Open Stack

Jay Pipes <jaypipes at gmail.com> wrote on 03/18/2014 12:02:50 PM:

> From: Jay Pipes <jaypipes at gmail.com>
> To: openstack-dev at lists.openstack.org,
> Date: 03/18/2014 12:15 PM
> Subject: Re: [openstack-dev] Offset support in REST API pagination
>
> On Tue, 2014-03-18 at 11:31 -0500, Steven Kaufer wrote:
> > First, here is some background on this topic:
> >  http://www.gossamer-threads.com/lists/openstack/dev/2777
> >
> > Does anyone have any insight as to why offset is not supported in the
> > REST API calls that support pagination?   I realize that there are
> > tradeoffs when using a offset (vs. marker) but I believe that there is
> > value in supporting both.  For example, if you want to jump to the
> > n-th page of data without having to traverse all of the previous
> > pages.
> >
> > Is there a reason why the APIs do not support either a marker or an
> > offset (obviously not both) on the API request?  It appears that
> > sqlalchemy has offset support.
> >
> > Also, it seems that cinder at least looks for the offset parameter
> > (but ignores it).  Does this mean that it was supported at one time
> > but later the support was removed?
> >  https://github.com/openstack/cinder/blob/master/cinder/api/v2/
> volumes.py#L214
> >
> > Thanks for the information.
>
> Hail to thee, stranger! Thou hast apparently not passed into the cave of
> marker/offset before!
>
> I humbly direct you to buried mailing list treasures which shall
> enlighten you!
>
> This lengthy thread shall show you how yours truly was defeated in
> written combat by the great Justin Santa Barbara, who doth exposed the
> perils of the offset:
>
> http://www.gossamer-threads.com/lists/openstack/dev/2803
>
> A most recent incantation of the marker/offset wars is giveth here:
>
>
http://lists.openstack.org/pipermail/openstack-dev/2013-November/018861.html

>
> Best of days to you,
> -jay

Jay:

Thanks for the feedback and the history on this topic. I understand that
the limit/marker
approach is superior when simply traversing all of the pages. However,
consider the
following:

- User knows that there are 1000 items (VMs, volumes, images, really
doesn't matter)
- User knows that the item that they want is in roughly the middle of the
data set (assume
everything is sorted by display name)
- User cannot remember the exact name so a filter will not help and
changing the sort
direction will not help (since the item they want it is in the middle of
the dataset)
- User supplies an offset of 500 to jump into the middle of the data set
- User then uses the marker approach to traverse the pages from this point
to find the
item that they want

In this case the offset approach is not used to traverse pages so there are
no issues with
missing an item or seeing a duplicate.

Why couldn't the APIs support either marker or offset on a given request?
Also, to encourage
the use of marker instead of offset, the next/previous links on any request
with an offset
supplied should contain the appropriate marker key values -- this should
help discourage
simply increasing the offset when traversing the pages.

I realize that if only one solution had to be chosen, then limit/marker
would always win
this war. But why can't both be supported?

Thanks,

Steven Kaufer

>
>
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