forking media files

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Peter Gervai

15 Aug 2011 15 Aug '11

1:45 a.m.

Let me retitle one of the topics nobody seems to touch. On Fri, Aug 12, 2011 at 13:44, Brion Vibber <brion(a)pobox.com> wrote:

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* media files -- these are freely copiable but I'm not sure the state of easily obtaing them in bulk. As the data set moved into TB it became impractical to just build .tar dumps. There are batch downloader tools available, and the metadata's all in dumps and api.

Right now it is basically locked: there is no way to bulk copy the media files, including doing simply a backup of one wikipedia, or commons. I've tried, I've asked, and the answer was basically to contact a dev and arrange it, which obviously could be done (I know many of the folks) but that isn't the point. Some explanations were mentioned, mostly mentioning that media and its metadata is quite detached, and thus it's hard to enforce licensing quirks like attribution, special licenses and such. I can guess this is a relevant comment since the text corpus is uniformly licensed under CC/GFDL while the media files are at best non-homogeneous (like commons, where everything's free in a way) and completely chaos at its worst (individual wikipedias, where there may be anything from leftover fair use to copyrighted by various entities to images to be deleted "soon"). Still, I do not believe it's a good method to make it close to impossible to bulk copy the data. I am not sure which technical means is best, as there are many competing ones. We could, for example, open up an API which would serve media file with its metadata together, possibly supporting mass operations. Still, it's pretty ineffective. Or we could support zsync, rsync and such (and I again recommend examining zsync's several interesting abilities to offload the work to the client), but there ought to be some pointers to image metadata, at least an oneliner file with every image linking to the license page. Or we could connect the bulk way to established editor accounts, so we could have at least a bit of an assurance that s/he knows what s/he's doing. -- byte-byte, grin

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Russell N. Nelson - rnnelson

15 Aug 15 Aug

9:40 a.m.

The problem is that 1) the files are bulky, 2) there are many of them, 3) they are in constant flux, and 4) it's likely that your connection would close for whatever reason part-way through the download.. Even taking a snapshot of the filenames is dicey. By the time you finish, it's likely that there will be new ones, and possible that some will be deleted. Probably the best way to make this work is to 1) make a snapshot of files periodically, 2) create an API which returns a tarball using the snapshot of files that also implements Range requests. The snapshot of filenames would have to include file sizes so the server would know where to restart. Once the snapshot had not been accessed in a week, it would be deleted. As a snapshot got older and older it would be less and less accurate, but hey, life is tough that way. Of course, this would result in a 12-terabyte file on the recipient's host. That wouldn't work very well. I'm pretty sure that the recipient would need an http client which would 1) keep track of the place in the bytestream and 2) split out files and write them to disk as separate files. It's possible that a program like getbot already implements this. ________________________________________ From: wikitech-l-bounces(a)lists.wikimedia.org [wikitech-l-bounces(a)lists.wikimedia.org] on behalf of Peter Gervai [grinapo(a)gmail.com] Sent: Monday, August 15, 2011 4:45 AM To: Wikimedia developers Subject: [Wikitech-l] forking media files Let me retitle one of the topics nobody seems to touch. On Fri, Aug 12, 2011 at 13:44, Brion Vibber <brion(a)pobox.com> wrote:

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Peter Gervai

2:40 p.m.

On Mon, Aug 15, 2011 at 18:40, Russell N. Nelson - rnnelson <rnnelson(a)clarkson.edu> wrote:

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The problem is that 1) the files are bulky,

That's expected. :-)

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2) there are many of them, 3) they are in constant flux,

That is not really a problem: since there are many of them statistically they are not in flux.

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and 4) it's likely that your connection would close for whatever reason part-way through the download..

I seem not to forgot to mention zsync/rsync. ;-)

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Even taking a snapshot of the filenames is dicey. By the time you finish, it's likely that there will be new ones, and possible that some will be deleted. Probably the best way to make this work is to 1) make a snapshot of files periodically,

Since I've been told they're backed up it naturally should exist.

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2) create an API which returns a tarball using the snapshot of files that also implements Range requests.

I would very much prefer ready-to-use format instead of a tarball, not to mention it's pretty resource consuming to create a tarball just for that.

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Of course, this would result in a 12-terabyte file on the recipient's host. That wouldn't work very well. I'm pretty sure that the recipient would need an http client which would 1) keep track of the place in the bytestream and 2) split out files and write them to disk as separate files. It's possible that a program like getbot already implements this.

I'd make a snapshot without tar especially because partial transfers aren't possible that way. -- byte-byte, grin

Russell N. Nelson - rnnelson

3:14 p.m.

I hate this email client. Hate, hate, hate. Thank you, Microsoft, for making my life that little bit worse. Anyway, you can't rely on the media files being stored in a filesystem. They could be stored in a database or an object storage. So *sync is not available. I don't know how the media files are backed up. If you only want the originals, that's a lot less than 12TB (or whatever is the current number for thumbs+origs). If you just want to fetch a tarball, wget or curl will automaticallly restart a connection and supply a range parameter if the server supports it. If you want a ready-to-use format, then you're going to need a client which can write individual files. But it's not particularly efficient to stream 120B files over a separate TCP connection. You'd have to have a client which can do TCP session reuse. No matter how you cut it, you're looking at a custom client. But there's no need to invent a new download protocol or stream format. That's why I suggest tarball and range. Standards ... they're not just for breakfast. ________________________________________ From: wikitech-l-bounces(a)lists.wikimedia.org [wikitech-l-bounces(a)lists.wikimedia.org] on behalf of Peter Gervai [grinapo(a)gmail.com] Sent: Monday, August 15, 2011 5:40 PM To: Wikimedia developers Subject: Re: [Wikitech-l] forking media files On Mon, Aug 15, 2011 at 18:40, Russell N. Nelson - rnnelson <rnnelson(a)clarkson.edu> wrote:

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The problem is that 1) the files are bulky,

That's expected. :-)

...

2) there are many of them, 3) they are in constant flux,

That is not really a problem: since there are many of them statistically they are not in flux.

...

and 4) it's likely that your connection would close for whatever reason part-way through the download..

I seem not to forgot to mention zsync/rsync. ;-)

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Since I've been told they're backed up it naturally should exist.

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2) create an API which returns a tarball using the snapshot of files that also implements Range requests.

I would very much prefer ready-to-use format instead of a tarball, not to mention it's pretty resource consuming to create a tarball just for that.

...

I'd make a snapshot without tar especially because partial transfers aren't possible that way. -- byte-byte, grin _______________________________________________ Wikitech-l mailing list Wikitech-l(a)lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikitech-l

Brion Vibber

3:31 p.m.

On Mon, Aug 15, 2011 at 3:14 PM, Russell N. Nelson - rnnelson < rnnelson(a)clarkson.edu> wrote:

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Anyway, you can't rely on the media files being stored in a filesystem. They could be stored in a database or an object storage. So *sync is not available.

Note that at this moment, upload file storage on the Wikimedia sites is still 'some big directories on an NFS server', but at some point is planned to migrate to a Swift cluster backend: http://www.mediawiki.org/wiki/Extension:SwiftMedia A file dump / bulk-fetching intermediary would probably need to speak to MediaWiki to get lists of available files and then bump back via the backend to actually obtain them. There's no reason why this couldn't speak something like the rsync protocol, of course. I don't know how the media files are backed up. If you only want the

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originals, that's a lot less than 12TB (or whatever is the current number for thumbs+origs). If you just want to fetch a tarball, wget or curl will automaticallly restart a connection and supply a range parameter if the server supports it. If you want a ready-to-use format, then you're going to need a client which can write individual files. But it's not particularly efficient to stream 120B files over a separate TCP connection. You'd have to have a client which can do TCP session reuse. No matter how you cut it, you're looking at a custom client. But there's no need to invent a new download protocol or stream format. That's why I suggest tarball and range. Standards ... they're not just for breakfast.

Range on a tarball assumes that you have a static tarball file -- or else predictable, unchanging snapshot of its contents that can be used to simulate one: 1) every filename in the data set, in order 2) every file's exact size and version 3) every other bit of file metadata that might go into constructing that tarball or else actually generating and storing a giant tarball, and then keeping it around long enough for all clients to download the whole thing -- obviously not very attractive. Since every tiny change (*any* new file, *any* changed file, *any* deleted file) would alter the generated tarball and shift terabytes of data around, this doesn't seem like it would be a big win for anything other than initial downloads of the full data set (or else batching up specifically-requested files). Anything that involves updating your mirror/copy/fork/backup needs to work in a more live fashion, that only needs to transfer new data for things that have changed. rsync can check for differences but still needs to go over the full file list (and so still takes a Long Time and lots of bandwidth just to do that). -- brion

Russell N. Nelson - rnnelson

4:16 p.m.

Exactly what I propose. Keep a list of files and their sizes, so that when somebody asks for a range, you can skip files up until you get to the range they've requested. Not worrying about new or already-downloaded changed files, or deleted files. You're not getting a "current" copy of the files, you're getting a copy of the files that were available when you started your download. Minus the deleted files, which by policy we shouldn't be handing out anyway. rsync doesn't have the MW database to consult for changes. ________________________________________ From: wikitech-l-bounces(a)lists.wikimedia.org [wikitech-l-bounces(a)lists.wikimedia.org] on behalf of Brion Vibber [brion(a)pobox.com] Sent: Monday, August 15, 2011 6:31 PM To: Wikimedia developers Subject: Re: [Wikitech-l] forking media files On Mon, Aug 15, 2011 at 3:14 PM, Russell N. Nelson - rnnelson < rnnelson(a)clarkson.edu> wrote:

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download protocol or stream format. That's why I suggest tarball and range. Standards ... they're not just for breakfast.

Brion Vibber

4:22 p.m.

On Mon, Aug 15, 2011 at 4:16 PM, Russell N. Nelson - rnnelson < rnnelson(a)clarkson.edu> wrote:

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Except the ones that weren't deleted when you started your download, I presume? Otherwise you've now got an inconsistent data set. And of course anything that has changed, you'll want to make sure you can access the original version, not the new version, or else the size or contents will be wrong and you'll end up sending bad info.

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rsync doesn't have the MW database to consult for changes.

That's an implementation detail, isn't it? GNU tar doesn't either. -- brion

Russell N. Nelson - rnnelson

4:30 p.m.

Wasn't suggesting that GNU tar or PDtar or any suchlike would be usable. I'm pretty sure that whatever protocol is used, you probably can't have a standard client or server simply because of the size of the data to be transfered. Maybe rsync would work with a custom rsyncd? Not so familiar with that protocol. Doesn't it compute an md5 for all files and ship it around? -russ ________________________________________ From: wikitech-l-bounces(a)lists.wikimedia.org [wikitech-l-bounces(a)lists.wikimedia.org] on behalf of Brion Vibber [brion(a)pobox.com] Sent: Monday, August 15, 2011 7:22 PM To: Wikimedia developers Subject: Re: [Wikitech-l] forking media files On Mon, Aug 15, 2011 at 4:16 PM, Russell N. Nelson - rnnelson < rnnelson(a)clarkson.edu> wrote:

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rsync doesn't have the MW database to consult for changes.

That's an implementation detail, isn't it? GNU tar doesn't either.

Brion Vibber

5:06 p.m.

On Mon, Aug 15, 2011 at 4:30 PM, Russell N. Nelson - rnnelson < rnnelson(a)clarkson.edu> wrote:

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rsync's wire protocol isn't very well documented, but roughly speaking, it builds a "file list" of every file that may need to be transferred, and the server and client compare notes to see which ones will actually need to get transferred (and then which pieces of the files need to be transferred, if they exist in both places). Since rsync 3 the file list can be built and sent incrementally, which made it possible to do batch rsyncs of Wikimedia uploads to/from a couple of ad-hoc off-site servers (I think Greg Maxwell ran one for a while? I do not know whether any of these are still in place -- other people manage these servers now and I just haven't paid attention). Older versions of rsync would build and transfer the entire file list first, which was impractical when a complete file list for millions and millions of files would be bigger than RAM and take hours just to generate. :) A custom rsync daemon could certainly speak to regular rsync clients to manage doing the file listings and pulling up the appropriate backend file. Simply re-starting the transfer can handle pulling updates or continuing a broken transfer with no additional trouble. For ideal incremental updates & recoveries you'd want to avoid having to transfer data about unchanged files -- rsync will still have to send that file list over so it can check if files need to be updated. A more customized protocol might end up better at that; offhand I'm not sure if rsync 3's protocol can be super convenient at that or whether something else would be needed. (For the most part we don't need rsync's ability to transfer pieces of large individual files, though it's a win if a transfer gets interrupted on a large video file; usually we just want to find *new* files or files that need to be deleted. It may be possible to optimize this on the existing protocol with timestamp limitations.) -- brion

Russell N. Nelson - rnnelson

7:10 p.m.

I see several paths forward on this: 1) Make an existing protocol and client work. Whether ftp or rsync or http or scp, they think they're copying a tree of filles. a) Give people access to something that looks like a tree of folders, and just let them recurse as needed using "wget -m". Doesn't quite make me want to barf. 2) Make an existing protocol work, even if a new client is needed for optimal use. E.g. wget -m with an extra parameter that only shows the client new files since the date of the last sync. 3) Devise a new protocol. Call it "BCD" for "Big Copy of Data". a) I'm thinking that the client should have the capability of asking for files with timestamps in a given range. b) The client would then be able to keep a record of the timestamp ranges for which it is currently accurate. c) A file deletion event would have a timestamp. Once deleted, the file would be unavailable even if its timestamp was requested. d) Any change in filename becomes an edit event. e) The idea is that a client would never have to re-ask for a timestamp range again. ______________________________________ From: wikitech-l-bounces(a)lists.wikimedia.org [wikitech-l-bounces(a)lists.wikimedia.org] on behalf of Brion Vibber [brion(a)pobox.com] Sent: Monday, August 15, 2011 8:06 PM To: Wikimedia developers Subject: Re: [Wikitech-l] forking media files A more customized protocol might end up better at that; offhand I'm not sure if rsync 3's protocol can be super convenient at that or whether something else would be needed.

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Brion Vibber
Peter Gervai
Russell N. Nelson - rnnelson