If a node is down, then it is ignored.<div>That is the whole point about 3 replicas.<br><br><div class="gmail_quote">On Fri, Jan 20, 2012 at 10:43 AM, Nikolaus Rath <span dir="ltr"><<a href="mailto:Nikolaus@rath.org">Nikolaus@rath.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi,<br>
<br>
What happens if one of the nodes is down? Especially if that node holds<br>
the newest copy?<br>
<br>
Thanks,<br>
Nikolaus<br>
<br>
On 01/20/2012 12:33 PM, Stephen Broeker wrote:<br>
> The X-Newest header can be used by a GET Operation to ensure that all of the<br>
> Storage Nodes (3 by default) are queried for the latest copy of the Object.<br>
> The COPY Object operation already has this functionality.<br>
><br>
> On Fri, Jan 20, 2012 at 9:12 AM, Nikolaus Rath <<a href="mailto:Nikolaus@rath.org">Nikolaus@rath.org</a><br>
> <mailto:<a href="mailto:Nikolaus@rath.org">Nikolaus@rath.org</a>>> wrote:<br>
><br>
> Hi,<br>
><br>
> No one able to further clarify this?<br>
><br>
> Does swift offer there read-after-create consistence like<br>
> non-us-standard S3? What are the precise syntax and semantics of<br>
> X-Newest header?<br>
><br>
> Best,<br>
> Nikolaus<br>
><br>
><br>
> On 01/18/2012 10:15 AM, Nikolaus Rath wrote:<br>
> > Michael Barton <<a href="mailto:mike-launchpad@weirdlooking.com">mike-launchpad@weirdlooking.com</a><br>
> <mailto:<a href="mailto:mike-launchpad@weirdlooking.com">mike-launchpad@weirdlooking.com</a>>> writes:<br>
> >> On Tue, Jan 17, 2012 at 4:55 PM, Nikolaus Rath <<a href="mailto:Nikolaus@rath.org">Nikolaus@rath.org</a><br>
> <mailto:<a href="mailto:Nikolaus@rath.org">Nikolaus@rath.org</a>>> wrote:<br>
> >>> Amazon S3 and Google Storage make very explicit (non-) consistency<br>
> >>> guarantees for stored objects. I'm looking for a similar<br>
> documentation<br>
> >>> about OpenStack's Swift, but haven't had much success.<br>
> >><br>
> >> I don't think there's any documentation on this, but it would<br>
> probably<br>
> >> be good to write up. Consistency in Swift is very similar to S3.<br>
> >> That is, there aren't many non-eventual consistency guarantees.<br>
> >><br>
> >> Listing updates can happen asynchronously (especially under<br>
> load), and<br>
> >> older versions of files can show up in requests (deletes are just a<br>
> >> new "deleted" version of the file).<br>
> ><br>
> > Ah, ok. Thanks a lot for stating this so explicitly. There seems<br>
> to be a<br>
> > lot of confusion about this, now I can at least point people to<br>
> > something.<br>
> ><br>
> >> Swift can generally be relied on for read-after-write consistency,<br>
> >> like S3's regions other than the the US Standard region. The reason<br>
> >> S3 in US Standard doesn't have this guarantee is because it's more<br>
> >> geographically widespread - something Swift isn't good at yet. I can<br>
> >> imagine we'll have the same limitation when we get there.<br>
> ><br>
> > Do you mean read-after-create consistency? Because below you say about<br>
> > read-after-write:<br>
> ><br>
> >>> - If I receive a (non-error) response to a PUT request, am I<br>
> guaranteed<br>
> >>> that the object will be immediately included in all object<br>
> listings in<br>
> >>> every possible situation?<br>
> >><br>
> >> Nope.<br>
> ><br>
> > ..so is there such a guarantee for PUTs of *new* objects (like S3 non<br>
> > us-classic), or does "can generally be relied on" just mean that the<br>
> > chances for new puts are better?<br>
> ><br>
> >> Also like S3, Swift can't make any strong guarantees about<br>
> >> read-after-update or read-after-delete consistency. We do have an<br>
> >> "X-Newest" header that can be added to GETs and HEADs to make the<br>
> >> proxy do a quorum of backend servers and return the newest available<br>
> >> version, which greatly improves these, at the cost of latency.<br>
> ><br>
> > That sounds very interesting. Could you give some more details on what<br>
> > exactly is guaranteed when using this header? What happens if the<br>
> server<br>
> > having the newest copy is down?<br>
> ><br>
> >>> - If the swift server looses an object, will the object name<br>
> still be<br>
> >>> returned in object listings? Will attempts to retrieve it result<br>
> in 404<br>
> >>> errors (as if it never existed) or a different error?<br>
> >><br>
> >> It will show up in listings, but give a 404 when you attempt to<br>
> >> retrieve it. I'm not sure how we can improve that with Swift's<br>
> >> general model, but feel free to make suggestions.<br>
> ><br>
> > From an application programmers point of view, it would be very<br>
> helpful<br>
> > if lost objects could be distinguished from non-existing object by a<br>
> > different HTTP error. Trying to access a non-existing object may<br>
> > indicate a bug in the application, so it would be nice to know when it<br>
> > happens.<br>
> ><br>
> > Also, it would be very helpful if there was a way to list all lost<br>
> > objects without having to issue HEAD requests for every stored object.<br>
> > Could this information be added to the XML and JSON output of<br>
> container<br>
> > listings? Then an application would have the chance to periodically<br>
> > check for lost data, rather than having to handle all lost objects at<br>
> > the instant they're required.<br>
> ><br>
> ><br>
> > I am working on a swift backend for S3QL<br>
> > (<a href="http://code.google.com/p/s3ql/" target="_blank">http://code.google.com/p/s3ql/</a>), a program that exposes online cloud<br>
> > storage as a local UNIX file system. To prevent data corruption, there<br>
> > are two requirements that I'm currently struggling to provide with the<br>
> > swift backend:<br>
> ><br>
> > - There needs to be a way to reliably check if one object (holding the<br>
> > file system metadata) is the newest version.<br>
> ><br>
> > The S3 backend does this by requiring storage in the non us-classic<br>
> > regions and using list-after-create consistency with a marker object<br>
> > that has has a "generation number" of the metadata embedded in its<br>
> > name.<br>
> ><br>
> > I'm not yet sure if this would work with swift as well (the google<br>
> > storage backend just relies on the strong read-after-write<br>
> > consistency).<br>
> ><br>
> > - The file system checker needs a way to identify lost objects.<br>
> ><br>
> > Here the S3 backend just relies on the durability guarantee that<br>
> > effectively no object will ever be lost.<br>
> ><br>
> > Again, I'm not sure how to implement this for swift.<br>
> ><br>
> ><br>
> > Any suggestions?<br>
> ><br>
> ><br>
> ><br>
> > Best,<br>
> ><br>
> > -Nikolaus<br>
> ><br>
><br>
><br>
> -Nikolaus<br>
><br>
> --<br>
> »Time flies like an arrow, fruit flies like a Banana.«<br>
><br>
> PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C<br>
><br>
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><br>
><br>
<span class="HOEnZb"><font color="#888888"><br>
<br>
-Nikolaus<br>
<br>
--<br>
»Time flies like an arrow, fruit flies like a Banana.«<br>
<br>
PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C<br>
</font></span></blockquote></div><br></div>