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From Emmanuel Lecharny <elecha...@apache.org>
Subject Re: [ApacheDS] Implementing isolation using multi-version concurrency control (MVCC)
Date Sat, 31 Jan 2009 10:25:17 GMT
Alex Karasulu wrote:
> Hi all,
>
> Emmanuel and I were having an interesting conversation about the kinds of
> transaction properties needed by ApacheDS to comply with various
> specification requirements. For example all LDAP operations must be atomic
> and must preserve consistency. Furthermore, one can easily debate the need
> for isolation where any operation does not see or get impacted by the
> partial changes of another concurrent operation.
>
> We started discussing these ACID properties and ways in which they could be
> implemented. Isolation is very interesting now thanks to directory
> versioning and the change log. We can easily implement isolation now. When a
> relatively lengthy operation like a search is being conducted, it should not
> see modifications within scope that occur after the search began. The search
> operation in the example can be replaced with any other operation minus all
> unbind, some extended, and all abandon requests.
>   
Atomicity and Isolation are both complex to guarantee in a LDAP server.

If we think about Atomicity, for instance, even if we can guarantee it 
somehow for somesimple operation like Modify, Add or Delete, for the 
ModDN operation is not that simple. We have to guarantee that all the 
potential renames are done - or reverted - as a whole. As this operation 
can impact a big part of the server, and could take several seconds 
(minutes, hours, dependening on the number of entries), this is 
obviously not trivial. However, moddn operation aren't the most frequent 
one. Regarding the most simpler operation (add, delete and modify), I 
think we should implement some kind of "transaction" in the backend : 
the modified entry has to be tagged as 'under modification' until the 
backend has updated correctly the modification (or rollbacked it). Then 
we can untag the entry, and it's available back. How the CL can come 
into play here is to be discussed. IMHO, the CL and this 'transaction' 
will work hand to hand at some point.

Regarding isolation, it's a bit more difficult, as a search can already 
have sent back some results which could be change by another modify 
operation. This is especially the case for a ModDN operation.
> The change log, not only tracks each change, but it allows the directory
> server to inject the "revisions" attribute into entries. The revisions
> attribute is multi-valued and lists all the revisions of changes which
> altered the entry. For the search example, we can conduct the search
> operation while locking it down to a revision. 
That does not work for deleted entries, obviously ...
>  This is best implemented by
> conditionally filtering out or injecting candidates with revisions greater
> than the revision at which the search operation started. Let's call the
> revision when search started S. So entries in the server which posses
> revision numbers greater than S need further evaluation. We have to evaluate
> if the filter matches these entries with revisions > S when their state was
> at revision S. This may require some reverse patching and re-evaluation of
> the filter on the patched entry in state S.  This is not so bad because
> there usually are not that many changes taking place at the same time on the
> same entry: meaning the number of LDIF's to patch on an entry to evaluate in
> it's former state at S will be small. This way we effectively lock down the
> search to a specific revision, giving the search operation what appears to
> be a snapshot of the DIT. The search results will not be impacted by any
> concurrent changes.
>   
Well, I don't think this is the best approach. In any case, a Ldap 
Search is considered as a dirty read. We have no way to lock down the 
modification on the read entries. So the user has _no_ guarantee that 
the entry he gets back will be valid. Usually, it doesn't matter, 
because the ratio of read vs writes on a LDAP server is just so big that 
we consider we don't have modifications. So we can simply return the 
latest revision, whatever it is. Anyway, there is another aspect we have 
to consider : once the user gets his entry, and before he uses it, 
before potentially send it back as a modification to the server, the 
very same entry can have been modified in between. As we don't lock 
entries, we can't protect the users from such a case.

<digression>
We have to remember here that we are not dealing with bank accounts and 
balances, or nuclear plants. Most of the case, we are using a LDAP 
server to handle identities. They rarely change, or when they do, it's 
because the person owning this identity is changing his own identity - 
thus limitating the odds that he is using it at the same time -. 
Usually, if we think about authorizations, which are subject to way more 
changes that identities, we can't consider it as a continuous flow of 
modification.

In other words, creations or deletion of entries might be frequent, 
modification should be quite rare.

I have gathered some stats from some of my clients, and the ration 
changes/reads is like 1/5000... I would be interested to get more 
numbers here !

Last, not least, I consider that if the ratio goes up to something like 
1/100, then it's time to consider using a transactional system, namely, 
a RDBMS.
</digression>

So far, I'm not saying that it's wrong to think about using a MVCC 
system, but I'm a bit sceptic about the gain in term of isolation (the I 
in ACID) it will offer in our case.

Let's discuss this anyway, it's interesting !

-- 
--
cordialement, regards,
Emmanuel L├ęcharny
www.iktek.com
directory.apache.org



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