Why not just use Cache-Control: no-cache ? That way, intervening caches will do the right thing too...
Cheers, On 21/01/2012, at 4:33 AM, 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> 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> writes: > >> On Tue, Jan 17, 2012 at 4:55 PM, Nikolaus Rath <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 > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp > > _______________________________________________ > Mailing list: https://launchpad.net/~openstack > Post to : openstack@lists.launchpad.net > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp -- Mark Nottingham http://www.mnot.net/ _______________________________________________ Mailing list: https://launchpad.net/~openstack Post to : openstack@lists.launchpad.net Unsubscribe : https://launchpad.net/~openstack More help : https://help.launchpad.net/ListHelp
