365 lines
13 KiB
Markdown
365 lines
13 KiB
Markdown
A few bits about the RCS backends
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[[toc ]]
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## Terminology
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``web-edit'' means that a page is edited by using the web (CGI) interface
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as opposed to using a editor and the RCS interface.
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## [[svn]]
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Subversion was the first RCS to be supported by ikiwiki.
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### How does it work internally?
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Master repository M.
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RCS commits from the outside are installed into M.
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There is a working copy of M (a checkout of M): W.
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HTML is generated from W. rcs_update() will update from M to W.
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CGI operates on W. rcs_commit() will commit from W to M.
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For all the gory details of how ikiwiki handles this behind the scenes,
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see [[commit-internals]].
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You browse and web-edit the wiki on W.
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W "belongs" to ikiwiki and should not be edited directly.
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## [darcs](http://darcs.net/) (not yet included)
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Support for using darcs as a backend is being worked on by [Thomas
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Schwinge](mailto:tschwinge@gnu.org), although development is on hold curretly.
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There is a patch in [[todo/darcs]].
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### How will it work internally?
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``Master'' repository R1.
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RCS commits from the outside are installed into R1.
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HTML is generated from R1. HTML is automatically generated (by using a
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``post-hook'') each time a new change is installed into R1. It follows
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that rcs_update() is not needed.
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There is a working copy of R1: R2.
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CGI operates on R2. rcs_commit() will push from R2 to R1.
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You browse the wiki on R1 and web-edit it on R2. This means for example
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that R2 needs to be updated from R1 if you are going to web-edit a page,
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as the user otherwise might be irritated otherwise...
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How do changes get from R1 to R2? Currently only internally in
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rcs\_commit(). Is rcs\_prepedit() suitable?
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It follows that the HTML rendering and the CGI handling can be completely
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separated parts in ikiwiki.
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What repository should [[RecentChanges]] and History work on? R1?
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#### Rationale for doing it differently than in the Subversion case
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darcs is a distributed RCS, which means that every checkout of a
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repository is equal to the repository it was checked-out from. There is
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no forced hierarchy.
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R1 is nevertheless called the master repository. It's used for
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collecting all the changes and publishing them: on the one hand via the
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rendered HTML and on the other via the standard darcs RCS interface.
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R2, the repository the CGI operates on, is just a checkout of R1 and
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doesn't really differ from the other checkouts that people will branch
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off from R1.
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(To be continued.)
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#### Another possible approach
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Here's what I (tuomov) think, would be a “cleaner” approach:
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1. Upon starting to edit, Ikiwiki gets a copy of the page, and `darcs changes --context`.
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This context _and_ the present version of the page are stored in as the “version” of the
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page in a hidden control of the HTML.
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Thus the HTML includes all that is needed to generate a patch wrt. to the state of the
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repository at the time the edit was started. This is of course all that darcs needs.
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2. Once the user is done with editing, _Ikiwiki generates a patch bundle_ for darcs.
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This should be easy with existing `Text::Diff` or somesuch modules, as the Web edits
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only concern single files. The reason why the old version of the page is stored in
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the HTML (possibly compressed) is that the diff can be generated.
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3. Now this patch bundle is applied with `darcs apply`, or sent by email for moderation…
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there are many possibilities.
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This approach avoids some of the problems of concurrent edits that the previous one may have,
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although there may be conflicts, which may or may not propagate to the displayed web page.
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(Unfortunately there is not an option to `darcs apply` to generate some sort of ‘confliction resolution
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bundle’.) Also, only one repository is needed, as it is never directly modified
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by Ikiwiki.
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This approach might be applicable to other distributed VCSs as well, although they're not as oriented
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towards transmitting changes with standalone patch bundles (often by email) as darcs is.
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> The mercurial plugin seems to just use one repo and edit it directly - is
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> there some reason that's okay there but not for darcs? I agree with tuomov
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> that having just the one repo would be preferable; the point of a dvcs is
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> that there's no difference between one repo and another. I've got a
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> darcs.pm based on mercurial.pm, that's almost usable... --bma
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>> IMHO it comes down to whatever works well for a given RCS. Seems like
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>> the darcs approach _could_ be done with most any distributed system, but
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>> it might be overkill for some (or all?) While there is the incomplete darcs
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>> plugin in [[todo/darcs]], if you submit one that's complete, I will
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>> probably accept it into ikiwiki.. --[[Joey]]
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>>> I'd like to help make a robust darcs (2) backend. I also think ikiwiki should use
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>>> exactly one darcs repo. I think we can simplify and say conflicting web
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>>> edits are not allowed, like most current wiki engines. I don't see that
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>>> saving (so much) context in the html is necessary, then.
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>>> bma, I would like to see your code. --[[Simon Michael]]
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>>> PS ah, there it is. Let's continue on the [[todo/darcs]] page.
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## [[Git]]
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Regarding the Git support, Recai says:
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I have been testing it for the past few days and it seems satisfactory. I
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haven't observed any race condition regarding the concurrent blog commits
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and it handles merge conflicts gracefully as far as I can see.
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(After about a year, git support is nearly as solid as subversion support --[[Joey]])
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As you may notice from the patch size, GIT support is not so trivial to
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implement (for me, at least). It has some drawbacks (especially wrt merge
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which was the hard part). GIT doesn't have a similar functionality like
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'svn merge -rOLD:NEW FILE' (please see the relevant comment in `_merge_past`
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for more details), so I had to invent an ugly hack just for the purpose.
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> I was looking at this, and WRT the problem of uncommitted local changes,
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> it seems to me you could just git-stash them now that git-stash exists.
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> I think it didn't when you first added the git support.. --[[Joey]]
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>> Yes, git-stash had not existed before. What about sth like below? It
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>> seems to work (I haven't given much thought on the specific implementation
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details). --[[roktas]]
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>> # create test files
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>> cd /tmp
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>> seq 6 >page
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>> cat page
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>> 1
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>> 2
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>> 3
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>> 4
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>> 5
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>> 6
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>> sed -e 's/2/2ME/' page >page.me # my changes
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>> cat page
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>> 1
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>> 2ME
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>> 3
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>> 4
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>> 5
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>> 6
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>> sed -e 's/5/5SOMEONE/' page >page.someone # someone's changes
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>> cat page
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>> 1
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>> 2
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>> 3
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>> 4
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>> 5SOMEONE
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>> 6
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>>
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>> # create a test repository
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>> mkdir t
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>> cd t
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>> cp ../page .
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>> git init
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>> git add .
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>> git commit -m init
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>>
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>> # save the current HEAD
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>> ME=$(git rev-list HEAD -- page)
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>> $EDITOR page # assume that I'm starting to edit page via web
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>>
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>> # simulates someone's concurrent commit
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>> cp ../page.someone page
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>> git commit -m someone -- page
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>>
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>> # My editing session ended, the resulting content is in page.me
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>> cp ../page.me page
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>> cat page
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>> 1
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>> 2ME
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>> 3
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>> 4
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>> 5
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>> 6
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>>
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>> # let's start to save my uncommitted changes
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>> git stash clear
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>> git stash save "changes by me"
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>> # we've reached a clean state
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>> cat page
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>> 1
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>> 2
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>> 3
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>> 4
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>> 5SOMEONE
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>> 6
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>>
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>> # roll-back to the $ME state
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>> git reset --soft $ME
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>> # now, the file is marked as modified
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>> git stash save "changes by someone"
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>>
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>> # now, we're at the $ME state
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>> cat page
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>> 1
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>> 2
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>> 3
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>> 4
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>> 5
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>> 6
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>> git stash list
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>> stash@{0}: On master: changes by someone
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>> stash@{1}: On master: changes by me
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>>
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>> # first apply my changes
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>> git stash apply stash@{1}
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>> cat page
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>> 1
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>> 2ME
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>> 3
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>> 4
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>> 5
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>> 6
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>> # ... and commit
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>> git commit -m me -- page
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>>
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>> # apply someone's changes
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>> git stash apply stash@{0}
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>> cat page
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>> 1
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>> 2ME
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>> 3
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>> 4
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>> 5SOMEONE
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>> 6
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>> # ... and commit
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>> git commit -m me+someone -- page
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By design, Git backend uses a "master-clone" repository pair approach in contrast
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to the single repository approach (here, _clone_ may be considered as the working
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copy of a fictious web user). Even though a single repository implementation is
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possible, it somewhat increases the code complexity of backend (I couldn't figure
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out a uniform method which doesn't depend on the prefered repository model, yet).
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By exploiting the fact that the master repo and _web user_'s repo (`srcdir`) are all
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on the same local machine, I suggest to create the latter with the "`git clone -l -s`"
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command to save disk space.
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Note that, as a rule of thumb, you should always put the rcs wrapper (`post-update`)
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into the master repository (`.git/hooks/`) as can be noticed in the Git wrappers of
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the sample [[ikiwiki.setup]].
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Here is how a web edit works with ikiwiki and git:
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* ikiwiki cgi modifies the page source in the clone
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* git-commit in the clone
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* git push origin master, pushes the commit from the clone to the master repo
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* the master repo's post-update hook notices this update, and runs ikiwiki
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* ikiwiki notices the modifies page source, and compiles it
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Here is a how a commit from a remote repository works:
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* git-commit in the remote repository
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* git-push, pushes the commit to the master repo on the server
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* the master repo's post-update hook notices this update, and runs ikiwiki
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* ikiwiki notices the modifies page source, and compiles it
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## [[Mercurial]]
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The Mercurial backend is still in a early phase, so it may not be mature
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enough, but it should be simple to understand and use.
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As Mercurial is a distributed RCS, it lacks the distinction between
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repository and working copy (every wc is a repo).
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This means that the Mercurial backend uses directly the repository as
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working copy (the master M and the working copy W described in the svn
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example are the same thing).
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You only need to specify 'srcdir' (the repository M) and 'destdir' (where
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the HTML will be generated).
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Master repository M.
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RCS commit from the outside are installed into M.
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M is directly used as working copy (M is also W).
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HTML is generated from the working copy in M. rcs_update() will update
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to the last committed revision in M (the same as 'hg update').
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If you use an 'update' hook you can generate automatically the HTML
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in the destination directory each time 'hg update' is called.
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CGI operates on M. rcs_commit() will commit directly in M.
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If you have any question or suggestion about the Mercurial backend
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please refer to [Emanuele](http://nerd.ocracy.org/em/)
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## [[tla]]
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## rcs
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There is a patch that needs a bit of work linked to from [[todo/rcs]].
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## [[Monotone]]
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In normal use, monotone has a local database as well as a workspace/working copy.
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In ikiwiki terms, the local database takes the role of the master repository, and
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the srcdir is the workspace. As all monotone workspaces point to a default
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database, there is no need to tell ikiwiki explicitly about the "master" database. It
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will know.
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The backend currently supports normal committing and getting the history of the page.
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To understand the parallel commit approach, you need to understand monotone's
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approach to conflicts:
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Monotone allows multiple micro-branches in the database. There is a command,
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`mtn merge`, that takes the heads of all these branches and merges them back together
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(turning the tree of branches into a dag). Conflicts in monotone (at time of writing)
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need to be resolved interactively during this merge process.
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It is important to note that having multiple heads is not an error condition in a
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monotone database. This condition will occur in normal use. In this case
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'update' will choose a head if it can, or complain and tell the user to merge.
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For the ikiwiki plugin, the monotone ikiwiki plugin borrows some ideas from the svn ikiwiki plugin.
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On prepedit() we record the revision that this change is based on (I'll refer to this as the prepedit revision). When the web user
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saves the page, we check if that is still the current revision. If it is, then we commit.
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If it isn't then we check to see if there were any changes by anyone else to the file
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we're editing while we've been editing (a diff bewteen the prepedit revision and the current rev).
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If there were no changes to the file we're editing then we commit as normal.
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It is only if there have been parallel changes to the file we're trying to commit that
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things get hairy. In this case the current approach is to
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commit the web changes as a branch from the prepedit revision. This
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will leave the repository with multiple heads. At this point, all data is saved.
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The system then tries to merge the heads with a merger that will fail if it cannot
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resolve the conflict. If the merge succeeds then everything is ok.
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If that merge failed then there are conflicts. In this case, the current code calls
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merge again with a merger that inserts conflict markers. It commits this new
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revision with conflict markers to the repository. It then returns the text to the
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user for cleanup. This is less neat than it could be, in that a conflict marked
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revision gets committed to the repository.
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## [[bzr]]
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