Re: [389-devel] fractional replication monitoring proposal

[thierry bordaz <tbordaz@xxxxxxxxxx>]

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.

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

-- 
Mark Reynolds
389 Development Team
Red Hat, Inc
mreynolds@xxxxxxxxxx

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