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Re: contextCSN interaction between syncrepl and syncprov

Rein Tollevik wrote:
The remaining errors and race condition that test058 demonstrates cannot
be solved unless syncrepl is changed to always store the contextCSN in
the suffix of the database where it is configured, not the suffix of its
glue database as it does today.

Assuming serverID 0 is reserved for the single master case, syncrepl and
syncprov can in that case only be configured within the same database
context if syncprov is a pure forwarding server  I.e, it will not update
any CSN value and syncrepl have no need to fetch any values from it.

In the multi-master case it is only the contextCSN whose SID matches the
current serverID that syncprov maintains, the other are all received by
syncrepl.  So, the only time syncrepl should need an updated CSN from
syncprov is when it is about to present it to its peer, i.e when it
initiates a refresh phase.  Actually, a race condition that would render
the state of the database undetermined could occur if syncrepl fetches
an updated CSN from syncprov during the initial refresh phase.  So, it
should be sufficient to read the contextCSN values from the database
before a new refresh phase is initiated, independent of whether syncprov
is in use or not.

Syncrepl will receive updates to the contextCSN value with its own SID
from its peers, at least with ITS#5972 and ITS#5973 in place.  I.e, the
normal ignoring of updates tagged with a too old contextCSN value will
continue to work.  It should also be safe to ignore all updates tagged
with a contextCSN or entryCSN value whose SID is the current servers
non-zero serverID, provided a complete refresh cycle is known to have
taken place.  I.e, when a contextCSN value with the current non-zero
serverID was read from the database before the refresh phase started, or
after the persistent phase have been entered.

The state of the database will be undetermined unless an initial refresh
(i.e starting from an empty database or CSN set) have been run to
completion.  I cannot see how this can be avoided, and as far as I know
it is so now too.  It might be worth mentioning in the doc. though
(unless it already is).

Syncprov must continue to monitor the contextCSN updates from syncrepl.
When it receives updates destined for the suffix of the database it
itself is configured it must replace any CSN value whose SID matches its
own non-zero serverID with the value it manages itself (which should be
greater or equal to the value syncrepl tried to store unless something
is seriously wrong).

Syncrepl should never be propagating contextCSN updates whose SID matches the current serverID. By definition, only the current server should ever be generating changes with the current serverID.

Updates to "foreign" contextCSN values (i.e those
with a SID not matching the current non-zero serverID) should be
imported into the set of contextCSN values syncprov itself maintain.
Syncprov could also short-circuit the contextCSN update and delay it to
its own checkpoint.  I'm not sure what effect the checkpoint feature
have today when syncrepl constantly updates the contextCSN..

The checkpoint probably only made sense for single-master.

Syncprov must, when syncrepl updates the contextCSN in the suffix of a
subordinate DB, update its own knowledge of the "foreign" CSNs to be the
*lowest* CSN with any given SID stored in all the subordinate DBs (where
syncrepl is configured).  And no update must take place unless a
contextCSN value have been stored in *all* the syncrepl-enabled
subordinate DBs.  Any  values matching the current non-zero serverID
should be updated in this case too, but a new value should probably not
be inserted.

Every source of updates to a DB must use its own unique SID. There should not be a lowest/highest foreign CSN to choose; there should only be one per SID. And as already noted, no syncrepl should ever be sending in a contextCSN update for the current serverID, those can only come from clients directly writing the local DB.

These changes should (unless I'm completely lost that is..) create a
cleaner interface between syncrepl and syncprov without harming the
current multi-master configurations, and make asymmetric multiple
masters configurations like the one in test058 work.  Comments please?


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  CTO, Symas Corp.           http://www.symas.com
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