By default there are 3 replicas. A PUT Object will return after 2 replicas are done. So if all nodes are up then there are at least 2 replicas. If all replica nodes are down, then the GET Object will fail.
On Fri, Jan 20, 2012 at 11:21 AM, Nikolaus Rath <nikol...@rath.org> wrote: > Hi, > > So if an object update has not yet been replicated on all nodes, and all > nodes that have been updated are offline, what will happen? Will swift > recognize this and give me an error, or will it silently return the > older version? > > Thanks, > Nikolaus > > > On 01/20/2012 02:14 PM, Stephen Broeker wrote: > > If a node is down, then it is ignored. > > That is the whole point about 3 replicas. > > > > On Fri, Jan 20, 2012 at 10:43 AM, Nikolaus Rath <nikol...@rath.org > > <mailto:nikol...@rath.org>> wrote: > > > > Hi, > > > > What happens if one of the nodes is down? Especially if that node > holds > > the newest copy? > > > > Thanks, > > Nikolaus > > > > On 01/20/2012 12:33 PM, Stephen Broeker wrote: > > > The X-Newest header can be used by a GET Operation to ensure that > > all of the > > > Storage Nodes (3 by default) are queried for the latest copy of > > the Object. > > > The COPY Object operation already has this functionality. > > > > > > On Fri, Jan 20, 2012 at 9:12 AM, Nikolaus Rath <nikol...@rath.org > > <mailto:nikol...@rath.org> > > > <mailto:nikol...@rath.org <mailto:nikol...@rath.org>>> wrote: > > > > > > Hi, > > > > > > No one able to further clarify this? > > > > > > Does swift offer there read-after-create consistence like > > > non-us-standard S3? What are the precise syntax and semantics > of > > > X-Newest header? > > > > > > Best, > > > Nikolaus > > > > > > > > > On 01/18/2012 10:15 AM, Nikolaus Rath wrote: > > > > Michael Barton <mike-launch...@weirdlooking.com > > <mailto:mike-launch...@weirdlooking.com> > > > <mailto:mike-launch...@weirdlooking.com > > <mailto:mike-launch...@weirdlooking.com>>> writes: > > > >> On Tue, Jan 17, 2012 at 4:55 PM, Nikolaus Rath > > <nikol...@rath.org <mailto:nikol...@rath.org> > > > <mailto:nikol...@rath.org <mailto:nikol...@rath.org>>> wrote: > > > >>> Amazon S3 and Google Storage make very explicit (non-) > > consistency > > > >>> guarantees for stored objects. I'm looking for a similar > > > documentation > > > >>> about OpenStack's Swift, but haven't had much success. > > > >> > > > >> I don't think there's any documentation on this, but it > would > > > probably > > > >> be good to write up. Consistency in Swift is very similar > > to S3. > > > >> That is, there aren't many non-eventual consistency > guarantees. > > > >> > > > >> Listing updates can happen asynchronously (especially under > > > load), and > > > >> older versions of files can show up in requests (deletes > > are just a > > > >> new "deleted" version of the file). > > > > > > > > Ah, ok. Thanks a lot for stating this so explicitly. There > seems > > > to be a > > > > lot of confusion about this, now I can at least point people > to > > > > something. > > > > > > > >> Swift can generally be relied on for read-after-write > > consistency, > > > >> like S3's regions other than the the US Standard region. > > The reason > > > >> S3 in US Standard doesn't have this guarantee is because > > it's more > > > >> geographically widespread - something Swift isn't good at > > yet. I can > > > >> imagine we'll have the same limitation when we get there. > > > > > > > > Do you mean read-after-create consistency? Because below you > > say about > > > > read-after-write: > > > > > > > >>> - If I receive a (non-error) response to a PUT request, am > I > > > guaranteed > > > >>> that the object will be immediately included in all object > > > listings in > > > >>> every possible situation? > > > >> > > > >> Nope. > > > > > > > > ..so is there such a guarantee for PUTs of *new* objects > > (like S3 non > > > > us-classic), or does "can generally be relied on" just mean > > that the > > > > chances for new puts are better? > > > > > > > >> Also like S3, Swift can't make any strong guarantees about > > > >> read-after-update or read-after-delete consistency. We do > > have an > > > >> "X-Newest" header that can be added to GETs and HEADs to > > make the > > > >> proxy do a quorum of backend servers and return the newest > > available > > > >> version, which greatly improves these, at the cost of > latency. > > > > > > > > That sounds very interesting. Could you give some more > > details on what > > > > exactly is guaranteed when using this header? What happens > > if the > > > server > > > > having the newest copy is down? > > > > > > > >>> - If the swift server looses an object, will the object > name > > > still be > > > >>> returned in object listings? Will attempts to retrieve it > > result > > > in 404 > > > >>> errors (as if it never existed) or a different error? > > > >> > > > >> It will show up in listings, but give a 404 when you > attempt to > > > >> retrieve it. I'm not sure how we can improve that with > Swift's > > > >> general model, but feel free to make suggestions. > > > > > > > > From an application programmers point of view, it would be > very > > > helpful > > > > if lost objects could be distinguished from non-existing > > object by a > > > > different HTTP error. Trying to access a non-existing object > may > > > > indicate a bug in the application, so it would be nice to > > know when it > > > > happens. > > > > > > > > Also, it would be very helpful if there was a way to list > > all lost > > > > objects without having to issue HEAD requests for every > > stored object. > > > > Could this information be added to the XML and JSON output of > > > container > > > > listings? Then an application would have the chance to > > periodically > > > > check for lost data, rather than having to handle all lost > > objects at > > > > the instant they're required. > > > > > > > > > > > > I am working on a swift backend for S3QL > > > > (http://code.google.com/p/s3ql/), a program that exposes > > online cloud > > > > storage as a local UNIX file system. To prevent data > > corruption, there > > > > are two requirements that I'm currently struggling to > > provide with the > > > > swift backend: > > > > > > > > - There needs to be a way to reliably check if one object > > (holding the > > > > file system metadata) is the newest version. > > > > > > > > The S3 backend does this by requiring storage in the non > > us-classic > > > > regions and using list-after-create consistency with a > > marker object > > > > that has has a "generation number" of the metadata > > embedded in its > > > > name. > > > > > > > > I'm not yet sure if this would work with swift as well > > (the google > > > > storage backend just relies on the strong read-after-write > > > > consistency). > > > > > > > > - The file system checker needs a way to identify lost > objects. > > > > > > > > Here the S3 backend just relies on the durability > > guarantee that > > > > effectively no object will ever be lost. > > > > > > > > Again, I'm not sure how to implement this for swift. > > > > > > > > > > > > Any suggestions? > > > > > > > > > > > > > > > > Best, > > > > > > > > -Nikolaus > > > > > > > > > > > > > -Nikolaus > > > > > > -- > > > »Time flies like an arrow, fruit flies like a Banana.« > > > > > > PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 > > AE4E 425C > > > > > > _______________________________________________ > > > Mailing list: https://launchpad.net/~openstack > > > Post to : openstack@lists.launchpad.net > > <mailto:openstack@lists.launchpad.net> > > > <mailto:openstack@lists.launchpad.net > > <mailto:openstack@lists.launchpad.net>> > > > Unsubscribe : https://launchpad.net/~openstack > > > More help : https://help.launchpad.net/ListHelp > > > > > > > > > > > > -Nikolaus > > > > -- > > »Time flies like an arrow, fruit flies like a Banana.« > > > > PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C > > > > > > > -Nikolaus > > -- > »Time flies like an arrow, fruit flies like a Banana.« > > PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C >
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