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.
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. 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.
Again similar strategies can be applied to any other operation in place of the search example with specific consideration for each operation.
This is a form of multi-version concurrency control , rather than using locking mechanisms to achieve isolation. This technique works pretty well in modern DBMS'.
 - http://en.wikipedia.org/wiki/Multiversion_concurrency_control