>
|
|
bos@559
|
559
|
|
bos@559
|
560 <sect3>
|
|
bos@559
|
561 <title>Network recompression</title>
|
|
bos@559
|
562
|
|
bos@584
|
563 <para id="x_327">When storing revisions on disk, Mercurial uses the
|
|
bos@559
|
564 <quote>deflate</quote> compression algorithm (the same one
|
|
bos@559
|
565 used by the popular <literal>zip</literal> archive format),
|
|
bos@559
|
566 which balances good speed with a respectable compression
|
|
bos@559
|
567 ratio. However, when transmitting revision data over a
|
|
bos@559
|
568 network connection, Mercurial uncompresses the compressed
|
|
bos@559
|
569 revision data.</para>
|
|
bos@559
|
570
|
|
bos@584
|
571 <para id="x_328">If the connection is over HTTP, Mercurial recompresses
|
|
bos@559
|
572 the entire stream of data using a compression algorithm that
|
|
bos@559
|
573 gives a better compression ratio (the Burrows-Wheeler
|
|
bos@559
|
574 algorithm from the widely used <literal>bzip2</literal>
|
|
bos@559
|
575 compression package). This combination of algorithm and
|
|
bos@559
|
576 compression of the entire stream (instead of a revision at a
|
|
bos@559
|
577 time) substantially reduces the number of bytes to be
|
|
bos@559
|
578 transferred, yielding better network performance over almost
|
|
bos@559
|
579 all kinds of network.</para>
|
|
bos@559
|
580
|
|
bos@584
|
581 <para id="x_329">(If the connection is over <command>ssh</command>,
|
|
bos@559
|
582 Mercurial <emphasis>doesn't</emphasis> recompress the
|
|
bos@559
|
583 stream, because <command>ssh</command> can already do this
|
|
bos@559
|
584 itself.)</para>
|
|
bos@559
|
585
|
|
bos@559
|
586 </sect3>
|
|
bos@559
|
587 </sect2>
|
|
bos@559
|
588 <sect2>
|
|
bos@559
|
589 <title>Read/write ordering and atomicity</title>
|
|
bos@559
|
590
|
|
bos@584
|
591 <para id="x_32a">Appending to files isn't the whole story when it comes to
|
|
bos@559
|
592 guaranteeing that a reader won't see a partial write. If you
|
|
bos@559
|
593 recall figure <xref linkend="fig:concepts:metadata"/>,
|
|
bos@559
|
594 revisions in the
|
|
bos@559
|
595 changelog point to revisions in the manifest, and revisions in
|
|
bos@559
|
596 the manifest point to revisions in filelogs. This hierarchy
|
|
bos@559
|
597 is deliberate.</para>
|
|
bos@559
|
598
|
|
bos@584
|
599 <para id="x_32b">A writer starts a transaction by writing filelog and
|
|
bos@559
|
600 manifest data, and doesn't write any changelog data until
|
|
bos@559
|
601 those are finished. A reader starts by reading changelog
|
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602 data, then manifest data, followed by filelog data.</para>
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603
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bos@584
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604 <para id="x_32c">Since the writer has always finished writing filelog and
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bos@559
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605 manifest data before it writes to the changelog, a reader will
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bos@559
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606 never read a pointer to a partially written manifest revision
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bos@559
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607 from the changelog, and it will never read a pointer to a
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608 partially written filelog revision from the manifest.</para>
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609
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bos@559
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610 </sect2>
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bos@559
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611 <sect2>
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bos@559
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612 <title>Concurrent access</title>
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613
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bos@584
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614 <para id="x_32d">The read/write ordering and atomicity guarantees mean that
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bos@559
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615 Mercurial never needs to <emphasis>lock</emphasis> a
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616 repository when it's reading data, even if the repository is
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617 being written to while the read is occurring. This has a big
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618 effect on scalability; you can have an arbitrary number of
|
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bos@559
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619 Mercurial processes safely reading data from a repository
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bos@559
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620 safely all at once, no matter whether it's being written to or
|
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bos@559
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621 not.</para>
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622
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bos@584
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623 <para id="x_32e">The lockless nature of reading means that if you're
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bos@559
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624 sharing a repository on a multi-user system, you don't need to
|
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bos@559
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625 grant other local users permission to
|
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bos@559
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626 <emphasis>write</emphasis> to your repository in order for
|
|
bos@559
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627 them to be able to clone it or pull changes from it; they only
|
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bos@559
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628 need <emphasis>read</emphasis> permission. (This is
|
|
bos@559
|
629 <emphasis>not</emphasis> a common feature among revision
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630 control systems, so don't take it for granted! Most require
|
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bos@559
|
631 readers to be able to lock a repository to access it safely,
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bos@559
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632 and this requires write permission on at least one directory,
|
|
bos@559
|
633 which of course makes for all kinds of nasty and annoying
|
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bos@559
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634 security and administrative problems.)</para>
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635
|
|
bos@584
|
636 <para id="x_32f">Mercurial uses locks to ensure that only one process can
|
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bos@559
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637 write to a repository at a time (the locking mechanism is safe
|
|
bos@559
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638 even over filesystems that are notoriously hostile to locking,
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bos@559
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639 such as NFS). If a repository is locked, a writer will wait
|
|
bos@559
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640 for a while to retry if the repository becomes unlocked, but
|
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bos@559
|
641 if the repository remains locked for too long, the process
|
|
bos@559
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642 attempting to write will time out after a while. This means
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bos@559
|
643 that your daily automated scripts won't get stuck forever and
|
|
bos@559
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644 pile up if a system crashes unnoticed, for example. (Yes, the
|
|
bos@559
|
645 timeout is configurable, from zero to infinity.)</para>
|
|
bos@559
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646
|
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bos@559
|
647 <sect3>
|
|
bos@559
|
648 <title>Safe dirstate access</title>
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649
|
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bos@584
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650 <para id="x_330">As with revision data, Mercurial doesn't take a lock to
|
|
bos@559
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651 read the dirstate file; it does acquire a lock to write it.
|
|
bos@559
|
652 To avoid the possibility of reading a partially written copy
|
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bos@559
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653 of the dirstate file, Mercurial writes to a file with a
|
|
bos@559
|
654 unique name in the same directory as the dirstate file, then
|
|
bos@559
|
655 renames the temporary file atomically to
|
|
bos@559
|
656 <filename>dirstate</filename>. The file named
|
|
bos@559
|
657 <filename>dirstate</filename> is thus guaranteed to be
|
|
bos@559
|
658 complete, not partially written.</para>
|
|
bos@559
|
659
|
|
bos@559
|
660 </sect3>
|
|
bos@559
|
661 </sect2>
|
|
bos@559
|
662 <sect2>
|
|
bos@559
|
663 <title>Avoiding seeks</title>
|
|
bos@559
|
664
|
|
bos@584
|
665 <para id="x_331">Critical to Mercurial's performance is the avoidance of
|
|
bos@559
|
666 seeks of the disk head, since any seek is far more expensive
|
|
bos@559
|
667 than even a comparatively large read operation.</para>
|
|
bos@559
|
668
|
|
bos@584
|
669 <para id="x_332">This is why, for example, the dirstate is stored in a
|
|
bos@559
|
670 single file. If there were a dirstate file per directory that
|
|
bos@559
|
671 Mercurial tracked, the disk would seek once per directory.
|
|
bos@559
|
672 Instead, Mercurial reads the entire single dirstate file in
|
|
bos@559
|
673 one step.</para>
|
|
bos@559
|
674
|
|
bos@584
|
675 <para id="x_333">Mercurial also uses a <quote>copy on write</quote> scheme
|
|
bos@559
|
676 when cloning a repository on local storage. Instead of
|
|
bos@559
|
677 copying every revlog file from the old repository into the new
|
|
bos@559
|
678 repository, it makes a <quote>hard link</quote>, which is a
|
|
bos@559
|
679 shorthand way to say <quote>these two names point to the same
|
|
bos@559
|
680 file</quote>. When Mercurial is about to write to one of a
|
|
bos@559
|
681 revlog's files, it checks to see if the number of names
|
|
bos@559
|
682 pointing at the file is greater than one. If it is, more than
|
|
bos@559
|
683 one repository is using the file, so Mercurial makes a new
|
|
bos@559
|
684 copy of the file that is private to this repository.</para>
|
|
bos@559
|
685
|
|
bos@584
|
686 <para id="x_334">A few revision control developers have pointed out that
|
|
bos@559
|
687 this idea of making a complete private copy of a file is not
|
|
bos@559
|
688 very efficient in its use of storage. While this is true,
|
|
bos@559
|
689 storage is cheap, and this method gives the highest
|
|
bos@559
|
690 performance while deferring most book-keeping to the operating
|
|
bos@559
|
691 system. An alternative scheme would most likely reduce
|
|
bos@559
|
692 performance and increase the complexity of the software, each
|
|
bos@559
|
693 of which is much more important to the <quote>feel</quote> of
|
|
bos@559
|
694 day-to-day use.</para>
|
|
bos@559
|
695
|
|
bos@559
|
696 </sect2>
|
|
bos@559
|
697 <sect2>
|
|
bos@559
|
698 <title>Other contents of the dirstate</title>
|
|
bos@559
|
699
|
|
bos@584
|
700 <para id="x_335">Because Mercurial doesn't force you to tell it when you're
|
|
bos@559
|
701 modifying a file, it uses the dirstate to store some extra
|
|
bos@559
|
702 information so it can determine efficiently whether you have
|
|
bos@559
|
703 modified a file. For each file in the working directory, it
|
|
bos@559
|
704 stores the time that it last modified the file itself, and the
|
|
bos@559
|
705 size of the file at that time.</para>
|
|
bos@559
|
706
|
|
bos@584
|
707 <para id="x_336">When you explicitly <command role="hg-cmd">hg
|
|
bos@559
|
708 add</command>, <command role="hg-cmd">hg remove</command>,
|
|
bos@559
|
709 <command role="hg-cmd">hg rename</command> or <command
|
|
bos@559
|
710 role="hg-cmd">hg copy</command> files, Mercurial updates the
|
|
bos@559
|
711 dirstate so that it knows what to do with those files when you
|
|
bos@559
|
712 commit.</para>
|
|
bos@559
|
713
|
|
bos@584
|
714 <para id="x_337">When Mercurial is checking the states of files in the
|
|
bos@559
|
715 working directory, it first checks a file's modification time.
|
|
bos@559
|
716 If that has not changed, the file must not have been modified.
|
|
bos@559
|
717 If the file's size has changed, the file must have been
|
|
bos@559
|
718 modified. If the modification time has changed, but the size
|
|
bos@559
|
719 has not, only then does Mercurial need to read the actual
|
|
bos@559
|
720 contents of the file to see if they've changed. Storing these
|
|
bos@559
|
721 few extra pieces of information dramatically reduces the
|
|
bos@559
|
722 amount of data that Mercurial needs to read, which yields
|
|
bos@559
|
723 large performance improvements compared to other revision
|
|
bos@559
|
724 control systems.</para>
|
|
bos@559
|
725
|
|
bos@559
|
726 </sect2>
|
|
bos@559
|
727 </sect1>
|
|
bos@559
|
728 </chapter>
|
|
bos@559
|
729
|
|
bos@559
|
730 <!--
|
|
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|
731 local variables:
|
|
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|
732 sgml-parent-document: ("00book.xml" "book" "chapter")
|
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|
733 end:
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734 -->
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