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RE: more editor v2 (3)

From: Bert Huijben <rhuijben_at_sharpsvn.net>
Date: Wed, 16 Sep 2009 09:57:45 +0200

> -----Original Message-----
> From: Greg Stein [mailto:gstein_at_gmail.com]
> Sent: woensdag 16 september 2009 5:57
> To: Neels J Hofmeyr
> Cc: Subversion Developers
> Subject: Re: more editor v2 (3)
>
> On Tue, Sep 15, 2009 at 19:57, Neels J Hofmeyr <neels_at_elego.de> wrote:
> >...
> > Greg Stein wrote:
> >> Sorry for the short reply; on phone; more later.
> >>
> >> Ev2 doesn't care about text deltas. The stream passed in set_text is
> the
> >> new, desired contents. If the receiver wants to construct a delta,
> then
> >> it needs to discover a base somehow, and use that. In the RA commit
> >> editor case, it wants a delta for the wire, so it would take a WC
> >> callback at editor-construction time to get a stream of the base
> contents.
> >
> > what? Isn't it just a revision number per file? The repos already has
> BASE
> > right there.
> >
> >> The update and merge editors in the WC don't need deltas; they just
> use
> >> the new contents.
> >
> > uh, what? -1.
> >
> > We've got deltas lying around everywhere, don't see how 'update'
> should send
> > entire source files: We've got BASE and HEAD, should be a
> straightforward
> > delta to apply to WORKING.
> >
> > Could you drive your point home for me? I don't buy it.
>
> The delta streams are *only* used over the wire.
>
> Ev2 is designed to pass the fulltext through the API. The receiver
> takes the fulltext and can store it, compute a delta, or even drop it
> on the floor. In the Ev2 world, I envision the following four
> scenarios around delta computation:
>
> 1) RA *sends* a delta to REPOS: the RA layer takes ACTUAL (as
> fulltext), computes a delta against BASE (fetched via WC callback),
> and sends that to REPOS.
> 2) RA *receives* a delta from REPOS: the RA layer fetches BASE from
> the WC, applies the delta, and passes the new fulltext to set_text()
>
> 3) REPOS *sends* a delta to RA: REPOS gets BASE and TARGET from the
> FS, computes the delta, and sends it over the wire
> 4) REPOS *receives* a delta from RA: REPOS gets BASE from the FS,
> applies the delta, and passes that to svn_fs_apply_text().
>
> I'll note that REPOS the server side is a bit looser with editors than
> the client. Conceptually, you could imagine Ev2 implementing
> set_text() as svn_fs_apply_text() when the FS is the receiver. When
> the FS is the driver, it would send TARGET into set_text(), and let
> REPOS fetch BASE *if needed*.
>
> That last "if needed" is the important point here. It is REPOS that
> determines whether a delta is needed. NOT FS. In Ev1, the driver (the
> FS) computed a delta and shoved it into the API. But if the client
> does not have a BASE, then REPOS would have to (re)generate the
> fulltext and send that down the wire. The delta computation was
> totally useless.
>
> Now, consider libsvn_ra_local. No delta computation ever has to occur.
> RA just writes the full text, which writes it into the FS. This stuff
> is all linked together, so no need for computing/sending/applying
> deltas exist. Just pass it through in-memory streams from the source
> into the FS.
>
> Consider 'svn import'. The RA networking layers can do a simple zlib
> compression on the wire, sending the fulltext up to the server. It
> *could* do a delta computation against the empty file for a bit of
> self-compression, but the space savings are comparable, so it is
> actually nicer to just go with standard compression (which helps with
> intermediary servers, proxies, etc; they may try to compress one of
> our delta-compressed files because they think none has occurred; so
> those intermediaries would be doing senseless work; but if the request
> was tagged as 'deflate', then all is good -- no extra compression and
> tools can look at the fulltext if desired).
>
> Consider 'svn merge': as changes are being applied to the working
> copy, why should a delta ever happen? The working copy wants fulltext.
> Sure, 'svn patch' will be applying a patch, but there isn't really any
> delta needs anywhere in merge/patch. If the RA needs to request a file
> source, then it can certainly provide a BASE, get a delta, and
> generate a fulltext; but I'll note this RA behavior is operating
> outside of the Editor API anyways.

But the editor probably has to provide the info to its callbacks on the
three url_at_rev locations that are related (wc-base, merge left, merge right)
to at least allow generating deltas.

These three files are always related in a merge. For a single line change in
a 15 MB file that relation is the difference between a 30 MB data transfer
and one of only a few KByte.

Just think about how blame would perform if it had to transfer all revisions
as full text instead of only the differences between them.
(AnkhSVN uses this api for showing the differences between two directly
following revision; which made showing diffs much faster than every other
GUI client I tried)

        Bert

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Received on 2009-09-16 09:58:04 CEST

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