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POWhat makes merging in DVCS easy?
 

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  1. POWhat makes merging in DVCS easy?
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    <p>I read at <a href="http://www.joelonsoftware.com/items/2010/03/17.html" rel="nofollow noreferrer">Joel on Software</a>:</p> <blockquote> <p>With distributed version control, the distributed part is actually not the most interesting part.</p> <p>The interesting part is that these systems think in terms of changes, not in terms of versions.</p> </blockquote> <p>and at <a href="http://hginit.com/00.html" rel="nofollow noreferrer">HgInit</a>:</p> <blockquote> <p>When we have to merge, Subversion tries to look at both revisions—my modified code, and your modified code—and it tries to guess how to smash them together in one big unholy mess. It usually fails, producing pages and pages of “merge conflicts” that aren’t really conflicts, simply places where Subversion failed to figure out what we did.</p> <p>By contrast, while we were working separately in Mercurial, Mercurial was busy keeping a series of changesets. And so, when we want to merge our code together, Mercurial actually has a whole lot more information: it knows what each of us changed and can reapply those changes, rather than just looking at the final product and trying to guess how to put it together.</p> </blockquote> <p>By looking at the SVN's repository folder, I have the impression that Subversion is maintaining each revisions as <em>changeset</em>. And from what I know, Hg is using both <em>changeset</em> and <em>snapshot</em> while Git is purely using <em>snapshot</em> to store the data.</p> <p>If my assumption is correct, then there must be other ways that make merging in DVCS easy. What are those?</p> <p><strong>* Update:</strong> </p> <ul> <li>I am more interested in the technical perspective, but answers from non-technical perspective are acceptable</li> <li>Corrections: <ol> <li>Git's <em>conceptual</em> model is purely based on snapshots. The snapshots can be stored as diffs of other snapshots, it's just that the diffs are purely for storage optimization. – <a href="https://stackoverflow.com/users/12166/rafal-dowgird">Rafał Dowgird</a>'s <a href="https://stackoverflow.com/questions/2613525/what-makes-merging-in-dvcs-easy#comment2626335_2613525">comment</a> </li> </ol></li> <li>From non-technical perspective: <ol> <li>It's simply cultural: a DVCS wouldn't work at all if merging were hard, so DVCS developers invest a lot of time and effort into making merging easy. CVCS users OTOH are used to crappy merging, so there's no incentive for the developers to make it work. (Why make something good when your users pay you equally well for something crap?)<br> ...<br> To recap: the whole point of a DVCS is to have many decentralized repositories and constantly merge changes back and forth. Without good merging, a DVCS simply is useless. A CVCS however, can still survive with crappy merging, especially if the vendor can condition its users to avoid branching. – <a href="https://stackoverflow.com/users/2988/">Jörg W Mittag</a>'s <a href="https://stackoverflow.com/questions/2613525/what-makes-merging-in-dvcs-easy#2613595">answer</a></li> </ol></li> <li>From technical perspective: <ol> <li>recording a real DAG of the history does help! I think the main difference is that CVCS didn't always record a merge as a changeset with several parents, losing some information. – <a href="https://stackoverflow.com/users/148532/tonfa">tonfa</a>'s <a href="https://stackoverflow.com/questions/2613525/what-makes-merging-in-dvcs-easy#comment2624709_2613595">comment</a></li> <li>because of <strong><em>merge tracking</em></strong>, and the more fundamental fact that <strong><em>each revisions knows its parents</em></strong>. ... When each revision (each commit), including merge commits, know its parents (for merge commits that means having/remembering more than one parent, i.e. merge tracking), you can reconstruct diagram (DAG = Direct Acyclic Graph) of revision history. If you know graph of revisions, you can find common ancestor of the commits you want to merge. And when your DVCS knows itself how to <strong><em>find common ancestor</em></strong>, you don't need to provide it as an argument, as for example in CVS.<br> .<br> Note that there might be more than one common ancestor of two (or more) commits. Git makes use of so called "recursive" merge strategy, which merges merge bases (common ancestor), till you are left with one virtual / effective common ancestor (in some simplification), and can the do simple 3-way merge. – <a href="https://stackoverflow.com/users/46058/jakub-narebski">Jakub Narębski</a>'s <a href="https://stackoverflow.com/questions/2613525/what-makes-merging-in-dvcs-easy#2614937">answer</a></li> </ol></li> </ul> <p>Check as well <a href="https://stackoverflow.com/questions/2471606/how-and-or-why-is-merging-in-git-better-than-in-svn">How and/or why is merging in Git better than in SVN?</a></p>
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    1. CO"Git is purely using snapshot to store the data" - this is only partially true. Git's *conceptual* model is purely based on snapshots. The snapshots can be stored as diffs of other snapshots, it's just that the diffs are purely for storage optimization.
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      1. USRafał Dowgird
    2. CONote that HgInit is wrong or at least misleading in one regard on that page: it claims that if you move a function around and change it at the same time, DVCSes will be able to merge that. In reality this kind of move/copy tracking only happens on a whole-file level.
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      1. POWhat makes merging in DVCS easy?
      1. USLaurens Holst
    3. CO@LaurensHolst: My understanding is that Git does track this sort of change (though I haven't run tests to confirm). I don't know if Mercurial does.
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      1. USMarnen Laibow-Koser
 

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