openstack team mailing list archive

[Mark Nottingham <mnot@xxxxxxxx>]

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 <Nikolaus@xxxxxxxx> 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-launchpad@xxxxxxxxxxxxxxxx> writes:
> >> On Tue, Jan 17, 2012 at 4:55 PM, Nikolaus Rath <Nikolaus@xxxxxxxx> 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
> 
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--
Mark Nottingham   http://www.mnot.net/