[SSSD] Design Discussion: Domains, users and groups over D-Bus

Jakub Hrozek jhrozek at redhat.com
Fri Jan 9 08:59:00 UTC 2015


On Thu, Jan 08, 2015 at 09:16:53PM +0100, Pavel Březina wrote:
> On 01/08/2015 07:44 PM, Jakub Hrozek wrote:
> >On Thu, Jan 08, 2015 at 09:35:42AM -0500, Dmitri Pal wrote:
> >>On 01/08/2015 04:05 AM, Jakub Hrozek wrote:
> >>>On Wed, Jan 07, 2015 at 03:41:01PM -0500, Simo Sorce wrote:
> >>>>On Wed, 07 Jan 2015 15:25:30 -0500
> >>>>Dmitri Pal <dpal at redhat.com> wrote:
> >>>>
> >>>>>On 01/07/2015 03:05 PM, Simo Sorce wrote:
> >>>>>>On Tue, 06 Jan 2015 09:59:08 -0500
> >>>>>>Dmitri Pal <dpal at redhat.com> wrote:
> >>>>>>
> >>>>>>>On 01/06/2015 05:54 AM, Jakub Hrozek wrote:
> >>>>>>>>On Tue, Jan 06, 2015 at 11:31:55AM +0100, Pavel Březina wrote:
> >>>>>>>>>>>*Users*
> >>>>>>>>>>>Do we want also to have methods ListDomainUsers() and
> >>>>>>>>>>>ListUsers() without the name filter?
> >>>>>>>>>>To list all? What about using '*' for that?
> >>>>>>>>>We can implement it this way internally, but exposing an easier
> >>>>>>>>>way to the consumers is nice, imho.
> >>>>>>>>I'm not too opposed, although I prefer minimal APIs.
> >>>>>>>>
> >>>>>>>>>However, do we actually want to allow to list all users? As
> >>>>>>>>>Dmitri suggested we may want to require the minimum filter
> >>>>>>>>>length since the number of users may be very high. The maximum
> >>>>>>>>>D-Bus message is 128MiB so I think we are good there but I think
> >>>>>>>>>it can be very time consuming to return all users without some
> >>>>>>>>>sort of paging.
> >>>>>>>>This feature is internally dependant on enumerate=true, where we
> >>>>>>>>already store all standard POSIX attributes (struct passwd, struct
> >>>>>>>>group) in-memory, do you think the D-Bus "enumeration" provides
> >>>>>>>>that much overhead?
> >>>>>>>>
> >>>>>>>>Paging would be really complex, we'd need to store the full
> >>>>>>>>results in-memory per-client anyway and then pass around some
> >>>>>>>>kind of cookie to resume iteration..
> >>>>>>>>
> >>>>>>>>In a centralized environment, I wouldn't expect the listing
> >>>>>>>>commands to be used that commonly. Greeters or login managers
> >>>>>>>>(gdm) would typically use the cached users instead. Some
> >>>>>>>>applications (Hi, RHEV-M!) choose to display all their users in
> >>>>>>>>some kind of table and then I would expect them to implement
> >>>>>>>>paging themselves:
> >>>>>>>>
> >>>>>>>>for letter in a..z:
> >>>>>>>>       users = ListUsersByNameFilter($letter)
> >>>>>>>>
> >>>>>>>>>>>Do we want some other filter options as well?
> >>>>>>>>>>In the design I wanted to keep the filtering simple. Unless we
> >>>>>>>>>>receive some other requirements..
> >>>>>>>>>Yes, you suggested to allow only asterisk. Implement full regular
> >>>>>>>>>expression efficiently as Dmitri would be quite problematic since
> >>>>>>>>>ldb doesn't support regex lookup thus we would have to do this
> >>>>>>>>>ourselves and therefore we would loose indices, or am I wrong?
> >>>>>>>>I guess we'd have to grab all the entries and filter them
> >>>>>>>>ourselves..
> >>>>>>>>
> >>>>>>>>(Yes, this is the reason I chose the asterisk notation in the
> >>>>>>>>first place) _______________________________________________
> >>>>>>>>sssd-devel mailing list
> >>>>>>>>sssd-devel at lists.fedorahosted.org
> >>>>>>>>https://lists.fedorahosted.org/mailman/listinfo/sssd-devel
> >>>>>>>Several points:
> >>>>>>>
> >>>>>>>- IMO having a full regular expression support will be an overhead.
> >>>>>>>- "Begins with" filtering with * to indicate the remaining part is
> >>>>>>>good enough
> >>>>>>reasonable
> >>>>>>
> >>>>>>>- I do not think we should rely on enumeration. I think we should
> >>>>>>>do a lookup since these operations will be rare.
> >>>>>>Nope.
> >>>>>>This is the same thinking the implementers of the nss interface went
> >>>>>>through. And then users started using enumeration all the time.
> >>>>>>
> >>>>>>It may be ok to let users programmatically force full enumeration
> >>>>>>somehow. But the default should be to return only what is in cache.
> >>>>>>If you do otherwise people will test applications using * with 3
> >>>>>>users on the system and then fail spectacularly when there are
> >>>>>>actually 100K users in the directory.
> >>>>>I think there is a problem with the approach you suggest.
> >>>>>
> >>>>>Say there is an application that allows you to list groups starting
> >>>>>with a letter. It is used to define roles for application
> >>>>>administration.
> >>>>>
> >>>>>Assume that there is a group "Agroup". It is a new group that does
> >>>>>not have users yet. But it was created for use with the application
> >>>>>so that roles can be associated with this group. The admin of the
> >>>>>application thus wants to start using it.
> >>>>>This group will never be looked up if "A*" query will be run against
> >>>>>cache because it will not end up in cache. That would force admin to
> >>>>>turn full enumeration on SSSD. This is bad.
> >>>>What matters is the '*' does not do enumeration, if 'ABC*' causes an
> >>>>online lookup it is fine imo.
> >>>Yes, this is exactly what I was proposing. However, handling 'ABC*' on
> >>>the back end side must be implemented, currently we only handle a single
> >>>entry or enumeration.
> >>>
> >>>I might have confused you because I proposed to first finish the DBus
> >>>*responder* work and temporarily require enumeration until we extend the
> >>>back end.
> >>>
> >>>I hope it makes sense now.
> >>
> >>As long as the work is bound to the same milestone the order does not
> >>matter.
> >
> >OK, I filed https://fedorahosted.org/sssd/ticket/2553 into the same
> >milestone.
> 
> This brings another question. Because we are talking about
> ListUsersByFilter() that returns a list of object paths the only thing that
> is needed to construct such path is UID. So we can avoid membership
> evaluation here which would reduce load and processing time.
> 
> The caller will be probably interested mostly in user name, is that correct?
> So we can skip membership evaluation at all unless group property is
> requested.

Yes, we know updating membership is a costly operation, that's why there is a
separate method for that.



More information about the sssd-devel mailing list