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Re: Swift Consistency Guarantees?



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?


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


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