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From Howard Chu <...@symas.com>
Subject Re: ContextCSN handling
Date Thu, 13 Feb 2014 15:13:30 GMT
Emmanuel L├ęcharny wrote:
> Hi guys,
>
> we have had a long chat with Kiran about the best way to handle the
> contextCSN attribute.
>
> This attribute is used for replication, as we need to know if one server
> is up to date. The way to do it is to compare the local contextCSN with
> the remote one. The pb is where to store this contextCSN, and how to get
> it when doing a search.
>
> o the contextCSN is associated with the root of a partition, it's
> associated with teh contextEntry (see further for some new ideas about
> where to store teh contextCSN).
> o it contains the newest entryCSN in the partition.
> o it's updated every time an update is done on the partition.
> o it's persisted (ie, when the server is restarted, it's present)
>
> Now, when one does a search, we have to have this contextCSN attrbute
> present in the contextEntry (assumng the user has requested the
> attribute, of course, as it's operationnal).
>
> If we update the contextEntry after *each* update, we wil double to time
> for updating an entry. No way...
>
> The solution is to store this value in memory, and to recompute it when
> the server is started (finding the newest entryCSN of each partition).
>
> That being said, we still have to inject this attribute in the
> contextEntry when a search is done, which means we have to detect that
> an entry is a context entry. Not easy, and costly.
>
> There are two ways to do that :
> o assume that the entry's cache always contain the contextEntry, which
> always contains the contextCsn. In this case, we just have to update the
> contextEntry after any modification (all in all, we just update the cache).
> o modify the OperationalAttributeInterceptor to injetc the contextCSN
> when we detect that a fetched entry is the contextEntry. In order to do
> that, we need a quick way to detect the contextEntry. We need to
> propagate the partition's DN into the SearchOperationContext, so that we
> can compare the length of this DN with the length of the fetched entry's
> DN : if they are equal, it's the contextEntry.
>
> IMO the second approach is a bit more complicated (but I don't know why,
> I thik it's the best approach...)

In OpenLDAP we cache the value in memory by default. There is a  configurable 
syncprov-checkpoint so after T time or N updates we do an actual write of the 
attribute into the underlying entry. We also have a flag on the backends 
indicating whether or not it previously had a clean shutdown; if this flag 
indicates an unclean shutdown then we can do a search for the highest entryCSN 
in the DB. (This flag is somewhat obsolete now with back-mdb.)

Aside from that, we of course write the attribute on clean shutdowns.
>
> thoughts ?
>


-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/

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