On Fri, 09 Jan 2015 16:07:21 +0100
Ludwig Krispenz <lkris...@redhat.com> wrote:

> 
> On 01/09/2015 03:50 PM, Simo Sorce wrote:
> > On Fri, 09 Jan 2015 15:29:02 +0100
> > Ludwig Krispenz <lkris...@redhat.com> wrote:
> >
> >> On 01/07/2015 05:35 PM, Simo Sorce wrote:
> >>> On Wed, 07 Jan 2015 17:23:08 +0100
> >>> Ludwig Krispenz <lkris...@redhat.com> wrote:
> >>>
> >>>> On 01/07/2015 05:13 PM, Simo Sorce wrote:
> >>>>> On Wed, 07 Jan 2015 17:11:53 +0100
> >>>>> Ludwig Krispenz <lkris...@redhat.com> wrote:
> >>>>>
> >>>>>>>>>> Now, with some effort this can be resolved, eg
> >>>>>>>>>> if the server is removed, keep it internally as removed
> >>>>>>>>>> server and for segments connecting this server trigger
> >>>>>>>>>> removal of replication agreements or mark a the last
> >>>>>>>>>> segment, when tried to remove, as pending and once the
> >>>>>>>>>> server is removed also remove the corresponding repl
> >>>>>>>>>> agreements
> >>>>>>>>> Why should we "keep it internally" ?
> >>>>>>>>> If you mark the agreements as managed by setting an
> >>>>>>>>> attribute on them, then you will never have any issue
> >>>>>>>>> recognizing a "managed" agreement in cn=config, and you
> >>>>>>>>> will also immediately find out it is "old" as it is not
> >>>>>>>>> backed by a segment so you will safely remove it.
> >>>>>>>> I didn't want to add new flags/fields to the replication
> >>>>>>>> agreements as long as anything can be handled by the data in
> >>>>>>>> the shared tree.
> >>>>>>> We have too. I think it is a must or we will find numerous
> >>>>>>> corner cases. Is there a specific reason why you do not want
> >>>>>>> to add flags to replication agreements in cn=config ?
> >>>>>> Simo and I had a discussion on this and had agreed that the
> >>>>>> "marking" of a replication agreement
> >>>>>> as controlled by the plugin could be done by a naming
> >>>>>> convention on the replication agreements.
> >>>>>> They are originally created as "cn=meTo<remote host>,..." and
> >>>>>> would be renamed to something like
> >>>>>> "cn=<local host>-to-<remote host>,....." avoiding to add a new
> >>>>>> attribute to the repl agreement schema.
> >>>>>>
> >>>>>> Unfortunately this does not work out of the box. I only
> >>>>>> discovered afetr implementing and testing (was not aware of
> >>>>>> before :-) that DS does not implement modrdn operation for
> >>>>>> internal backends, It just returns unwilling_to_perform.
> >>>>>> And if it will be implemented the replication plugin will have
> >>>>>> to be changed as well to catch the mordrdn to update the in
> >>>>>> memory objects with the new name (which is used in logging).
> >>>>>>
> >>>>>> So, if there is no objection, I will go back to the "flag"
> >>>>>> solution.
> >>>>> What about simply deleting the agreement and adding it back with
> >>>>> the new name ?
> >>>> it will stop replication and the restart it again, unnecessary
> >>>> interrupting replication for some time.
> >>>> Assume you have a working topology and then raise the domain
> >>>> level and the plugin becomes active,
> >>>> creates segments and "marks" agreements as controlled. This
> >>>> should happen as smooth as
> >>>> possible.
> >>> While this is true, it is also a rare operation. I do not see it
> >>> as a big deal to be honest.
> >>> However if you prefer to add a flag attribute that is fine by me
> >>> too.
> >> after thinking a bit more about it, I don't think we need the mark
> >> at all.
> > We discussed this already and we came to the conclusion we need
> > it :)
> ok, I think in the discussion we came up with the mark was with my 
> concerns about
> removing a replica, and you more or less destroyed my concerns.
> I agree there could be some corner cases like the ones you sketch
> below. But it will still be difficult to do the right thing in all
> cases. Take your example of a marked agreement and no segment. we
> could arrive at this state
> in two different scenarios:
> 1]
> -  take a backup of the shared database
> - add a segment, a RA will be created and marked
> - the database gets corrupted and you restore the backup
> - the agmt is marked and no segment exists - should we really delete
> the agmt ?

Yes.
The shared tree is the authoritative repository. If you restore you
are required to do due diligence and check what was lost and restore
that too.

> 2]
> -  have a segment and a marked agreement
> - take a backup of the dse.ldif
> - delete the segment, agmt is removed
> - restore the dse.ldif,
> - the agmt is marked, no segment exists

Yes, this is the scenario I was hinting at, it the agreement is marked
we know it is not something we need to import (it was already imported)
so we just go on and delete it.

> >> The agreement would have been marked in two scenarios
> >> - the agreement exists and the dom level is raised, so that a
> >> segment is created from the agreement
> >> - the dom level is set, the plugin active and a segment is addded
> >> to the shared tree so that a replication agreement is generated.
> >> In all cases where an agreement is marked, there is a 1:1
> >> corresponding segment,  so the existence of a segment should be
> >> marking enough. I will make the mark_agreement and check_mark as
> >> noops, so if we really run into a
> >> scenario where a mark would be required, it can be added in one of
> >> the methods discussed
> >> so far.
> > I recall the problems came in corner cases. Like when a replica
> > starts and finds an agreement.
> > If it is marked and a segment does not exist, we simply delete it.
> > However if it not marked and a segment does not exist, what do you
> > do ? Without markings you can only import it, and this way you may
> > end up reviving deleted agreements by mistake.
> >
> > The reason this may happen is that cn=config and the main database
> > are separate, and a backup dse.ldif may be use upon start even
> > though the shared database had been already updated, so we cannot
> > count on the replica starting and finding the
> > segment-to-be-deleted-via-replication still there.
> >
> > The marking was needed to make sure that once an agreement was
> > imported we wouldn't do it again later on.
> > I still maintain that marking it is safer and will lead to less
> > issues with phantom segments coming back to life.
> >
> > Simo.
> >
> >
> 



-- 
Simo Sorce * Red Hat, Inc * New York

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