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