On Sat, Jan 31, 2009 at 5:25 AM, Emmanuel Lecharny <email@example.com> wrote:
Alex Karasulu wrote:Atomicity and Isolation are both complex to guarantee in a LDAP server.
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.
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.That does not work for deleted entries, obviously ...
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.
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.
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
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.
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.