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From Howard Chu <...@symas.com>
Subject Re: [Mitosis] Push or pul, plus some random thoughts
Date Thu, 15 Jan 2009 08:33:53 GMT
Emmanuel Lecharny wrote:
> Quanah Gibson-Mount wrote:
>> --On Thursday, January 15, 2009 1:28 AM +0100 Emmanuel Lecharny
>> <elecharny@gmail.com>  wrote:
>>
>>
>>> The queue will be limited in size, obviously. I can even be empty, all
>>> the modification being stored on disk.
>>>
>>> However, we don't necessarily have to keep a track of pending modify for
>>> disconnect replicas, as it's easy to know which entries has not been
>>> replicated since the last time the replica was on. The connecting
>>> replica
>>> could send the last entryCSN received, and then a search can be done on
>>> the server for every CSN with a higher CSN. Then you don't need to keep
>>> any modification on disk, as they are already stored in the DiT.
>> Sounds a lot like syncrepl. ;)
> yeah, this is what I realized :)
>> That search doesn't sound like it handles deletes though, which is
>> always a PITA.
> it does. Any modification done somewhere which might impact the elements
> you are supposed to get back from this persistent search will be send
> back to the recipient. The way Mitosis works atm is to keep the deleted
> entries in the DiT with a added attribute telling if the entry has been
> deleted, so we keep them in the DiT ( but not available for standard
> operations) until all the replicas has been updated. So a disconnected
> replicas which reconnect will get the deleted entry info when it connect
> back.

Tombstones. Yuck. If you're leaving the entry in the DIT, what do you do if an 
entry with the same DN is added again? Just resurrect the deleted entry and 
replace its values with the newly added attributes? Will anyone care if a 
replica never sees the delete, in this case?

> How to handle the real deletion is the problem, as we have to keep a
> state of each replica...

Don't leave the tombstone in the original location, move it to a reserved area 
of the DIT. That will make housekeeping a lot simpler later; you can just 
purge entries older than X under that particular branch rather than having to 
search the entire DIT for cleanup targets.
-- 
   -- 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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