rev |
line source |
bos@559
|
1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
|
bos@26
|
2
|
bos@559
|
3 <preface id="chap:preface">
|
bos@559
|
4 <title>Preface</title>
|
bos@26
|
5
|
bos@583
|
6 <sect1>
|
bos@583
|
7 <title>Why revision control? Why Mercurial?</title>
|
bos@583
|
8
|
bos@583
|
9 <para>Revision control is the process of managing multiple
|
bos@583
|
10 versions of a piece of information. In its simplest form, this
|
bos@583
|
11 is something that many people do by hand: every time you modify
|
bos@583
|
12 a file, save it under a new name that contains a number, each
|
bos@583
|
13 one higher than the number of the preceding version.</para>
|
bos@583
|
14
|
bos@583
|
15 <para>Manually managing multiple versions of even a single file is
|
bos@583
|
16 an error-prone task, though, so software tools to help automate
|
bos@583
|
17 this process have long been available. The earliest automated
|
bos@583
|
18 revision control tools were intended to help a single user to
|
bos@583
|
19 manage revisions of a single file. Over the past few decades,
|
bos@583
|
20 the scope of revision control tools has expanded greatly; they
|
bos@583
|
21 now manage multiple files, and help multiple people to work
|
bos@583
|
22 together. The best modern revision control tools have no
|
bos@583
|
23 problem coping with thousands of people working together on
|
bos@583
|
24 projects that consist of hundreds of thousands of files.</para>
|
bos@583
|
25
|
bos@583
|
26 <para>The arrival of distributed revision control is relatively
|
bos@583
|
27 recent, and so far this new field has grown due to people's
|
bos@583
|
28 willingness to explore ill-charted territory.</para>
|
bos@583
|
29
|
bos@583
|
30 <para>I am writing a book about distributed revision control
|
bos@583
|
31 because I believe that it is an important subject that deserves
|
bos@583
|
32 a field guide. I chose to write about Mercurial because it is
|
bos@583
|
33 the easiest tool to learn the terrain with, and yet it scales to
|
bos@583
|
34 the demands of real, challenging environments where many other
|
bos@583
|
35 revision control tools buckle.</para>
|
bos@583
|
36
|
bos@583
|
37 <sect2>
|
bos@583
|
38 <title>Why use revision control?</title>
|
bos@583
|
39
|
bos@583
|
40 <para>There are a number of reasons why you or your team might
|
bos@583
|
41 want to use an automated revision control tool for a
|
bos@583
|
42 project.</para>
|
bos@583
|
43
|
bos@583
|
44 <itemizedlist>
|
bos@583
|
45 <listitem><para>It will track the history and evolution of
|
bos@583
|
46 your project, so you don't have to. For every change,
|
bos@583
|
47 you'll have a log of <emphasis>who</emphasis> made it;
|
bos@583
|
48 <emphasis>why</emphasis> they made it;
|
bos@583
|
49 <emphasis>when</emphasis> they made it; and
|
bos@583
|
50 <emphasis>what</emphasis> the change
|
bos@583
|
51 was.</para></listitem>
|
bos@583
|
52 <listitem><para>When you're working with other people,
|
bos@583
|
53 revision control software makes it easier for you to
|
bos@583
|
54 collaborate. For example, when people more or less
|
bos@583
|
55 simultaneously make potentially incompatible changes, the
|
bos@583
|
56 software will help you to identify and resolve those
|
bos@583
|
57 conflicts.</para></listitem>
|
bos@583
|
58 <listitem><para>It can help you to recover from mistakes. If
|
bos@583
|
59 you make a change that later turns out to be in error, you
|
bos@583
|
60 can revert to an earlier version of one or more files. In
|
bos@583
|
61 fact, a <emphasis>really</emphasis> good revision control
|
bos@583
|
62 tool will even help you to efficiently figure out exactly
|
bos@583
|
63 when a problem was introduced (see section <xref
|
bos@583
|
64 linkend="sec:undo:bisect"/> for details).</para></listitem>
|
bos@583
|
65 <listitem><para>It will help you to work simultaneously on,
|
bos@583
|
66 and manage the drift between, multiple versions of your
|
bos@583
|
67 project.</para></listitem>
|
bos@583
|
68 </itemizedlist>
|
bos@583
|
69
|
bos@583
|
70 <para>Most of these reasons are equally valid---at least in
|
bos@583
|
71 theory---whether you're working on a project by yourself, or
|
bos@583
|
72 with a hundred other people.</para>
|
bos@583
|
73
|
bos@583
|
74 <para>A key question about the practicality of revision control
|
bos@583
|
75 at these two different scales (<quote>lone hacker</quote> and
|
bos@583
|
76 <quote>huge team</quote>) is how its
|
bos@583
|
77 <emphasis>benefits</emphasis> compare to its
|
bos@583
|
78 <emphasis>costs</emphasis>. A revision control tool that's
|
bos@583
|
79 difficult to understand or use is going to impose a high
|
bos@583
|
80 cost.</para>
|
bos@583
|
81
|
bos@583
|
82 <para>A five-hundred-person project is likely to collapse under
|
bos@583
|
83 its own weight almost immediately without a revision control
|
bos@583
|
84 tool and process. In this case, the cost of using revision
|
bos@583
|
85 control might hardly seem worth considering, since
|
bos@583
|
86 <emphasis>without</emphasis> it, failure is almost
|
bos@583
|
87 guaranteed.</para>
|
bos@583
|
88
|
bos@583
|
89 <para>On the other hand, a one-person <quote>quick hack</quote>
|
bos@583
|
90 might seem like a poor place to use a revision control tool,
|
bos@583
|
91 because surely the cost of using one must be close to the
|
bos@583
|
92 overall cost of the project. Right?</para>
|
bos@583
|
93
|
bos@583
|
94 <para>Mercurial uniquely supports <emphasis>both</emphasis> of
|
bos@583
|
95 these scales of development. You can learn the basics in just
|
bos@583
|
96 a few minutes, and due to its low overhead, you can apply
|
bos@583
|
97 revision control to the smallest of projects with ease. Its
|
bos@583
|
98 simplicity means you won't have a lot of abstruse concepts or
|
bos@583
|
99 command sequences competing for mental space with whatever
|
bos@583
|
100 you're <emphasis>really</emphasis> trying to do. At the same
|
bos@583
|
101 time, Mercurial's high performance and peer-to-peer nature let
|
bos@583
|
102 you scale painlessly to handle large projects.</para>
|
bos@583
|
103
|
bos@583
|
104 <para>No revision control tool can rescue a poorly run project,
|
bos@583
|
105 but a good choice of tools can make a huge difference to the
|
bos@583
|
106 fluidity with which you can work on a project.</para>
|
bos@583
|
107
|
bos@583
|
108 </sect2>
|
bos@583
|
109
|
bos@583
|
110 <sect2>
|
bos@583
|
111 <title>The many names of revision control</title>
|
bos@583
|
112
|
bos@583
|
113 <para>Revision control is a diverse field, so much so that it is
|
bos@583
|
114 referred to by many names and acronyms. Here are a few of the
|
bos@583
|
115 more common variations you'll encounter:</para>
|
bos@583
|
116 <itemizedlist>
|
bos@583
|
117 <listitem><para>Revision control (RCS)</para></listitem>
|
bos@583
|
118 <listitem><para>Software configuration management (SCM), or
|
bos@583
|
119 configuration management</para></listitem>
|
bos@583
|
120 <listitem><para>Source code management</para></listitem>
|
bos@583
|
121 <listitem><para>Source code control, or source
|
bos@583
|
122 control</para></listitem>
|
bos@583
|
123 <listitem><para>Version control
|
bos@583
|
124 (VCS)</para></listitem></itemizedlist>
|
bos@583
|
125 <para>Some people claim that these terms actually have different
|
bos@583
|
126 meanings, but in practice they overlap so much that there's no
|
bos@583
|
127 agreed or even useful way to tease them apart.</para>
|
bos@583
|
128
|
bos@583
|
129 </sect2>
|
bos@583
|
130 </sect1>
|
bos@26
|
131
|
bos@559
|
132 <sect1>
|
bos@559
|
133 <title>This book is a work in progress</title>
|
bos@26
|
134
|
bos@559
|
135 <para>I am releasing this book while I am still writing it, in the
|
bos@583
|
136 hope that it will prove useful to others. I am writing under an
|
bos@583
|
137 open license in the hope that you, my readers, will contribute
|
bos@583
|
138 feedback and perhaps content of your own.</para>
|
bos@200
|
139
|
bos@559
|
140 </sect1>
|
bos@559
|
141 <sect1>
|
bos@559
|
142 <title>About the examples in this book</title>
|
bos@200
|
143
|
bos@559
|
144 <para>This book takes an unusual approach to code samples. Every
|
bos@559
|
145 example is <quote>live</quote>---each one is actually the result
|
bos@559
|
146 of a shell script that executes the Mercurial commands you see.
|
bos@559
|
147 Every time an image of the book is built from its sources, all
|
bos@559
|
148 the example scripts are automatically run, and their current
|
bos@559
|
149 results compared against their expected results.</para>
|
bos@200
|
150
|
bos@559
|
151 <para>The advantage of this approach is that the examples are
|
bos@559
|
152 always accurate; they describe <emphasis>exactly</emphasis> the
|
bos@559
|
153 behaviour of the version of Mercurial that's mentioned at the
|
bos@559
|
154 front of the book. If I update the version of Mercurial that
|
bos@559
|
155 I'm documenting, and the output of some command changes, the
|
bos@559
|
156 build fails.</para>
|
bos@200
|
157
|
bos@559
|
158 <para>There is a small disadvantage to this approach, which is
|
bos@559
|
159 that the dates and times you'll see in examples tend to be
|
bos@559
|
160 <quote>squashed</quote> together in a way that they wouldn't be
|
bos@559
|
161 if the same commands were being typed by a human. Where a human
|
bos@559
|
162 can issue no more than one command every few seconds, with any
|
bos@559
|
163 resulting timestamps correspondingly spread out, my automated
|
bos@559
|
164 example scripts run many commands in one second.</para>
|
bos@200
|
165
|
bos@559
|
166 <para>As an instance of this, several consecutive commits in an
|
bos@559
|
167 example can show up as having occurred during the same second.
|
bos@559
|
168 You can see this occur in the <literal
|
bos@559
|
169 role="hg-ext">bisect</literal> example in section <xref
|
bos@559
|
170 id="sec:undo:bisect"/>, for instance.</para>
|
bos@200
|
171
|
bos@559
|
172 <para>So when you're reading examples, don't place too much weight
|
bos@559
|
173 on the dates or times you see in the output of commands. But
|
bos@559
|
174 <emphasis>do</emphasis> be confident that the behaviour you're
|
bos@559
|
175 seeing is consistent and reproducible.</para>
|
bos@26
|
176
|
bos@559
|
177 </sect1>
|
bos@583
|
178
|
bos@583
|
179 <sect1>
|
bos@583
|
180 <title>Trends in the field</title>
|
bos@583
|
181
|
bos@583
|
182 <para>There has been an unmistakable trend in the development and
|
bos@583
|
183 use of revision control tools over the past four decades, as
|
bos@583
|
184 people have become familiar with the capabilities of their tools
|
bos@583
|
185 and constrained by their limitations.</para>
|
bos@583
|
186
|
bos@583
|
187 <para>The first generation began by managing single files on
|
bos@583
|
188 individual computers. Although these tools represented a huge
|
bos@583
|
189 advance over ad-hoc manual revision control, their locking model
|
bos@583
|
190 and reliance on a single computer limited them to small,
|
bos@583
|
191 tightly-knit teams.</para>
|
bos@583
|
192
|
bos@583
|
193 <para>The second generation loosened these constraints by moving
|
bos@583
|
194 to network-centered architectures, and managing entire projects
|
bos@583
|
195 at a time. As projects grew larger, they ran into new problems.
|
bos@583
|
196 With clients needing to talk to servers very frequently, server
|
bos@583
|
197 scaling became an issue for large projects. An unreliable
|
bos@583
|
198 network connection could prevent remote users from being able to
|
bos@583
|
199 talk to the server at all. As open source projects started
|
bos@583
|
200 making read-only access available anonymously to anyone, people
|
bos@583
|
201 without commit privileges found that they could not use the
|
bos@583
|
202 tools to interact with a project in a natural way, as they could
|
bos@583
|
203 not record their changes.</para>
|
bos@583
|
204
|
bos@583
|
205 <para>The current generation of revision control tools is
|
bos@583
|
206 peer-to-peer in nature. All of these systems have dropped the
|
bos@583
|
207 dependency on a single central server, and allow people to
|
bos@583
|
208 distribute their revision control data to where it's actually
|
bos@583
|
209 needed. Collaboration over the Internet has moved from
|
bos@583
|
210 constrained by technology to a matter of choice and consensus.
|
bos@583
|
211 Modern tools can operate offline indefinitely and autonomously,
|
bos@583
|
212 with a network connection only needed when syncing changes with
|
bos@583
|
213 another repository.</para>
|
bos@583
|
214
|
bos@583
|
215 </sect1>
|
bos@583
|
216 <sect1>
|
bos@583
|
217 <title>A few of the advantages of distributed revision
|
bos@583
|
218 control</title>
|
bos@583
|
219
|
bos@583
|
220 <para>Even though distributed revision control tools have for
|
bos@583
|
221 several years been as robust and usable as their
|
bos@583
|
222 previous-generation counterparts, people using older tools have
|
bos@583
|
223 not yet necessarily woken up to their advantages. There are a
|
bos@583
|
224 number of ways in which distributed tools shine relative to
|
bos@583
|
225 centralised ones.</para>
|
bos@583
|
226
|
bos@583
|
227 <para>For an individual developer, distributed tools are almost
|
bos@583
|
228 always much faster than centralised tools. This is for a simple
|
bos@583
|
229 reason: a centralised tool needs to talk over the network for
|
bos@583
|
230 many common operations, because most metadata is stored in a
|
bos@583
|
231 single copy on the central server. A distributed tool stores
|
bos@583
|
232 all of its metadata locally. All else being equal, talking over
|
bos@583
|
233 the network adds overhead to a centralised tool. Don't
|
bos@583
|
234 underestimate the value of a snappy, responsive tool: you're
|
bos@583
|
235 going to spend a lot of time interacting with your revision
|
bos@583
|
236 control software.</para>
|
bos@583
|
237
|
bos@583
|
238 <para>Distributed tools are indifferent to the vagaries of your
|
bos@583
|
239 server infrastructure, again because they replicate metadata to
|
bos@583
|
240 so many locations. If you use a centralised system and your
|
bos@583
|
241 server catches fire, you'd better hope that your backup media
|
bos@583
|
242 are reliable, and that your last backup was recent and actually
|
bos@583
|
243 worked. With a distributed tool, you have many backups
|
bos@583
|
244 available on every contributor's computer.</para>
|
bos@583
|
245
|
bos@583
|
246 <para>The reliability of your network will affect distributed
|
bos@583
|
247 tools far less than it will centralised tools. You can't even
|
bos@583
|
248 use a centralised tool without a network connection, except for
|
bos@583
|
249 a few highly constrained commands. With a distributed tool, if
|
bos@583
|
250 your network connection goes down while you're working, you may
|
bos@583
|
251 not even notice. The only thing you won't be able to do is talk
|
bos@583
|
252 to repositories on other computers, something that is relatively
|
bos@583
|
253 rare compared with local operations. If you have a far-flung
|
bos@583
|
254 team of collaborators, this may be significant.</para>
|
bos@583
|
255
|
bos@583
|
256 <sect2>
|
bos@583
|
257 <title>Advantages for open source projects</title>
|
bos@583
|
258
|
bos@583
|
259 <para>If you take a shine to an open source project and decide
|
bos@583
|
260 that you would like to start hacking on it, and that project
|
bos@583
|
261 uses a distributed revision control tool, you are at once a
|
bos@583
|
262 peer with the people who consider themselves the
|
bos@583
|
263 <quote>core</quote> of that project. If they publish their
|
bos@583
|
264 repositories, you can immediately copy their project history,
|
bos@583
|
265 start making changes, and record your work, using the same
|
bos@583
|
266 tools in the same ways as insiders. By contrast, with a
|
bos@583
|
267 centralised tool, you must use the software in a <quote>read
|
bos@583
|
268 only</quote> mode unless someone grants you permission to
|
bos@583
|
269 commit changes to their central server. Until then, you won't
|
bos@583
|
270 be able to record changes, and your local modifications will
|
bos@583
|
271 be at risk of corruption any time you try to update your
|
bos@583
|
272 client's view of the repository.</para>
|
bos@583
|
273
|
bos@583
|
274 <sect3>
|
bos@583
|
275 <title>The forking non-problem</title>
|
bos@583
|
276
|
bos@583
|
277 <para>It has been suggested that distributed revision control
|
bos@583
|
278 tools pose some sort of risk to open source projects because
|
bos@583
|
279 they make it easy to <quote>fork</quote> the development of
|
bos@583
|
280 a project. A fork happens when there are differences in
|
bos@583
|
281 opinion or attitude between groups of developers that cause
|
bos@583
|
282 them to decide that they can't work together any longer.
|
bos@583
|
283 Each side takes a more or less complete copy of the
|
bos@583
|
284 project's source code, and goes off in its own
|
bos@583
|
285 direction.</para>
|
bos@583
|
286
|
bos@583
|
287 <para>Sometimes the camps in a fork decide to reconcile their
|
bos@583
|
288 differences. With a centralised revision control system, the
|
bos@583
|
289 <emphasis>technical</emphasis> process of reconciliation is
|
bos@583
|
290 painful, and has to be performed largely by hand. You have
|
bos@583
|
291 to decide whose revision history is going to
|
bos@583
|
292 <quote>win</quote>, and graft the other team's changes into
|
bos@583
|
293 the tree somehow. This usually loses some or all of one
|
bos@583
|
294 side's revision history.</para>
|
bos@583
|
295
|
bos@583
|
296 <para>What distributed tools do with respect to forking is
|
bos@583
|
297 they make forking the <emphasis>only</emphasis> way to
|
bos@583
|
298 develop a project. Every single change that you make is
|
bos@583
|
299 potentially a fork point. The great strength of this
|
bos@583
|
300 approach is that a distributed revision control tool has to
|
bos@583
|
301 be really good at <emphasis>merging</emphasis> forks,
|
bos@583
|
302 because forks are absolutely fundamental: they happen all
|
bos@583
|
303 the time.</para>
|
bos@583
|
304
|
bos@583
|
305 <para>If every piece of work that everybody does, all the
|
bos@583
|
306 time, is framed in terms of forking and merging, then what
|
bos@583
|
307 the open source world refers to as a <quote>fork</quote>
|
bos@583
|
308 becomes <emphasis>purely</emphasis> a social issue. If
|
bos@583
|
309 anything, distributed tools <emphasis>lower</emphasis> the
|
bos@583
|
310 likelihood of a fork:</para>
|
bos@583
|
311 <itemizedlist>
|
bos@583
|
312 <listitem><para>They eliminate the social distinction that
|
bos@583
|
313 centralised tools impose: that between insiders (people
|
bos@583
|
314 with commit access) and outsiders (people
|
bos@583
|
315 without).</para></listitem>
|
bos@583
|
316 <listitem><para>They make it easier to reconcile after a
|
bos@583
|
317 social fork, because all that's involved from the
|
bos@583
|
318 perspective of the revision control software is just
|
bos@583
|
319 another merge.</para></listitem></itemizedlist>
|
bos@583
|
320
|
bos@583
|
321 <para>Some people resist distributed tools because they want
|
bos@583
|
322 to retain tight control over their projects, and they
|
bos@583
|
323 believe that centralised tools give them this control.
|
bos@583
|
324 However, if you're of this belief, and you publish your CVS
|
bos@583
|
325 or Subversion repositories publicly, there are plenty of
|
bos@583
|
326 tools available that can pull out your entire project's
|
bos@583
|
327 history (albeit slowly) and recreate it somewhere that you
|
bos@583
|
328 don't control. So while your control in this case is
|
bos@583
|
329 illusory, you are forgoing the ability to fluidly
|
bos@583
|
330 collaborate with whatever people feel compelled to mirror
|
bos@583
|
331 and fork your history.</para>
|
bos@583
|
332
|
bos@583
|
333 </sect3>
|
bos@583
|
334 </sect2>
|
bos@583
|
335 <sect2>
|
bos@583
|
336 <title>Advantages for commercial projects</title>
|
bos@583
|
337
|
bos@583
|
338 <para>Many commercial projects are undertaken by teams that are
|
bos@583
|
339 scattered across the globe. Contributors who are far from a
|
bos@583
|
340 central server will see slower command execution and perhaps
|
bos@583
|
341 less reliability. Commercial revision control systems attempt
|
bos@583
|
342 to ameliorate these problems with remote-site replication
|
bos@583
|
343 add-ons that are typically expensive to buy and cantankerous
|
bos@583
|
344 to administer. A distributed system doesn't suffer from these
|
bos@583
|
345 problems in the first place. Better yet, you can easily set
|
bos@583
|
346 up multiple authoritative servers, say one per site, so that
|
bos@583
|
347 there's no redundant communication between repositories over
|
bos@583
|
348 expensive long-haul network links.</para>
|
bos@583
|
349
|
bos@583
|
350 <para>Centralised revision control systems tend to have
|
bos@583
|
351 relatively low scalability. It's not unusual for an expensive
|
bos@583
|
352 centralised system to fall over under the combined load of
|
bos@583
|
353 just a few dozen concurrent users. Once again, the typical
|
bos@583
|
354 response tends to be an expensive and clunky replication
|
bos@583
|
355 facility. Since the load on a central server---if you have
|
bos@583
|
356 one at all---is many times lower with a distributed tool
|
bos@583
|
357 (because all of the data is replicated everywhere), a single
|
bos@583
|
358 cheap server can handle the needs of a much larger team, and
|
bos@583
|
359 replication to balance load becomes a simple matter of
|
bos@583
|
360 scripting.</para>
|
bos@583
|
361
|
bos@583
|
362 <para>If you have an employee in the field, troubleshooting a
|
bos@583
|
363 problem at a customer's site, they'll benefit from distributed
|
bos@583
|
364 revision control. The tool will let them generate custom
|
bos@583
|
365 builds, try different fixes in isolation from each other, and
|
bos@583
|
366 search efficiently through history for the sources of bugs and
|
bos@583
|
367 regressions in the customer's environment, all without needing
|
bos@583
|
368 to connect to your company's network.</para>
|
bos@583
|
369
|
bos@583
|
370 </sect2>
|
bos@583
|
371 </sect1>
|
bos@583
|
372 <sect1>
|
bos@583
|
373 <title>Why choose Mercurial?</title>
|
bos@583
|
374
|
bos@583
|
375 <para>Mercurial has a unique set of properties that make it a
|
bos@583
|
376 particularly good choice as a revision control system.</para>
|
bos@583
|
377 <itemizedlist>
|
bos@583
|
378 <listitem><para>It is easy to learn and use.</para></listitem>
|
bos@583
|
379 <listitem><para>It is lightweight.</para></listitem>
|
bos@583
|
380 <listitem><para>It scales excellently.</para></listitem>
|
bos@583
|
381 <listitem><para>It is easy to
|
bos@583
|
382 customise.</para></listitem></itemizedlist>
|
bos@583
|
383
|
bos@583
|
384 <para>If you are at all familiar with revision control systems,
|
bos@583
|
385 you should be able to get up and running with Mercurial in less
|
bos@583
|
386 than five minutes. Even if not, it will take no more than a few
|
bos@583
|
387 minutes longer. Mercurial's command and feature sets are
|
bos@583
|
388 generally uniform and consistent, so you can keep track of a few
|
bos@583
|
389 general rules instead of a host of exceptions.</para>
|
bos@583
|
390
|
bos@583
|
391 <para>On a small project, you can start working with Mercurial in
|
bos@583
|
392 moments. Creating new changes and branches; transferring changes
|
bos@583
|
393 around (whether locally or over a network); and history and
|
bos@583
|
394 status operations are all fast. Mercurial attempts to stay
|
bos@583
|
395 nimble and largely out of your way by combining low cognitive
|
bos@583
|
396 overhead with blazingly fast operations.</para>
|
bos@583
|
397
|
bos@583
|
398 <para>The usefulness of Mercurial is not limited to small
|
bos@583
|
399 projects: it is used by projects with hundreds to thousands of
|
bos@583
|
400 contributors, each containing tens of thousands of files and
|
bos@583
|
401 hundreds of megabytes of source code.</para>
|
bos@583
|
402
|
bos@583
|
403 <para>If the core functionality of Mercurial is not enough for
|
bos@583
|
404 you, it's easy to build on. Mercurial is well suited to
|
bos@583
|
405 scripting tasks, and its clean internals and implementation in
|
bos@583
|
406 Python make it easy to add features in the form of extensions.
|
bos@583
|
407 There are a number of popular and useful extensions already
|
bos@583
|
408 available, ranging from helping to identify bugs to improving
|
bos@583
|
409 performance.</para>
|
bos@583
|
410
|
bos@583
|
411 </sect1>
|
bos@583
|
412 <sect1>
|
bos@583
|
413 <title>Mercurial compared with other tools</title>
|
bos@583
|
414
|
bos@583
|
415 <para>Before you read on, please understand that this section
|
bos@583
|
416 necessarily reflects my own experiences, interests, and (dare I
|
bos@583
|
417 say it) biases. I have used every one of the revision control
|
bos@583
|
418 tools listed below, in most cases for several years at a
|
bos@583
|
419 time.</para>
|
bos@583
|
420
|
bos@583
|
421
|
bos@583
|
422 <sect2>
|
bos@583
|
423 <title>Subversion</title>
|
bos@583
|
424
|
bos@583
|
425 <para>Subversion is a popular revision control tool, developed
|
bos@583
|
426 to replace CVS. It has a centralised client/server
|
bos@583
|
427 architecture.</para>
|
bos@583
|
428
|
bos@583
|
429 <para>Subversion and Mercurial have similarly named commands for
|
bos@583
|
430 performing the same operations, so if you're familiar with
|
bos@583
|
431 one, it is easy to learn to use the other. Both tools are
|
bos@583
|
432 portable to all popular operating systems.</para>
|
bos@583
|
433
|
bos@583
|
434 <para>Prior to version 1.5, Subversion had no useful support for
|
bos@583
|
435 merges. At the time of writing, its merge tracking capability
|
bos@583
|
436 is new, and known to be <ulink
|
bos@583
|
437 url="http://svnbook.red-bean.com/nightly/en/svn.branchmerge.advanced.html#svn.branchmerge.advanced.finalword">complicated
|
bos@583
|
438 and buggy</ulink>.</para>
|
bos@583
|
439
|
bos@583
|
440 <para>Mercurial has a substantial performance advantage over
|
bos@583
|
441 Subversion on every revision control operation I have
|
bos@583
|
442 benchmarked. I have measured its advantage as ranging from a
|
bos@583
|
443 factor of two to a factor of six when compared with Subversion
|
bos@583
|
444 1.4.3's <emphasis>ra_local</emphasis> file store, which is the
|
bos@583
|
445 fastest access method available. In more realistic
|
bos@583
|
446 deployments involving a network-based store, Subversion will
|
bos@583
|
447 be at a substantially larger disadvantage. Because many
|
bos@583
|
448 Subversion commands must talk to the server and Subversion
|
bos@583
|
449 does not have useful replication facilities, server capacity
|
bos@583
|
450 and network bandwidth become bottlenecks for modestly large
|
bos@583
|
451 projects.</para>
|
bos@583
|
452
|
bos@583
|
453 <para>Additionally, Subversion incurs substantial storage
|
bos@583
|
454 overhead to avoid network transactions for a few common
|
bos@583
|
455 operations, such as finding modified files
|
bos@583
|
456 (<literal>status</literal>) and displaying modifications
|
bos@583
|
457 against the current revision (<literal>diff</literal>). As a
|
bos@583
|
458 result, a Subversion working copy is often the same size as,
|
bos@583
|
459 or larger than, a Mercurial repository and working directory,
|
bos@583
|
460 even though the Mercurial repository contains a complete
|
bos@583
|
461 history of the project.</para>
|
bos@583
|
462
|
bos@583
|
463 <para>Subversion is widely supported by third party tools.
|
bos@583
|
464 Mercurial currently lags considerably in this area. This gap
|
bos@583
|
465 is closing, however, and indeed some of Mercurial's GUI tools
|
bos@583
|
466 now outshine their Subversion equivalents. Like Mercurial,
|
bos@583
|
467 Subversion has an excellent user manual.</para>
|
bos@583
|
468
|
bos@583
|
469 <para>Because Subversion doesn't store revision history on the
|
bos@583
|
470 client, it is well suited to managing projects that deal with
|
bos@583
|
471 lots of large, opaque binary files. If you check in fifty
|
bos@583
|
472 revisions to an incompressible 10MB file, Subversion's
|
bos@583
|
473 client-side space usage stays constant The space used by any
|
bos@583
|
474 distributed SCM will grow rapidly in proportion to the number
|
bos@583
|
475 of revisions, because the differences between each revision
|
bos@583
|
476 are large.</para>
|
bos@583
|
477
|
bos@583
|
478 <para>In addition, it's often difficult or, more usually,
|
bos@583
|
479 impossible to merge different versions of a binary file.
|
bos@583
|
480 Subversion's ability to let a user lock a file, so that they
|
bos@583
|
481 temporarily have the exclusive right to commit changes to it,
|
bos@583
|
482 can be a significant advantage to a project where binary files
|
bos@583
|
483 are widely used.</para>
|
bos@583
|
484
|
bos@583
|
485 <para>Mercurial can import revision history from a Subversion
|
bos@583
|
486 repository. It can also export revision history to a
|
bos@583
|
487 Subversion repository. This makes it easy to <quote>test the
|
bos@583
|
488 waters</quote> and use Mercurial and Subversion in parallel
|
bos@583
|
489 before deciding to switch. History conversion is incremental,
|
bos@583
|
490 so you can perform an initial conversion, then small
|
bos@583
|
491 additional conversions afterwards to bring in new
|
bos@583
|
492 changes.</para>
|
bos@583
|
493
|
bos@583
|
494
|
bos@583
|
495 </sect2>
|
bos@583
|
496 <sect2>
|
bos@583
|
497 <title>Git</title>
|
bos@583
|
498
|
bos@583
|
499 <para>Git is a distributed revision control tool that was
|
bos@583
|
500 developed for managing the Linux kernel source tree. Like
|
bos@583
|
501 Mercurial, its early design was somewhat influenced by
|
bos@583
|
502 Monotone.</para>
|
bos@583
|
503
|
bos@583
|
504 <para>Git has a very large command set, with version 1.5.0
|
bos@583
|
505 providing 139 individual commands. It has something of a
|
bos@583
|
506 reputation for being difficult to learn. Compared to Git,
|
bos@583
|
507 Mercurial has a strong focus on simplicity.</para>
|
bos@583
|
508
|
bos@583
|
509 <para>In terms of performance, Git is extremely fast. In
|
bos@583
|
510 several cases, it is faster than Mercurial, at least on Linux,
|
bos@583
|
511 while Mercurial performs better on other operations. However,
|
bos@583
|
512 on Windows, the performance and general level of support that
|
bos@583
|
513 Git provides is, at the time of writing, far behind that of
|
bos@583
|
514 Mercurial.</para>
|
bos@583
|
515
|
bos@583
|
516 <para>While a Mercurial repository needs no maintenance, a Git
|
bos@583
|
517 repository requires frequent manual <quote>repacks</quote> of
|
bos@583
|
518 its metadata. Without these, performance degrades, while
|
bos@583
|
519 space usage grows rapidly. A server that contains many Git
|
bos@583
|
520 repositories that are not rigorously and frequently repacked
|
bos@583
|
521 will become heavily disk-bound during backups, and there have
|
bos@583
|
522 been instances of daily backups taking far longer than 24
|
bos@583
|
523 hours as a result. A freshly packed Git repository is
|
bos@583
|
524 slightly smaller than a Mercurial repository, but an unpacked
|
bos@583
|
525 repository is several orders of magnitude larger.</para>
|
bos@583
|
526
|
bos@583
|
527 <para>The core of Git is written in C. Many Git commands are
|
bos@583
|
528 implemented as shell or Perl scripts, and the quality of these
|
bos@583
|
529 scripts varies widely. I have encountered several instances
|
bos@583
|
530 where scripts charged along blindly in the presence of errors
|
bos@583
|
531 that should have been fatal.</para>
|
bos@583
|
532
|
bos@583
|
533 <para>Mercurial can import revision history from a Git
|
bos@583
|
534 repository.</para>
|
bos@583
|
535
|
bos@583
|
536
|
bos@583
|
537 </sect2>
|
bos@583
|
538 <sect2>
|
bos@583
|
539 <title>CVS</title>
|
bos@583
|
540
|
bos@583
|
541 <para>CVS is probably the most widely used revision control tool
|
bos@583
|
542 in the world. Due to its age and internal untidiness, it has
|
bos@583
|
543 been only lightly maintained for many years.</para>
|
bos@583
|
544
|
bos@583
|
545 <para>It has a centralised client/server architecture. It does
|
bos@583
|
546 not group related file changes into atomic commits, making it
|
bos@583
|
547 easy for people to <quote>break the build</quote>: one person
|
bos@583
|
548 can successfully commit part of a change and then be blocked
|
bos@583
|
549 by the need for a merge, causing other people to see only a
|
bos@583
|
550 portion of the work they intended to do. This also affects
|
bos@583
|
551 how you work with project history. If you want to see all of
|
bos@583
|
552 the modifications someone made as part of a task, you will
|
bos@583
|
553 need to manually inspect the descriptions and timestamps of
|
bos@583
|
554 the changes made to each file involved (if you even know what
|
bos@583
|
555 those files were).</para>
|
bos@583
|
556
|
bos@583
|
557 <para>CVS has a muddled notion of tags and branches that I will
|
bos@583
|
558 not attempt to even describe. It does not support renaming of
|
bos@583
|
559 files or directories well, making it easy to corrupt a
|
bos@583
|
560 repository. It has almost no internal consistency checking
|
bos@583
|
561 capabilities, so it is usually not even possible to tell
|
bos@583
|
562 whether or how a repository is corrupt. I would not recommend
|
bos@583
|
563 CVS for any project, existing or new.</para>
|
bos@583
|
564
|
bos@583
|
565 <para>Mercurial can import CVS revision history. However, there
|
bos@583
|
566 are a few caveats that apply; these are true of every other
|
bos@583
|
567 revision control tool's CVS importer, too. Due to CVS's lack
|
bos@583
|
568 of atomic changes and unversioned filesystem hierarchy, it is
|
bos@583
|
569 not possible to reconstruct CVS history completely accurately;
|
bos@583
|
570 some guesswork is involved, and renames will usually not show
|
bos@583
|
571 up. Because a lot of advanced CVS administration has to be
|
bos@583
|
572 done by hand and is hence error-prone, it's common for CVS
|
bos@583
|
573 importers to run into multiple problems with corrupted
|
bos@583
|
574 repositories (completely bogus revision timestamps and files
|
bos@583
|
575 that have remained locked for over a decade are just two of
|
bos@583
|
576 the less interesting problems I can recall from personal
|
bos@583
|
577 experience).</para>
|
bos@583
|
578
|
bos@583
|
579 <para>Mercurial can import revision history from a CVS
|
bos@583
|
580 repository.</para>
|
bos@583
|
581
|
bos@583
|
582
|
bos@583
|
583 </sect2>
|
bos@583
|
584 <sect2>
|
bos@583
|
585 <title>Commercial tools</title>
|
bos@583
|
586
|
bos@583
|
587 <para>Perforce has a centralised client/server architecture,
|
bos@583
|
588 with no client-side caching of any data. Unlike modern
|
bos@583
|
589 revision control tools, Perforce requires that a user run a
|
bos@583
|
590 command to inform the server about every file they intend to
|
bos@583
|
591 edit.</para>
|
bos@583
|
592
|
bos@583
|
593 <para>The performance of Perforce is quite good for small teams,
|
bos@583
|
594 but it falls off rapidly as the number of users grows beyond a
|
bos@583
|
595 few dozen. Modestly large Perforce installations require the
|
bos@583
|
596 deployment of proxies to cope with the load their users
|
bos@583
|
597 generate.</para>
|
bos@583
|
598
|
bos@583
|
599
|
bos@583
|
600 </sect2>
|
bos@583
|
601 <sect2>
|
bos@583
|
602 <title>Choosing a revision control tool</title>
|
bos@583
|
603
|
bos@583
|
604 <para>With the exception of CVS, all of the tools listed above
|
bos@583
|
605 have unique strengths that suit them to particular styles of
|
bos@583
|
606 work. There is no single revision control tool that is best
|
bos@583
|
607 in all situations.</para>
|
bos@583
|
608
|
bos@583
|
609 <para>As an example, Subversion is a good choice for working
|
bos@583
|
610 with frequently edited binary files, due to its centralised
|
bos@583
|
611 nature and support for file locking.</para>
|
bos@583
|
612
|
bos@583
|
613 <para>I personally find Mercurial's properties of simplicity,
|
bos@583
|
614 performance, and good merge support to be a compelling
|
bos@583
|
615 combination that has served me well for several years.</para>
|
bos@583
|
616
|
bos@583
|
617
|
bos@583
|
618 </sect2>
|
bos@583
|
619 </sect1>
|
bos@583
|
620 <sect1>
|
bos@583
|
621 <title>Switching from another tool to Mercurial</title>
|
bos@583
|
622
|
bos@583
|
623 <para>Mercurial is bundled with an extension named <literal
|
bos@583
|
624 role="hg-ext">convert</literal>, which can incrementally
|
bos@583
|
625 import revision history from several other revision control
|
bos@583
|
626 tools. By <quote>incremental</quote>, I mean that you can
|
bos@583
|
627 convert all of a project's history to date in one go, then rerun
|
bos@583
|
628 the conversion later to obtain new changes that happened after
|
bos@583
|
629 the initial conversion.</para>
|
bos@583
|
630
|
bos@583
|
631 <para>The revision control tools supported by <literal
|
bos@583
|
632 role="hg-ext">convert</literal> are as follows:</para>
|
bos@583
|
633 <itemizedlist>
|
bos@583
|
634 <listitem><para>Subversion</para></listitem>
|
bos@583
|
635 <listitem><para>CVS</para></listitem>
|
bos@583
|
636 <listitem><para>Git</para></listitem>
|
bos@583
|
637 <listitem><para>Darcs</para></listitem></itemizedlist>
|
bos@583
|
638
|
bos@583
|
639 <para>In addition, <literal role="hg-ext">convert</literal> can
|
bos@583
|
640 export changes from Mercurial to Subversion. This makes it
|
bos@583
|
641 possible to try Subversion and Mercurial in parallel before
|
bos@583
|
642 committing to a switchover, without risking the loss of any
|
bos@583
|
643 work.</para>
|
bos@583
|
644
|
bos@583
|
645 <para>The <command role="hg-ext-convert">convert</command> command
|
bos@583
|
646 is easy to use. Simply point it at the path or URL of the
|
bos@583
|
647 source repository, optionally give it the name of the
|
bos@583
|
648 destination repository, and it will start working. After the
|
bos@583
|
649 initial conversion, just run the same command again to import
|
bos@583
|
650 new changes.</para>
|
bos@583
|
651 </sect1>
|
bos@583
|
652
|
bos@583
|
653 <sect1>
|
bos@583
|
654 <title>A short history of revision control</title>
|
bos@583
|
655
|
bos@583
|
656 <para>The best known of the old-time revision control tools is
|
bos@583
|
657 SCCS (Source Code Control System), which Marc Rochkind wrote at
|
bos@583
|
658 Bell Labs, in the early 1970s. SCCS operated on individual
|
bos@583
|
659 files, and required every person working on a project to have
|
bos@583
|
660 access to a shared workspace on a single system. Only one
|
bos@583
|
661 person could modify a file at any time; arbitration for access
|
bos@583
|
662 to files was via locks. It was common for people to lock files,
|
bos@583
|
663 and later forget to unlock them, preventing anyone else from
|
bos@583
|
664 modifying those files without the help of an
|
bos@583
|
665 administrator.</para>
|
bos@583
|
666
|
bos@583
|
667 <para>Walter Tichy developed a free alternative to SCCS in the
|
bos@583
|
668 early 1980s; he called his program RCS (Revision Control System).
|
bos@583
|
669 Like SCCS, RCS required developers to work in a single shared
|
bos@583
|
670 workspace, and to lock files to prevent multiple people from
|
bos@583
|
671 modifying them simultaneously.</para>
|
bos@583
|
672
|
bos@583
|
673 <para>Later in the 1980s, Dick Grune used RCS as a building block
|
bos@583
|
674 for a set of shell scripts he initially called cmt, but then
|
bos@583
|
675 renamed to CVS (Concurrent Versions System). The big innovation
|
bos@583
|
676 of CVS was that it let developers work simultaneously and
|
bos@583
|
677 somewhat independently in their own personal workspaces. The
|
bos@583
|
678 personal workspaces prevented developers from stepping on each
|
bos@583
|
679 other's toes all the time, as was common with SCCS and RCS. Each
|
bos@583
|
680 developer had a copy of every project file, and could modify
|
bos@583
|
681 their copies independently. They had to merge their edits prior
|
bos@583
|
682 to committing changes to the central repository.</para>
|
bos@583
|
683
|
bos@583
|
684 <para>Brian Berliner took Grune's original scripts and rewrote
|
bos@583
|
685 them in C, releasing in 1989 the code that has since developed
|
bos@583
|
686 into the modern version of CVS. CVS subsequently acquired the
|
bos@583
|
687 ability to operate over a network connection, giving it a
|
bos@583
|
688 client/server architecture. CVS's architecture is centralised;
|
bos@583
|
689 only the server has a copy of the history of the project. Client
|
bos@583
|
690 workspaces just contain copies of recent versions of the
|
bos@583
|
691 project's files, and a little metadata to tell them where the
|
bos@583
|
692 server is. CVS has been enormously successful; it is probably
|
bos@583
|
693 the world's most widely used revision control system.</para>
|
bos@583
|
694
|
bos@583
|
695 <para>In the early 1990s, Sun Microsystems developed an early
|
bos@583
|
696 distributed revision control system, called TeamWare. A
|
bos@583
|
697 TeamWare workspace contains a complete copy of the project's
|
bos@583
|
698 history. TeamWare has no notion of a central repository. (CVS
|
bos@583
|
699 relied upon RCS for its history storage; TeamWare used
|
bos@583
|
700 SCCS.)</para>
|
bos@583
|
701
|
bos@583
|
702 <para>As the 1990s progressed, awareness grew of a number of
|
bos@583
|
703 problems with CVS. It records simultaneous changes to multiple
|
bos@583
|
704 files individually, instead of grouping them together as a
|
bos@583
|
705 single logically atomic operation. It does not manage its file
|
bos@583
|
706 hierarchy well; it is easy to make a mess of a repository by
|
bos@583
|
707 renaming files and directories. Worse, its source code is
|
bos@583
|
708 difficult to read and maintain, which made the <quote>pain
|
bos@583
|
709 level</quote> of fixing these architectural problems
|
bos@583
|
710 prohibitive.</para>
|
bos@583
|
711
|
bos@583
|
712 <para>In 2001, Jim Blandy and Karl Fogel, two developers who had
|
bos@583
|
713 worked on CVS, started a project to replace it with a tool that
|
bos@583
|
714 would have a better architecture and cleaner code. The result,
|
bos@583
|
715 Subversion, does not stray from CVS's centralised client/server
|
bos@583
|
716 model, but it adds multi-file atomic commits, better namespace
|
bos@583
|
717 management, and a number of other features that make it a
|
bos@583
|
718 generally better tool than CVS. Since its initial release, it
|
bos@583
|
719 has rapidly grown in popularity.</para>
|
bos@583
|
720
|
bos@583
|
721 <para>More or less simultaneously, Graydon Hoare began working on
|
bos@583
|
722 an ambitious distributed revision control system that he named
|
bos@583
|
723 Monotone. While Monotone addresses many of CVS's design flaws
|
bos@583
|
724 and has a peer-to-peer architecture, it goes beyond earlier (and
|
bos@583
|
725 subsequent) revision control tools in a number of innovative
|
bos@583
|
726 ways. It uses cryptographic hashes as identifiers, and has an
|
bos@583
|
727 integral notion of <quote>trust</quote> for code from different
|
bos@583
|
728 sources.</para>
|
bos@583
|
729
|
bos@583
|
730 <para>Mercurial began life in 2005. While a few aspects of its
|
bos@583
|
731 design are influenced by Monotone, Mercurial focuses on ease of
|
bos@583
|
732 use, high performance, and scalability to very large
|
bos@583
|
733 projects.</para>
|
bos@583
|
734
|
bos@583
|
735 </sect1>
|
bos@583
|
736
|
bos@583
|
737 <sect1>
|
bos@583
|
738 <title>Colophon&emdash;this book is Free</title>
|
bos@26
|
739
|
bos@559
|
740 <para>This book is licensed under the Open Publication License,
|
bos@559
|
741 and is produced entirely using Free Software tools. It is
|
bos@580
|
742 typeset with DocBook XML. Illustrations are drawn and rendered with
|
bos@559
|
743 <ulink url="http://www.inkscape.org/">Inkscape</ulink>.</para>
|
bos@26
|
744
|
bos@559
|
745 <para>The complete source code for this book is published as a
|
bos@559
|
746 Mercurial repository, at <ulink
|
bos@559
|
747 url="http://hg.serpentine.com/mercurial/book">http://hg.serpentine.com/mercurial/book</ulink>.</para>
|
bos@559
|
748
|
bos@559
|
749 </sect1>
|
bos@559
|
750 </preface>
|
bos@559
|
751 <!--
|
bos@559
|
752 local variables:
|
bos@559
|
753 sgml-parent-document: ("00book.xml" "book" "preface")
|
bos@559
|
754 end:
|
bos@559
|
755 -->
|