Mercurial > CVu-Mercurial
annotate Hg.txt @ 5:2ec53c0ed5d8
Musings on Merging and Mergeability.
| author | Jim Hague <jim.hague@icc-atcsolutions.com> |
|---|---|
| date | Fri, 06 Mar 2009 14:07:34 +0000 |
| parents | 561edf852797 |
| children | a942bf7bc2ab |
| rev | line source |
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| 0 | 1 Inside a distributed version control system |
| 2 =========================================== | |
| 3 | |
| 4 Grinton Lodge is a Youth Hostel that sits on an exposed hillside just | |
| 5 above the small hamlet of Grinton in Swaledale, in the Yorkshire Dales | |
| 6 National Park. A former Victorian shooting lodge, it now welcomes | |
| 7 walkers and other travellers from around the world. | |
| 8 | |
| 9 Tonight, a Wednesday in mid-November, is not one of its busiest | |
| 10 nights. Kat, the duty staff member, tells me that there is a small | |
| 11 corporate team-building group in the annex. There's no sign of them at | |
| 12 present. Otherwise, that portion of the world that has beaten a path | |
| 13 to the door of this grand building today consists of just me. And Kat | |
| 14 goes home soon. | |
| 15 | |
| 16 The November CVu, removed from its wrappers and read yesterday, lies | |
| 17 in my bag. Taunting me. Go on, it says, if you've ever going to put | |
| 18 finger to keyboard in the name of CVu, well, tonight you are out of | |
| 19 excuses. | |
| 20 | |
| 21 Bugger. | |
| 22 | |
| 23 Let's look into Mercurial | |
| 24 ------------------------- | |
| 25 | |
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26 If you're at all interested in version control systems - and any |
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27 software developer not using one daily is a strange beast indeed - |
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28 you'll at least have become vaguely aware in the last few years of the |
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29 growing maturity of the latest group of version control systems |
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30 offering funky new stuff. These are the distributed version control |
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31 systems (DVCS). There is more to them than just their headline |
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32 attributes, being able to check history and do checkins while |
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33 disconnected from a central server, but these are damm useful to start |
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34 with. |
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35 |
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36 When I first heard about DVCS, it wasn't immediately obvious to me (to |
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37 put it mildly) how they would work. After years of using a centralised |
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38 version control system, I had rough mental model of what went on. But |
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39 how do you cope without the central server forcing ordering onto the |
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40 changes? |
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41 |
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42 Since then I've started using Mercurial. Mercurial is a DVCS. It's one |
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43 of three DVCSs that have gained significant popularity in the last few |
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44 years, the other two being Git and Bazaar. I switched a significant |
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45 work project over to Mercurial (from Subversion) over a year ago, |
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46 because a customer site required on-site work but could not allow |
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47 access back to the company VPN. I chose Mercurial for a variety of |
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48 reasons which I won't bore you with here. If you must know, see the |
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49 box. |
| 0 | 50 |
| 51 What I want to do in this article is give you an insight into how a | |
| 52 DVCS works. OK, so specifically I'm going to be talking about | |
| 53 Mercurial, but Git and Bazaar attack the problem in a similar way. But | |
| 54 first I'd better give you some idea of how you use Mercurial. | |
| 55 | |
| 56 :::: | |
| 57 Box: OK, if you must know: | |
| 58 | |
| 59 o Implementability. I needed the system to work on Windows, Linux and | |
| 60 AIX. The latter was not one of the directly supported platforms for | |
| 61 any of the candidates. Git's implementation uses a horde of | |
| 62 tools. Bazaar requires only Python, but required Python 2.4 while IBM | |
| 63 stubbornly still supplies only Python 2.3. Mercurial requires Python | |
| 64 2.3 or greater, and uses some C for speed. | |
| 65 | |
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66 o Simplicity. My users used Subversion daily, but did not generally |
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67 have much experience with other VCS. From the command line, |
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68 Mercurial's core operations will be familiar to a Subversion |
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69 user. This is also true of Bazaar, but was less true of Git. Git has |
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70 improved in this matter since then, but a Mr Winder of this parish |
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71 tells me that it's still possible to seriously embarass |
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72 yourself. There was also a lack of Windows support for Git at the |
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73 time. |
| 0 | 74 |
| 75 o Speed. Mercurial is fast. In the same ballpark as Git. Bazaar | |
| 76 wasn't, and although it has improved significantly, has, in my | |
| 77 estimation, added user complexity in the process, and is still off the | |
| 78 pace for some operations. | |
| 79 | |
| 80 o Documentation. At the time, Bryan O'Sullivan's excellent Mercurial | |
| 81 book (http://hgbook.red-bean.com) was a clear winner for best | |
| 82 documentation. | |
| 83 :::: | |
| 84 | |
| 85 The 5 minute Mercurial overview | |
| 86 ------------------------------- | |
| 87 | |
| 88 I think it unlikely that someone possessing the taste and discernment | |
| 89 to be reading CVu would not be familiar with at least one version | |
| 90 control system. So, while I want to give you a flavour of what it's | |
| 91 like to use, I'm not going to hang about. If you'd like a proper | |
| 92 introduction, or you don't follow something, I thoroughly recommend | |
| 93 you consult the Mercurial book. | |
| 94 | |
| 95 To start using Mercurial to keep track of a project. | |
| 96 | |
| 97 $ hg init | |
| 98 $ | |
| 99 | |
| 100 This creates the repository root in the current directory. | |
| 101 | |
| 102 Like CVS with its CVS directory and Subversion with its .svn | |
| 103 directory, Mercurial keeps its private data in a directory. Mercifully | |
| 104 there is only one of these, in the top level of your project. And | |
| 105 rather than holding details of where the actual repository is to be | |
| 106 found, the .hg directory holds the entire repository. | |
| 107 | |
| 108 Next you need to specify the files you want Mercurial to track. | |
| 109 | |
| 110 $ echo "There was a gibbon one morning" > pome.txt | |
| 111 $ hg add pome.txt | |
| 112 $ | |
| 113 | |
| 114 As you might expect, this marks the files as to be added. And as you | |
| 115 might also expect, you need to commit to record the added files in the | |
| 116 repository. The commit comment can be supplied on the command line; if | |
| 117 you don't supply a comment, you'll be dropped into an editor to | |
| 118 provide one. | |
| 119 | |
| 120 There is a suggested format for these messages - a one line summary | |
| 121 followed by any more required detail on following lines. By default | |
| 122 Mercurial will only display the first line of commit messages when | |
| 123 listing changes. In these examples I'll stick to terse messages, and | |
| 124 I'll enter them from the command line. | |
| 125 | |
| 126 $ hg commit -m "My Pome" -u "Jim Hague <jim.hague@acm.org>" | |
| 127 $ | |
| 128 | |
| 129 Mercurial records the user making the change as part of the change | |
| 130 information. It is usual to give your name and email address as I've | |
| 131 done here. You can imagine, though, that constantly having to repeat | |
| 132 this is a bit tedious, so you can set a default user name in a | |
| 133 configuration file. Mercurial keeps global, user and repository | |
| 134 configurations, and it can go in any of those. | |
| 135 | |
| 136 As with Subversion, after further edits you see how your working copy | |
| 137 differs from the repository. | |
| 138 | |
| 139 $ hg status | |
| 140 M pome.txt | |
| 141 $ hg diff | |
| 142 diff -r 33596ef855c1 pome.txt | |
| 143 --- a/pome.txt Wed Apr 23 22:36:33 2008 +0100 | |
| 144 +++ b/pome.txt Wed Apr 23 22:48:01 2008 +0100 | |
| 145 @@ -1,1 +1,2 @@ There was a gibbon one morning | |
| 146 There was a gibbon one morning | |
| 147 +said "I think I will fly to the moon". | |
| 148 $ hg commit -m "A great second line" | |
| 149 $ | |
| 150 | |
| 151 And look through a log of changes. | |
| 152 | |
| 153 $ hg log | |
| 154 changeset: 1:3d65e7a57890 | |
| 155 tag: tip | |
| 156 user: Jim Hague <jim.hague@acm.org> | |
| 157 date: Wed Apr 23 22:49:10 2008 +0100 | |
| 158 summary: A great second line | |
| 159 | |
| 160 changeset: 0:33596ef855c1 | |
| 161 user: Jim Hague <jim.hague@acm.org> | |
| 162 date: Wed Apr 23 22:36:33 2008 +0100 | |
| 163 summary: My Pome | |
| 164 | |
| 165 $ | |
| 166 | |
| 167 There are some items here that need an explanation. | |
| 168 | |
| 169 The changeset identifer is in fact two identifiers separated by a | |
| 170 colon. The first is the sequence number of the changeset in the | |
| 171 repository, and is directly comparable to the change number in a | |
| 172 Subversion repository. The second is a globally unique identifier for | |
| 173 that change. As the change is copied from one repository to another | |
| 174 (this is a distributed system, remember, even if we haven't come to | |
| 175 that bit yet), its sequence number in any particular repository will | |
| 176 change, but the global identifier will always remain the same. | |
| 177 | |
| 178 'tip' is a Mercurial term. It means simply the most recent change. | |
| 179 | |
| 180 Want to rename a file? | |
| 181 | |
| 182 $ hg mv pome.txt poem.txt | |
| 183 $ hg status | |
| 184 A poem.txt | |
| 185 R pome.txt | |
| 186 $ hg commit -m "Rename my file" | |
| 187 $ | |
| 188 | |
| 189 (The command to rename a file is actually 'hg rename', but Mercurial | |
| 190 saves Unix-trained fingers from typing embarrassment.) | |
| 191 | |
| 192 At this point you may be wondering about directories. 'hg mkdir' | |
| 193 perhaps? Well, no. Mercurial only tracks files. To be sure, the | |
| 194 directory a file occupies is tracked, but effectively only as a | |
| 195 component of the file name. This has the slightly unexpected result | |
| 196 that you can't record an empty directory in your repository. | |
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197 |
| 0 | 198 (Footnote: I tripped over this converting a work Subversion |
| 199 repository. One possibility is to create a placemaker file in the | |
| 200 directory. In the event I created the directory (which receives build | |
| 201 products) as part of the build instead.) | |
| 202 | |
| 203 Given this, and the status output above that suggests strongly that | |
| 204 Mercurial treats a rename as a copy followed by a delete, you may be | |
| 205 worried that Mercurial won't cope at all well with rearranging your | |
| 206 repository. Relax. Mercurial does store the details of the rename as | |
| 207 part of the changeset, and copes very well with rearrangements. | |
| 208 | |
| 209 (Footnote: The Mercurial designers justify not dealing with | |
| 210 directories as first class objects by pointing out that provided you | |
| 211 can correctly move files about in the tree, the other reasons for | |
| 212 tracking directories are uncommon and do not in their opinion justify | |
| 213 the considerable added complexity. So far I've found no reason to | |
| 214 doubt that judgement.) | |
| 215 | |
| 216 Want to rewind the working copy to a previous revision? | |
| 217 | |
| 218 $ hg update -r 1 | |
| 219 1 files updated, 0 files merged, 1 files removed, 0 files unresolved | |
| 220 $ | |
| 221 | |
| 222 'hg update' updates the working files. In this case I'm specifying | |
| 223 that I want to go back to local changeset 1. I could also have typed | |
| 224 '-r 3d65e7a57890', or even '-r 3d'; when specifying the global change | |
| 225 identifier you only need to type enough digits to make it unique. | |
| 226 | |
| 227 This is all very well, but it's not exactly distributed, is it? | |
| 228 | |
| 229 Copy an existing repository: | |
| 230 | |
| 231 elsewhere$ hg clone ssh://jim.home.net/Poem Jim-Poem | |
| 232 updating working directory | |
| 233 1 files updated, 0 files merged, 0 files removed, 0 files unresolved | |
| 234 | |
| 235 (You can access other repositories via the file system, over http or | |
| 236 over ssh). | |
| 237 | |
| 238 elsewhere$ cd Jim-Poem | |
| 239 elsewhere$ hg log | |
| 240 changeset: 3:a065eb26e6b9 | |
| 241 tag: tip | |
| 242 user: Jim Hague <jim.hague@acm.org> | |
| 243 date: Thu Apr 24 18:52:31 2008 +0100 | |
| 244 summary: Rename my file | |
| 245 | |
| 246 changeset: 2:ff97668b7422 | |
| 247 user: Jim Hague <jim.hague@acm.org> | |
| 248 date: Thu Apr 24 18:50:22 2008 +0100 | |
| 249 summary: Finished first verse | |
| 250 | |
| 251 changeset: 1:3d65e7a57890 | |
| 252 user: Jim Hague <jim.hague@acm.org> | |
| 253 date: Wed Apr 23 22:49:10 2008 +0100 | |
| 254 summary: A great second line | |
| 255 | |
| 256 changeset: 0:33596ef855c1 | |
| 257 user: Jim Hague <jim.hague@acm.org> | |
| 258 date: Wed Apr 23 22:36:33 2008 +0100 | |
| 259 summary: My Pome | |
| 260 | |
| 261 'hg clone' is aptly named. It creates a new repository that contains | |
| 262 exactly the same changes as the source repository. You can make a | |
| 263 clone just by copying your project directory, if you're confident | |
| 264 nothing else will access it during the copy. 'hg clone' saves you this | |
| 265 worry, and sets the default push/pull location in the new repo to the | |
| 266 cloned repo. | |
| 267 | |
| 268 From that point, you use 'hg pull' to collect changes from other | |
| 269 places into your repo (though note it does not by default update your | |
| 270 working copy), and, as you might guess, 'hg push' shoves your changes | |
| 271 into a foreign repository. By default these will act on the repository | |
| 272 you cloned from, but you can specify any other repository. | |
| 273 | |
| 274 More on those in a moment. First, though, I want to show you something | |
| 275 you can't do in Subversion. Start with the repository with 4 changes | |
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276 we just cloned. I want to focus on the first couple of lines, so I'll |
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277 wind the working copy back to the point where only those lines exist. |
| 0 | 278 |
| 279 $ hg update -r 1 | |
| 280 1 files updated, 0 files merged, 1 files removed, 0 files unresolved | |
| 281 | |
| 282 And make a change. | |
| 283 | |
| 284 $ hg diff | |
| 285 diff -r 3d65e7a57890 pome.txt | |
| 286 --- a/pome.txt Wed Apr 23 22:49:10 2008 +0100 | |
| 287 +++ b/pome.txt Thu Apr 24 19:13:14 2008 +0100 | |
| 288 @@ -1,2 +1,2 @@ There was a gibbon one morning | |
| 289 -There was a gibbon one morning | |
| 290 -said "I think I will fly to the moon". | |
| 291 +There was a baboon who one afternoon | |
| 292 +said "I think I will fly to the sun". | |
| 293 $ hg commit -m "Better first two lines" | |
| 294 $ | |
| 295 | |
| 296 The alert among you will have sat up at that. Well done! Yes, there's | |
| 297 something very worrying. How can I commit a change at an old point? | |
| 298 If you try this in Subversion, it will complain mightily about your | |
| 299 file being out of date. But Mercurial just went ahead and did | |
| 300 something. The Bazaar experts among you will know that in Bazaar, if | |
| 301 you use 'bzr revert -r' to bring the working copy to a past revision, | |
| 302 make a change and commit, then your latest version will be the past | |
| 303 revision plus your change. Perhaps that's what Mercurial did? | |
| 304 | |
| 305 No. What Mercurial did is central to Mercurial's view of the | |
| 306 world. You took your working copy back to an old changeset, and the | |
| 307 committed a fresh change based at that changeset. Mercurial actually | |
| 308 did just what you asked it to do, no more and no less. Let's see the | |
| 309 initial evidence. | |
| 310 | |
| 311 $ hg heads | |
| 312 changeset: 4:267d32f158b3 | |
| 313 tag: tip | |
| 314 parent: 1:3d65e7a57890 | |
| 315 user: Jim Hague <jim.hague@acm.org> | |
| 316 date: Thu Apr 24 19:13:59 2008 +0100 | |
| 317 summary: Better first two lines | |
| 318 | |
| 319 changeset: 3:a065eb26e6b9 | |
| 320 user: Jim Hague <jim.hague@acm.org> | |
| 321 date: Thu Apr 24 18:52:31 2008 +0100 | |
| 322 summary: Rename my file | |
| 323 | |
| 324 $ | |
| 325 | |
| 326 Time for some more Mercurial terminology. You can think of a 'head' in | |
| 327 Mercurial as the most recent change on a branch. In Mercurial, a | |
| 328 branch is simply what happens when you commit a change that has as its | |
| 329 parent a change that already has a child. Mercurial has a standard | |
| 330 extension 'hg glog' which uses some ASCII art to show the current | |
| 331 state: | |
| 332 | |
| 333 $ hg glog | |
| 334 @ changeset: 4:267d32f158b3 | |
| 335 | tag: tip | |
| 336 | parent: 1:3d65e7a57890 | |
| 337 | user: Jim Hague <jim.hague@acm.org> | |
| 338 | date: Thu Apr 24 19:13:59 2008 +0100 | |
| 339 | summary: Better first two lines | |
| 340 | | |
| 341 | o changeset: 3:a065eb26e6b9 | |
| 342 | | user: Jim Hague <jim.hague@acm.org> | |
| 343 | | date: Thu Apr 24 18:52:31 2008 +0100 | |
| 344 | | summary: Rename my file | |
| 345 | | | |
| 346 | o changeset: 2:ff97668b7422 | |
| 347 |/ user: Jim Hague <jim.hague@acm.org> | |
| 348 | date: Thu Apr 24 18:50:22 2008 +0100 | |
| 349 | summary: Finished first verse | |
| 350 | | |
| 351 o changeset: 1:3d65e7a57890 | |
| 352 | user: Jim Hague <jim.hague@acm.org> | |
| 353 | date: Wed Apr 23 22:49:10 2008 +0100 | |
| 354 | summary: A great second line | |
| 355 | | |
| 356 o changeset: 0:33596ef855c1 | |
| 357 user: Jim Hague <jim.hague@acm.org> | |
| 358 date: Wed Apr 23 22:36:33 2008 +0100 | |
| 359 summary: My Pome | |
| 360 | |
| 361 $ | |
| 362 | |
| 363 'hg view' shows a nicer graphical view. (Footnote: Though, being | |
| 364 Tcl/Tk based, not that much nicer.) | |
| 365 | |
| 366 So the change is in there. It's the latest change, and is simply on a | |
| 367 different branch to the other changes. | |
| 368 | |
| 369 Almost invariably, you will want to bring everything back together and | |
| 370 merge the branches. A merge is a change that combines two heads back | |
| 371 into one. It prepares an updated working directory with the merged | |
| 372 contents of the two heads for you to review and, if satisfactory, commit. | |
| 373 | |
| 374 $ hg merge | |
| 375 merging pome.txt and poem.txt | |
| 376 0 files updated, 1 files merged, 0 files removed, 0 files unresolved | |
| 377 (branch merge, don't forget to commit) | |
| 378 $ cat poem.txt | |
| 379 There was a baboon who one afternoon | |
| 380 said "I think I will fly to the sun". | |
| 381 So with two great palms strapped to his arms, | |
| 382 he started his takeoff run. | |
| 383 $ hg commit -m "Merge first line branch" | |
| 384 $ | |
| 385 | |
| 386 (Footnote: I'm no poet. The poem is, of course, 'Silly Old Baboon' by | |
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387 the late, great, Spike Milligan. From 'A Book of Milliganimals', |
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388 Puffin, 1971.) |
| 0 | 389 |
| 390 Here's the ASCII art again showing what just happened. Oh, and notice | |
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391 in the above that Mercurial has done the right thing with regard to |
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392 the rename. |
| 0 | 393 |
| 394 $ hg glog | |
| 395 @ changeset: 5:792ab970fc80 | |
| 396 |\ tag: tip | |
| 397 | | parent: 4:267d32f158b3 | |
| 398 | | parent: 3:a065eb26e6b9 | |
| 399 | | user: Jim Hague <jim.hague@acm.org> | |
| 400 | | date: Thu Apr 24 19:29:53 2008 +0100 | |
| 401 | | summary: Merge first line branch | |
| 402 | | | |
| 403 | o changeset: 4:267d32f158b3 | |
| 404 | | parent: 1:3d65e7a57890 | |
| 405 | | user: Jim Hague <jim.hague@acm.org> | |
| 406 | | date: Thu Apr 24 19:13:59 2008 +0100 | |
| 407 | | summary: Better first two lines | |
| 408 | | | |
| 409 o | changeset: 3:a065eb26e6b9 | |
| 410 | | user: Jim Hague <jim.hague@acm.org> | |
| 411 | | date: Thu Apr 24 18:52:31 2008 +0100 | |
| 412 | | summary: Rename my file | |
| 413 | | | |
| 414 o | changeset: 2:ff97668b7422 | |
| 415 |/ user: Jim Hague <jim.hague@acm.org> | |
| 416 | date: Thu Apr 24 18:50:22 2008 +0100 | |
| 417 | summary: Finished first verse | |
| 418 | | |
| 419 o changeset: 1:3d65e7a57890 | |
| 420 | user: Jim Hague <jim.hague@acm.org> | |
| 421 | date: Wed Apr 23 22:49:10 2008 +0100 | |
| 422 | summary: A great second line | |
| 423 | | |
| 424 o changeset: 0:33596ef855c1 | |
| 425 user: Jim Hague <jim.hague@acm.org> | |
| 426 date: Wed Apr 23 22:36:33 2008 +0100 | |
| 427 summary: My Pome | |
| 428 | |
| 429 $ | |
| 430 | |
| 431 So, our little branch change has now been merged back, and we have a | |
| 432 single line of development again. Notice that unlike the other | |
| 433 changesets, changeset 5 has two parent changesets, indicating it is a | |
| 434 merge changeset. You can only merge two branches in one operation; or | |
| 435 putting it another way, a changeset can have a maximum of two parents. | |
| 436 | |
| 437 This behaviour is absolutely central to Mercurial's philosophy. If a | |
| 438 change is committed that takes as its starting point a change that | |
| 439 already has a child, then a branch gets created. Working with | |
| 440 Mercurial, branches get created frequently, and equally frequently | |
| 441 merged back. As befits any frequent operation, both are easy to do. | |
| 442 | |
| 443 You're probably thinking at this point that this making a commit onto | |
| 444 an old version is a slightly strange thing to do, and you'd be right. | |
| 445 But that's exactly what's going to happen the moment you go | |
| 446 distributed. Two people working independently with their own | |
| 447 repositories are going to make commits based, typically, on the latest | |
| 448 changes they happen to have incorporated into their tree. To be | |
| 449 Distributed, a DVCS has to deal with this. Mercurial faces it head-on. | |
| 450 When you pull changes into your repo (or someone else pushes them), if | |
| 451 any of the changes overlap - are both based on the same base change - | |
| 452 you get extra heads, and it's up to you to let these extra heads live | |
| 453 or merge, as you please. | |
| 454 | |
| 455 In practice this is more manageable then you might think. Consider a | |
| 456 typical Mercurial usage, where the 'master' repo sits on a known | |
| 457 server, and everyone pulls changes from the master and pushes their | |
| 458 own efforts the master. But default Mercurial won't let you push if | |
| 459 the receiving repo will gain an extra head as a result, so you | |
| 460 typically pull (and do any required merging) just before | |
| 461 pushing. Subversion users will recognised this pattern. Subversion | |
| 462 won't let you commit a change if your working copy is not at the very | |
| 463 latest revision, so the Subversion user will update, and merge if | |
| 464 necessary, just before committing. | |
| 465 | |
| 466 What, then, about a branch in the conventional sense of '1.0 | |
| 467 maintenance branch'? Typically in Mercurial you'd handle this by | |
| 468 keeping a separate cloned repository for those changes. Cloning is | |
| 469 fast, and if local uses hard links where possible on filesystems that | |
| 470 support them, so isn't necessarily extravagant on disc space. You can, | |
| 471 if you prefer, handle them all in a single repo with 'named | |
| 472 branches', but cloning is definitely simpler. | |
| 473 | |
| 474 OK, so now you know the basics of using Mercurial. We can proceed to | |
| 475 looking at how this magic is achieved. In particular, where does this | |
| 476 magic globally unique identifier for a change come from? | |
| 477 | |
| 478 Inside the Mercurial repo | |
| 479 ------------------------- | |
| 480 | |
| 481 The way Mercurial handles its repo is really quite simple. | |
| 482 | |
| 483 That's simple, as in 'most things are simple once you know the | |
| 484 answer'. I found the explanation helpful, so this section attempts | |
| 485 the 10,000ft (FL100 if you prefer) view of Mercurial. | |
| 486 | |
| 487 (Foornote: Bryan O'Sullivan's excellent Mercurial book has a chapter | |
| 488 on the subject, and the Mercurial website has a fair amount of detail | |
| 489 too. This is 'research', OK?) | |
| 490 | |
| 491 First remember that any file or component can only have one or two | |
| 492 parents. You can't merge more than one other branch at once. | |
| 493 | |
| 494 We start with the basic building block, which Mercurial calls a | |
| 495 revlog. A revlog is a thing that holds a file and all the changes in | |
| 496 the file history. (Footnote: For any non-trivial file, this will | |
| 497 actually be two files on the disc, a data file and an index). The | |
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498 revlog stores the differences between successive versions |
| 0 | 499 of the file, though it will periodically store a complete version of |
| 500 the file instead of a difference, so that the content of any | |
| 501 particular file version can always be reconstructed without excessive | |
| 502 effort. | |
| 503 | |
| 504 Under the secret-squirrel Mercurial .hg directory at the top of your | |
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505 project is a store which holds a revlog for each file in your |
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506 project. So you have the complete history of the project locally. No |
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507 more round trips to the server. |
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508 |
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509 Both the differences between successive versions and the periodic |
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510 complete versions of a file are compressed before storing. This is |
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511 surprisingly effective at minimising the storage requirements this |
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512 entire history of your project. <!!!Comparison of .svn space |
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513 requirements for Waldo>. |
| 0 | 514 |
| 515 Any point in the evolution of a revlog can be uniquely identified with | |
| 516 a nodeid. This is simply the SHA1 hash of the current file contents | |
| 517 concatenated with the nodeids of one or both parents of the current | |
| 518 revision. Note that this way, two file states are identical if and | |
| 519 only if the file contents are the same *and* the file has the | |
| 520 same history. | |
| 521 | |
| 522 Here's a dump of a revlog index: | |
| 523 | |
| 524 $ hg debugindex .hg/store/data/pome.txt.i | |
| 525 rev offset length base linkrev nodeid p1 p2 | |
| 526 0 0 32 0 0 6bbbd5d6cc53 000000000000 000000000000 | |
| 527 1 32 51 0 1 83d266583303 6bbbd5d6cc53 000000000000 | |
| 528 2 83 84 0 2 14a54ec34bb6 83d266583303 000000000000 | |
| 529 3 167 76 3 4 dc4df776b38b 83d266583303 000000000000 | |
| 530 $ | |
| 531 | |
| 532 Note here that a file state can have two parents. If both the parent | |
| 533 nodeids are non-null, the file state has two parents, and the state is | |
| 534 therefore the result of a merge. | |
| 535 | |
| 536 Let's dump out a revlog at a particular revision: | |
| 537 | |
| 538 $ hg debugdata .hg/store/data/pome.txt.i 2 | |
| 539 There was a gibbon one morning | |
| 540 said "I think I will fly to the moon". | |
| 541 So with two great palms strapped to his arms, | |
| 542 he started his takeoff run. | |
| 543 $ | |
| 544 | |
| 545 The next component is the manifest. This is simply a list of all the | |
| 546 files in the project, together with their current nodeids. The | |
| 547 manifest is a file, held in a revlog. The nodeid of the manifest, | |
| 548 therefore, identifies the project filesystem at a particular point. | |
| 549 | |
| 550 $ hg debugdata .hg/store/00manifest.i 5 | |
| 551 poem.txt5168b1a5e2f44aa4e0f164e170820845183f50c8 | |
| 552 $ | |
| 553 | |
| 554 Finally we have the changeset. This is the atomic collection of | |
| 555 changes to a repository that leads to a new revision. The changeset | |
| 556 info includes the nodeid of the corresponding manifest, the timestamp | |
| 557 and committer ID, a list of changed files and a comment. The changeset | |
| 558 also includes the nodeid of the parent changeset, or the two parents | |
| 559 if the change is a merge. The changeset description is held in a | |
| 560 revlog, the changelog. | |
| 561 | |
| 562 $ hg debugdata .hg/store/00changelog.i 5 | |
| 563 1ccc11b6f7308cc8fa1573c2f3811a4710c91e3e | |
| 564 Jim Hague <jim.hague@acm.org> | |
| 565 1209061793 -3600 | |
| 566 poem.txt | |
| 567 pome.txt | |
| 568 | |
| 569 Merge first line branch | |
| 570 $ | |
| 571 | |
| 572 The nodeid of the changeset, therefore, gives us a globally unique | |
| 573 identifier for any particular change. Changesets have a | |
| 574 Subversion-like incrementing change number, but it is peculiar to that | |
| 575 repository. The nodeid, however, is global. | |
| 576 | |
| 577 One more detail remains to complete the picture. How do we get back | |
| 578 from a particular file change to find the responsible changeset? Each | |
| 579 revlog change has a linkrev entry that does just this. | |
| 580 | |
| 581 So, now we have a repository with a history of the changes applied to | |
| 582 that repository. Each change has a unique identifier. If we find that | |
| 583 change in another repository, it means that at the point in the other | |
| 584 repository we have exactly the same state; the file contents and | |
| 585 history are identical. | |
| 586 | |
| 587 At this point we can see how pulling changes from another repository | |
| 588 works. Mercurial has to determine which changesets in the source | |
| 589 repository are missing in the target repository. To do this, for each | |
| 590 head in the source repo it has to find the most recent change in that | |
| 591 head that it already present in the target repo, and get any remaining | |
| 592 changes after that point. These changes are then copied over and | |
| 593 applied. | |
| 594 | |
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595 The Mercurial revlog format has proved remarkably durable. Since the |
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596 first release of Mercurial in April 2005, these have been a total of 5 |
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597 changes to the file format. However, of those, all but one have been |
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598 changes to the handling of file names. The most recent change, in |
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599 October 2008, and its predecessor in December 2006, were both |
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600 introduced purely to cope with Windows specific issues. The one change |
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601 that touched the datastructures described above was in April 2006. The |
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602 format introduced, RevLogNG, changed only the details of index data |
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603 held, not the overall design. The chief Mercurial developer, Matt |
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604 Mackall, notes that the code in present-day Mercurial devoted to |
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605 reading the old format comprises 28 lines of Python. Compared with, |
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606 say, the early tribulations of Subversion and the switch from bdfs to |
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607 fsfs, this is an impressive record. |
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608 |
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609 Reflections on going distributed |
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610 -------------------------------- |
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611 |
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612 It's nearly traditional at this stage in an introduction to DVCS to |
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613 demonstrate several differenet workflow scanarios that you can build |
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614 with a DVCS. Which makes the important point that a DVCS can be |
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615 adapted to your workflow in a way that is at best unwieldy with a |
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616 CVCS. I intend, though, to break with tradition here. |
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617 |
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618 By this stage, I hope you can see that distributing version control |
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619 works by introducing branches where development takes place in |
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620 parallel. Mercurial treats these branches as arising naturally from |
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621 the commits made and transferred between repositories. Both Git and |
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622 Bazaar take a slightly different viewpoint, and explicitly generate a |
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623 fresh branch for work in a particular repositories. But in both cases |
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624 the underlying principle of identifying changes by a globally unique |
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625 identifier and resolving parallel development by merges between |
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626 overlapping changes is the same. And all three can be used in a truly |
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627 distributed manner, with full history and the ability to commit being |
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628 available locally. |
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629 |
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630 I want now to reflect on the consequences all this has for that |
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631 all-important question of whether a DVCS is a suitable vehicle for |
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632 your data. |
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633 |
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634 The first is a minor and rather obvious point. If you want to store |
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635 files that are both very large and which change often in your DVCS, |
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636 then all the compression in the world is unlikely to stop the storage |
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637 requirements for the full project history from becoming |
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638 uncomfortably large. |
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639 |
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640 The second, and main, point is that there is an important question you |
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641 need to ask about your data. We've seen that a DVCS relies on |
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642 branching and merging to weave its magic. So take a close look at your |
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643 data, and ask: |
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644 |
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645 Will It Merge? |
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646 |
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647 The subset of plain old text which comprises program source |
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648 code requires some human oversight, but will merge automatically |
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649 well enough for the process to be well within the bounds of the |
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650 possible. |
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651 |
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652 Unfortunately when we move further afield mergeability becomes a rarer |
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653 commodity. I nearly began the previous paragraph by stating that |
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654 plain old text will merge well enough. Then Doubt set in - what about |
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655 XML? Or BASE64 encoded content? |
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656 |
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657 Of course, merge doesn't necessarily have to be textual merge. I am |
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658 told that Word can be used to diff and merge two Word .doc files, a |
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659 data format notorious for its binary impenetrability. As long as some |
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660 suitable merge agent is available, and the DVCS can be configured to |
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661 use it for data of a particular type (Footnote: Mercurial can have the |
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662 merge and diff tools specified with reference to the file extension on |
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663 which they operate - I assume Bazaar and Git are similar.), then there |
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664 is no bar to successful DVCS use. |
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665 |
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666 Before this reliance on mergeability causes you to dismiss DVCS out of |
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667 hand, reflect. A CVCS can only handle non-mergeable data by acting as |
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668 a versioned file store; in other words, having as the only available |
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669 merge option the use of one or other of the merge candidates in its |
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670 entireity. Useful though a versioned file store can be, it cannot be |
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671 considered a full-featured version control system. By treating the |
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672 offending unmergeable files as external to the DVCS, or with careful |
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673 workflow - disabling the distributed and mergeable potentials - a DVCS |
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674 can deal with these files, but only at a cost of its distributedness |
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675 or its version control system-ness. In this it differs little from a |
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676 CVCS. |
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677 |
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678 So, for all data you want to version control, let your battle cry be |
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679 |
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680 Will It Merge? |
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681 |
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682 At this point, I have an urge to don lab coat and safety goggles and |
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683 be videoed attempting to mechanically merge data in a variety of |
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684 different formats. Frankly, this is unlike to be as exciting at |
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685 blending iPhones (Ref: www.willitblend.com), but from a system |
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686 development point of view it's rather more important. And, I think |
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687 gives us a large clue as to one of the reasons for the continuing |
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688 popularity of Plain Old Text as a source code representation mechanism. |