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1 \chapter{Collaborating with other people}
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2 \label{cha:collab}
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3
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4 As a completely decentralised tool, Mercurial doesn't impose any
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5 policy on how people ought to work with each other. However, if
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6 you're new to distributed revision control, it helps to have some
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7 tools and examples in mind when you're thinking about possible
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8 workflow models.
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9
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10 \section{Collaboration models}
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11
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12 With a suitably flexible tool, making decisions about workflow is much
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13 more of a social engineering challenge than a technical one.
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14 Mercurial imposes few limitations on how you can structure the flow of
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15 work in a project, so it's up to you and your group to set up and live
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16 with a model that matches your own particular needs.
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17
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18 \subsection{Factors to keep in mind}
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19
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20 The most important aspect of any model that you must keep in mind is
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21 how well it matches the needs and capabilities of the people who will
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22 be using it. This might seem self-evident; even so, you still can't
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23 afford to forget it for a moment.
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24
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25 I once put together a workflow model that seemed to make perfect sense
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26 to me, but that caused a considerable amount of consternation and
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27 strife within my development team. In spite of my attempts to explain
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28 why we needed a complex set of branches, and how changes ought to flow
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29 between them, a few team members revolted. Even though they were
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30 smart people, they didn't want to pay attention to the constraints we
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31 were operating under, or face the consequences of those constraints in
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32 the details of the model that I was advocating.
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33
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34 Don't sweep foreseeable social or technical problems under the rug.
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35 Whatever scheme you put into effect, you should plan for mistakes and
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36 problem scenarios. Consider adding automated machinery to prevent, or
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37 quickly recover from, trouble that you can anticipate. As an example,
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38 if you intend to have a branch with not-for-release changes in it,
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39 you'd do well to think early about the possibility that someone might
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40 accidentally merge those changes into a release branch. You could
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41 avoid this particular problem by writing a hook that prevents changes
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42 from being merged from an inappropriate branch.
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43
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44 \subsection{Informal anarchy}
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45
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46 I wouldn't suggest an ``anything goes'' approach as something
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47 sustainable, but it's a model that's easy to grasp, and it works
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48 perfectly well in a few unusual situations.
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49
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50 As one example, many projects have a loose-knit group of collaborators
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51 who rarely physically meet each other. Some groups like to overcome
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52 the isolation of working at a distance by organising occasional
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53 ``sprints''. In a sprint, a number of people get together in a single
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54 location (a company's conference room, a hotel meeting room, that kind
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55 of place) and spend several days more or less locked in there, hacking
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56 intensely on a handful of projects.
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57
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58 A sprint is the perfect place to use the \hgcmd{serve} command, since
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59 \hgcmd{serve} does not requires any fancy server infrastructure. You
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60 can get started with \hgcmd{serve} in moments, by reading
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61 section~\ref{sec:collab:serve} below. Then simply tell the person
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62 next to you that you're running a server, send the URL to them in an
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63 instant message, and you immediately have a quick-turnaround way to
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64 work together. They can type your URL into their web browser and
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65 quickly review your changes; or they can pull a bugfix from you and
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66 verify it; or they can clone a branch containing a new feature and try
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67 it out.
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68
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69 The charm, and the problem, with doing things in an ad hoc fashion
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70 like this is that only people who know about your changes, and where
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71 they are, can see them. Such an informal approach simply doesn't
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72 scale beyond a handful people, because each individual needs to know
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73 about $n$ different repositories to pull from.
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74
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75 \subsection{A single central repository}
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76
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77 For smaller projects migrating from a centralised revision control
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78 tool, perhaps the easiest way to get started is to have changes flow
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79 through a single shared central repository. This is also the
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80 most common ``building block'' for more ambitious workflow schemes.
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81
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82 Contributors start by cloning a copy of this repository. They can
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83 pull changes from it whenever they need to, and some (perhaps all)
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84 developers have permission to push a change back when they're ready
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85 for other people to see it.
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86
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87 Under this model, it can still often make sense for people to pull
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88 changes directly from each other, without going through the central
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89 repository. Consider a case in which I have a tentative bug fix, but
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90 I am worried that if I were to publish it to the central repository,
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91 it might subsequently break everyone else's trees as they pull it. To
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92 reduce the potential for damage, I can ask you to clone my repository
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93 into a temporary repository of your own and test it. This lets us put
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94 off publishing the potentially unsafe change until it has had a little
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95 testing.
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96
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97 In this kind of scenario, people usually use the \command{ssh}
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98 protocol to securely push changes to the central repository, as
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99 documented in section~\ref{sec:collab:ssh}. It's also usual to
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100 publish a read-only copy of the repository over HTTP using CGI, as in
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101 section~\ref{sec:collab:cgi}. Publishing over HTTP satisfies the
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102 needs of people who don't have push access, and those who want to use
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103 web browsers to browse the repository's history.
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104
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105 \subsection{Working with multiple branches}
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106
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107 Projects of any significant size naturally tend to make progress on
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108 several fronts simultaneously. In the case of software, it's common
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109 for a project to go through periodic official releases. A release
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110 might then go into ``maintenance mode'' for a while after its first
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111 publication; maintenance releases tend to contain only bug fixes, not
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112 new features. In parallel with these maintenance releases, one or
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113 more future releases may be under development. People normally use
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114 the word ``branch'' to refer to one of these many slightly different
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115 directions in which development is proceeding.
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116
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117 Mercurial is particularly well suited to managing a number of
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118 simultaneous, but not identical, branches. Each ``development
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119 direction'' can live in its own central repository, and you can merge
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120 changes from one to another as the need arises. Because repositories
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121 are independent of each other, unstable changes in a development
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122 branch will never affect a stable branch unless someone explicitly
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123 merges those changes in.
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124
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125 Here's an example of how this can work in practice. Let's say you
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126 have one ``main branch'' on a central server.
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127 \interaction{branching.init}
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128 People clone it, make changes locally, test them, and push them back.
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129
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130 Once the main branch reaches a release milestone, you can use the
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131 \hgcmd{tag} command to give a permanent name to the milestone
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132 revision.
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133 \interaction{branching.tag}
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134 Let's say some ongoing development occurs on the main branch.
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135 \interaction{branching.main}
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136 Using the tag that was recorded at the milestone, people who clone
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137 that repository at any time in the future can use \hgcmd{update} to
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138 get a copy of the working directory exactly as it was when that tagged
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139 revision was committed.
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140 \interaction{branching.update}
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141
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142 In addition, immediately after the main branch is tagged, someone can
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143 then clone the main branch on the server to a new ``stable'' branch,
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144 also on the server.
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145 \interaction{branching.clone}
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146
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147 Someone who needs to make a change to the stable branch can then clone
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148 \emph{that} repository, make their changes, commit, and push their
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149 changes back there.
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150 \interaction{branching.stable}
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151 Because Mercurial repositories are independent, and Mercurial doesn't
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152 move changes around automatically, the stable and main branches are
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153 \emph{isolated} from each other. The changes that you made on the
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154 main branch don't ``leak'' to the stable branch, and vice versa.
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155
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156 You'll often want all of your bugfixes on the stable branch to show up
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157 on the main branch, too. Rather than rewrite a bugfix on the main
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158 branch, you can simply pull and merge changes from the stable to the
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159 main branch, and Mercurial will bring those bugfixes in for you.
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160 \interaction{branching.merge}
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161 The main branch will still contain changes that are not on the stable
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162 branch, but it will also contain all of the bugfixes from the stable
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163 branch. The stable branch remains unaffected by these changes.
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164
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165 \subsection{Feature branches}
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166
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167 For larger projects, an effective way to manage change is to break up
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168 a team into smaller groups. Each group has a shared branch of its
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169 own, cloned from a single ``master'' branch used by the entire
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170 project. People working on an individual branch are typically quite
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171 isolated from developments on other branches.
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172
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173 \begin{figure}[ht]
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174 \centering
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175 \grafix{feature-branches}
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176 \caption{Feature branches}
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177 \label{fig:collab:feature-branches}
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178 \end{figure}
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179
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180 When a particular feature is deemed to be in suitable shape, someone
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181 on that feature team pulls and merges from the master branch into the
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182 feature branch, then pushes back up to the master branch.
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183
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184 \subsection{The release train}
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185
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186 Some projects are organised on a ``train'' basis: a release is
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187 scheduled to happen every few months, and whatever features are ready
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188 when the ``train'' is ready to leave are allowed in.
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189
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190 This model resembles working with feature branches. The difference is
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191 that when a feature branch misses a train, someone on the feature team
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192 pulls and merges the changes that went out on that train release, and
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193 the team continues its work on top of that release so that their
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194 feature can make the next release.
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195
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196 \subsection{The Linux kernel model}
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197
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198 The development of the Linux kernel has a shallow hierarchical
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199 structure, surrounded by a cloud of apparent chaos. Because most
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200 Linux developers use \command{git}, a distributed revision control
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201 tool with capabilities similar to Mercurial, it's useful to describe
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202 the way work flows in that environment; if you like the ideas, the
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203 approach translates well across tools.
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204
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205 At the center of the community sits Linus Torvalds, the creator of
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206 Linux. He publishes a single source repository that is considered the
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207 ``authoritative'' current tree by the entire developer community.
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208 Anyone can clone Linus's tree, but he is very choosy about whose trees
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209 he pulls from.
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210
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211 Linus has a number of ``trusted lieutenants''. As a general rule, he
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212 pulls whatever changes they publish, in most cases without even
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213 reviewing those changes. Some of those lieutenants are generally
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214 agreed to be ``maintainers'', responsible for specific subsystems
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215 within the kernel. If a random kernel hacker wants to make a change
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216 to a subsystem that they want to end up in Linus's tree, they must
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217 find out who the subsystem's maintainer is, and ask that maintainer to
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218 take their change. If the maintainer reviews their changes and agrees
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219 to take them, they'll pass them along to Linus in due course.
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220
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221 Individual lieutenants have their own approaches to reviewing,
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222 accepting, and publishing changes; and for deciding when to feed them
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223 to Linus. In addition, there are several well known branches that
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224 people use for different purposes. For example, a few people maintain
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225 ``stable'' repositories of older versions of the kernel, to which they
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226 apply critical fixes as needed.
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227
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228 This model has two notable features. The first is that it's ``pull
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229 only''. You have to ask, convince, or beg another developer to take a
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230 change from you, because there are no shared trees, and there's no way
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231 to push changes into a tree that someone else controls.
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232
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233 The second is that it's based on reputation and acclaim. If you're an
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234 unknown, Linus will probably ignore changes from you without even
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235 responding. But a subsystem maintainer will probably review them, and
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236 will likely take them if they pass their criteria for suitability.
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237 The more ``good'' changes you contribute to a maintainer, the more
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238 likely they are to trust your judgment and accept your changes. If
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239 you're well-known and maintain a long-lived branch for something Linus
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240 hasn't yet accepted, people with similar interests may pull your
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241 changes regularly to keep up with your work.
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242
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243 Reputation and acclaim don't necessarily cross subsystem or ``people''
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244 boundaries. If you're a respected but specialised storage hacker, and
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245 you try to fix a networking bug, that change will receive a level of
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246 scrutiny from a network maintainer comparable to a change from a
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247 complete stranger.
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248
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249 To people who come from more orderly project backgrounds, the
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250 comparatively chaotic Linux kernel development process often seems
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251 completely insane. It's subject to the whims of individuals; people
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252 make sweeping changes whenever they deem it appropriate; and the pace
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253 of development is astounding. And yet Linux is a highly successful,
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254 well-regarded piece of software.
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255
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256 \section{The technical side of sharing}
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257
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258 \subsection{Informal sharing with \hgcmd{serve}}
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259 \label{sec:collab:serve}
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260
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261 Mercurial's \hgcmd{serve} command is wonderfully suited to small,
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262 tight-knit, and fast-paced group environments. It also provides a
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263 great way to get a feel for using Mercurial commands over a network.
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264
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265 Run \hgcmd{serve} inside a repository, and in under a second it will
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266 bring up a specialised HTTP server; this will accept connections from
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267 any client, and serve up data for that repository until you terminate
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268 it. Anyone who knows the URL of the server you just started, and can
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269 talk to your computer over the network, can then use a web browser or
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270 Mercurial to read data from that repository. A URL for a
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271 \hgcmd{serve} instance running on a laptop is likely to look something
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272 like \Verb|http://my-laptop.local:8000/|.
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273
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274 The \hgcmd{serve} command is \emph{not} a general-purpose web server.
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275 It can do only two things:
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276 \begin{itemize}
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277 \item Allow people to browse the history of the repository it's
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278 serving, from their normal web browsers.
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279 \item Speak Mercurial's wire protocol, so that people can
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280 \hgcmd{clone} or \hgcmd{pull} changes from that repository.
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281 \end{itemize}
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282 In particular, \hgcmd{serve} won't allow remote users to \emph{modify}
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283 your repository. It's intended for read-only use.
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284
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285 If you're getting started with Mercurial, there's nothing to prevent
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286 you from using \hgcmd{serve} to serve up a repository on your own
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287 computer, then use commands like \hgcmd{clone}, \hgcmd{incoming}, and
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288 so on to talk to that server as if the repository was hosted remotely.
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289 This can help you to quickly get acquainted with using commands on
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290 network-hosted repositories.
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291
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292 \subsubsection{A few things to keep in mind}
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293
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294 Because it provides unauthenticated read access to all clients, you
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295 should only use \hgcmd{serve} in an environment where you either don't
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296 care, or have complete control over, who can access your network and
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297 pull data from your repository.
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298
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299 The \hgcmd{serve} command knows nothing about any firewall software
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300 you might have installed on your system or network. It cannot detect
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301 or control your firewall software. If other people are unable to talk
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302 to a running \hgcmd{serve} instance, the second thing you should do
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303 (\emph{after} you make sure that they're using the correct URL) is
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304 check your firewall configuration.
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305
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306 By default, \hgcmd{serve} listens for incoming connections on
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307 port~8000. If another process is already listening on the port you
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308 want to use, you can specify a different port to listen on using the
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309 \hgopt{serve}{-p} option.
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310
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311 Normally, when \hgcmd{serve} starts, it prints no output, which can be
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312 a bit unnerving. If you'd like to confirm that it is indeed running
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313 correctly, and find out what URL you should send to your
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314 collaborators, start it with the \hggopt{-v} option.
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315
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316 \subsection{Using \command{ssh} as a tunnel}
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317 \label{sec:collab:ssh}
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318
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319 \subsection{Serving HTTP with a CGI script}
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320 \label{sec:collab:cgi}
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321
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322
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323
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324 %%% Local Variables:
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325 %%% mode: latex
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326 %%% TeX-master: "00book"
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327 %%% End:
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