openstack team mailing list archive

[Nikolaus Rath <Nikolaus@xxxxxxxx>]

Hi,

Sorry for being so persistent, but I'm still not sure what happens if
the 2 servers that carry the new replica are down, but the 1 server that
has the old replica is up. Will GET fail or return the old replica?

Best,
Niko

On 01/20/2012 02:52 PM, Stephen Broeker wrote:
> 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 <Nikolaus@xxxxxxxx
> <mailto:Nikolaus@xxxxxxxx>> 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 <Nikolaus@xxxxxxxx
>     <mailto:Nikolaus@xxxxxxxx>
>     > <mailto:Nikolaus@xxxxxxxx <mailto:Nikolaus@xxxxxxxx>>> 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
>     <Nikolaus@xxxxxxxx <mailto:Nikolaus@xxxxxxxx>
>     >     <mailto:Nikolaus@xxxxxxxx <mailto:Nikolaus@xxxxxxxx>>
>     >     > <mailto:Nikolaus@xxxxxxxx <mailto:Nikolaus@xxxxxxxx>
>     <mailto:Nikolaus@xxxxxxxx <mailto: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
>     <mailto:mike-launchpad@xxxxxxxxxxxxxxxx>
>     >     <mailto:mike-launchpad@xxxxxxxxxxxxxxxx
>     <mailto:mike-launchpad@xxxxxxxxxxxxxxxx>>
>     >     >     <mailto:mike-launchpad@xxxxxxxxxxxxxxxx
>     <mailto:mike-launchpad@xxxxxxxxxxxxxxxx>
>     >     <mailto:mike-launchpad@xxxxxxxxxxxxxxxx
>     <mailto:mike-launchpad@xxxxxxxxxxxxxxxx>>>> writes:
>     >     >     >> On Tue, Jan 17, 2012 at 4:55 PM, Nikolaus Rath
>     >     <Nikolaus@xxxxxxxx <mailto:Nikolaus@xxxxxxxx>
>     <mailto:Nikolaus@xxxxxxxx <mailto:Nikolaus@xxxxxxxx>>
>     >     >     <mailto:Nikolaus@xxxxxxxx <mailto:Nikolaus@xxxxxxxx>
>     <mailto:Nikolaus@xxxxxxxx <mailto: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
>     >     >
>     >     >     _______________________________________________
>     >     >     Mailing list: https://launchpad.net/~openstack
>     >     >     Post to     : openstack@xxxxxxxxxxxxxxxxxxx
>     <mailto:openstack@xxxxxxxxxxxxxxxxxxx>
>     >     <mailto:openstack@xxxxxxxxxxxxxxxxxxx
>     <mailto:openstack@xxxxxxxxxxxxxxxxxxx>>
>     >     >     <mailto:openstack@xxxxxxxxxxxxxxxxxxx
>     <mailto:openstack@xxxxxxxxxxxxxxxxxxx>
>     >     <mailto:openstack@xxxxxxxxxxxxxxxxxxx
>     <mailto:openstack@xxxxxxxxxxxxxxxxxxx>>>
>     >     >     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
> 
> 


   -Nikolaus

-- 
 »Time flies like an arrow, fruit flies like a Banana.«

  PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6  02CF A9AD B7F8 AE4E 425C