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From Howard Chu <...@symas.com>
Subject Re: [Schema Refactoring] Serious side effect...
Date Mon, 23 Nov 2009 23:31:43 GMT
Emmanuel Lecharny wrote:
> We discussed this point with Alex extensively lately. This Observer/Listener pattern
sounds good, but IMO, it does not solve the problem.
> 1) We have to deal with all the serverEntries being currently processed, and maybe being
modified. A ServerEntry is not an atomic object, and we don't want to deal with the extra
complexity of a event occuring while we are in the middle of a modification of this ServerEntry
> 2) Many ServerEntries might be stuck in memory, waiting for a thread to be free. This
is the case for every partial ServerEntry waiting for some more bytes from the client.
> 3) We have many places where we store cached ServerEntries. The question is how do we
update them ?
> 4) What do we do with the Original entry, which is stored into each OpContext, as we
may need it later ? Do we update it too ? IMO, that would defeat the purpose of this object
> 5) ServerEntries are serialized in the Backend, and if we modify the schema, it will
most certainly impact them. If they are not migrated, they might not be usable anymore after
the schema modification. Also if we have millions of entries, changing them online is probably
not realistic. Anyway, the admin has to deal with this problem in any case
>
> I don't see how possibly we can deal with a schema modification live, except for a few
modifications :
> - AT, OC, S, MR, C, N and SC, and only for Add or Move operations
> - schema enabling
>
> Any other operation (delete, modify, rename, disabling a schema) are most certainly leading
to dire errors, something an administrator will not want to experiment in production. IMO,
they should be forbidden on a working base. Such operation is like manipulating a loaded weapon
with no safety...

This is pretty much the same conclusion we reached, which is why in OpenLDAP 
2.3 we only supported dynamic adding of schema. In 2.4 we support 
delete/modify but it's a hack - the deleted elements are kept around. If you 
do a Modify to alter an existing value (e.g. Modify/delete foo=1/add foo=2) we 
make sure the subsequent add applies to the corresponding deleted element. 
Since we don't refcount AttributeDescriptions, these things are kept around 
for the life of the server process, and only get purged on a restart.

> For the reason I mentionned, I don't think that any alternative is ok.
> We probably don't have a perfect solution because there are none. As we
> say : "any problem vanishes when there is no solution"...
>
> More seriously, I don't think we need a dynamic schemaManager for a LDAP
> server in production : admins don't change such a critical thing in
> production, except those who are insane or desesperate. We must accept
> the idea that we might have a downtime, we just have to minimize it.

That was my generally my perspective as well. IMO, admins should be able to 
dynamically Add tested schema to a production server. If they're experimenting 
and need to alter the schema on-the-fly, they should be testing in a dedicated 
test environment. This was the rationale behind the 2.3 implementation.

But it seems that admins like to whine a lot about things they think they 
need, even if they're more likely to shoot themselves in the foot. So we added 
dynamic delete/modify support in 2.4.

Either way, 2.3 or 2.4, it's possible to end up with entries in the DB using 
schema that no longer exist - we don't search them out and remove them when 
deleting schema elements. (Indeed, you really shouldn't; someone may be adding 
a new version of the deleted schema in the next operation. You have no way to 
know when a delete is really final, and spontaneously deleting user data is 
always a mistake...)

> Unless someone has a genious idea !
>> Regards,
>>
>>
>
>


-- 
   -- 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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