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On 10/17/2013 11:06 AM, Ludwig Krispenz
wrote:
On 10/17/2013 10:56 AM, thierry
bordaz wrote:
On 10/17/2013 10:49 AM, Ludwig
Krispenz wrote:
On 10/17/2013 10:15 AM, thierry
bordaz wrote:
On 10/16/2013 05:41 PM, Ludwig
Krispenz wrote:
On 10/16/2013 05:28 PM, Mark
Reynolds wrote:
On 10/16/2013 11:05 AM,
Ludwig Krispenz wrote:
On 10/15/2013 10:41 PM,
Mark Reynolds wrote:
https://fedorahosted.org/389/ticket/47368
So we run into issues when trying to figure out if
replicas are in synch(if those replicas use
fractional replication and "strip mods"). What
happens is that an update is made on master A, but
due to fractional replication there is no update
made to any replicas. So if you look at the ruv in
the tombstone entry on each server, it would
appear they are out of synch. So using the ruv in
the db tombstone is no longer accurate when using
fractional replication.
I'm proposing a new ruv to be stored in the
backend replica entry: e.g.
cn=replica,cn="dc=example,dc=com",cn=mapping
tree,cn=config. I'm calling this the "replicated
ruv". So whenever we actually send an update to a
replica, this ruv will get updated.
I don't see how this will help, you have an additional
info on waht has been replicated (which is available
on the consumer as well) and you have a max csn, but
you don't know if there are outstanding fractional
changes to be sent.
Well you will know on master A what operations get
replicated(this updates the new ruv before sending any
changes), and you can use this ruv to compare against
the other master B's ruv(in its replication
agreement). Maybe I am missing your point?
MY point is that the question is, what is NOT yet
replicated. Without fractional replication you have states
of the ruv on all servers, and if ruv(A) > ruv(B) you
know there are updates missing on B. With fractional, if
(ruv(A) > ruv(B) this might be ok or not. If you keep
an additional ruv on A when sending updates to be, you can
only record what ws sent or attempted to send, but not
what still has to be sent
I agree with you Ludwig, but unless I missed something would
not be enough to know that the replica B is late or in sync
?
For example, we have updates U1 U2 U3 and U4. U3 should be
skipped by fractional replication.
replica RUV (tombstone) on master_A contains U4 and master_B
replica RUV contains U1.
Let's assume that as initial value of the "replicated ruv"
on master_A we have U1.
Starting a replication session, master_A should send U2 and
update the "replicated ruv" to U2.
If the update is successfully applied on master_B, master_B
replica ruv is U2 and monitoring the two ruv shoud show they
are in sync.
They are not, since U4 is not yet replicated, in master_A you
see the "normal" ruv as U4 and the "replicated" ruv as U2, but
you don't know how many changes are between U2 and U4 an if
any of them should be replicated, the replicated ruv is more
or less a local copy of the remote ruv
Yes I agree they are not this is a transient status. Transient
because the RA will continue going through the changelog until
it hits U4. At this point it will write U4 in the "replicated
RUV" and until master_B will apply U4 both server will appear
out of sync.
My understanding is that this "replicated RUV" only says it is
in sync or not, but does not address how far a server is out of
sync from the other (how many updates are missing). When you say
it is more or less a copy, it is exactly what it is. If it is a
copy => in sync, if it different => out of sync.
maybe we need to define what "in sync" means. For me in sync means
both servers have the same set of updates applied.
Forget fractional for a moment, if we have standard replication
and master A is at U4 and master B is at U2, we say they are not
in sync - or not ? You could keep a replicated ruv for thos as
well, but this wouldn't change things.
I agree we need to agree of what "in sync" means :-)
I would prefer to speak of 'fractional ruv' (in place of 'replicated
ruv') for the new ruv proposed by Mark.
'replica ruv' being for the traditional ruv (tombstone) used in
standard replication.
With 'replica ruv' we are in sync when the 'replica ruv' on both
side have the same value.
With 'fractional ruv' we are in sync when the 'fractional ruv' on
the supplier and the 'replica ruv' have the same value.
In fractional replication, we have updates U1, U2, U3 and U4. Let's
U3 and U4 being skipped by fractional
Let master_A 'replica ruv' is U4 and master_B 'replica ruv' is U2.
And no new updates.
From a standard replication point of view they are out of sync, but
for fractional they are in sync.
For fractional, how to know that that both masters are in sync. With
Mark solution 'fractional ruv' shows U2.
Now a new update arrives U5 that is not skipped by fractional.
master_A 'replicat ruv' is U5 and master_B 'replica ruv' is U2.
until the replica agreement starts a new replication session,
'fractional ruv' shows U2.
The servers are shown 'in sync', because the RA has not yet started.
From my understanding, the solution proposed by Mark has a drawback
where for a transient period (time to the RA to start its jobs,
evaluate and send U5, store it into the 'fractional ruv'), the
servers will appear 'in sync' although they are not. It could be an
issue with schedule replication but should be transient wrong status
under normal condition.
If the update is not applierd, master_B replica ruv stays at
U1 and the two ruv will show out of sync.
In the first case, we have a transient status of 'in sync'
because the replica agreement will evaluate U3 then U4 then
send U4 and store it into the "replicated ruv". At this
point master_A and master_B will appear out of sync until
master_B will apply U4.
If U4 was to be skipped by fractional we have master_B ruv
and Master_A replicated ruv both showing U2 and that is
correct both servers are in sync.
Mark instead of storing the replicated ruv in the replica,
would not be possible to store it into the replica agreement
(one replicated ruv per RA). So that it can solve the
problem of different fractional replication policy ?
Do you mean changes that have not been read
from the changelog yet? My plan was to update the new
ruv in perform_operation() - right after all the
"stripping" has been done and there is something to
replicate. We need to have a ruv for replicated
operations.
I guess there are other scenarios I didn't think of,
like if replication is in a backoff state, and valid
changes are coming in. Maybe, we could do test
"stripping" earlier in the replication process(when
writing to the changelog?), and then update the new ruv
there instead of waiting until we try and send the
changes.
Since
we can not compare this "replicated ruv" to the
replicas tombstone ruv, we can instead compare the
"replicated ruv" to the ruv in the replica's repl
agreement(unless it is a dedicated consumer - here
we might be able to still look at the db tombstone
ruv to determine the status).
Problems with this approach:
- All the servers need to have the same
replication configuration(the same fractional
replication policy and attribute stripping) to
give accurate results.
- If one replica has an agreement that does NOT
filter the updates, but has agreements that do
filter updates, then we can not correctly
determine its synchronization state with the
fractional replicas.
- Performance hit from updating another ruv(in
cn=config)?
Fractional replication simply breaks our
monitoring process. I'm not sure, not without
updating the repl protocol, that we can cover all
deployment scenarios(mixed fractional repl agmts,
etc). However, I "think" this approach would work
for most deployments(compared to none at the
moment). For IPA, since they don't use consumers,
this approach would work for them. And finally,
all of this would have to be handled by a updated
version of repl-monitor.pl.
This is just my preliminary idea on how to handle
this. Feedback is welcome!!
Thanks in advance,
Mark
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Mark Reynolds
389 Development Team
Red Hat, Inc
mreynolds@xxxxxxxxxx
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Mark Reynolds
389 Development Team
Red Hat, Inc
mreynolds@xxxxxxxxxx
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