Re: (ITS#8768) Syncprov shouldn't send a new cookie at the end of delete phase

[ondra@mistotebe.net]

On Tue, Oct 31, 2017 at 05:34:05PM +0000, ondra@openldap.org wrote:
> If sessionlog data is available and found useful, syncprov will send a cookie at
> the end of delete phase, itself followed by the entries modified since time
> recorded in the client's original cookie.
> 
> Some of those entries might have been last modified before the new cookie's
> recorded time and if the connection is severed before this is communicated, they
> would not be re-sent under the new cookie.

There are other problems with this. I have always assumed that CSN of
each write is globally unique in a well-configured system and that this
is preserved across replication, since MMR needs that to function
properly. This assumption is clearly invalid if UUIDs are sent in a
delete SyncInfo message (consumer that needs to determine CSNs that
apply can only pick a single CSN for all of the deletes).

So this is a problem in MMR situations where the cookie carries semantic
information between MMR nodes.

An MMR member receiving such a message has to pick a CSN to apply here:
- either the cookie (if present at all) - leads to problems described
  above
- or some other CSN - the deletes could be lost or propagate to other
  masters as a fresh mod, either smells of replication problems down the
  line

This shouldn't affect deltaMMR environments, though, AFAIK they never use
sessionlog in any way, so batched deletes don't get sent over the wire
at all.

-- 
OndÅ?ej Kuzník
Senior Software Engineer
Symas Corporation                       http://www.symas.com
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