[389-users] sub-tree synchronization/watching: persistent search questions

[Petr Spacek]

On 7.6.2013 16:11, Rich Megginson wrote:
> On 06/07/2013 05:42 AM, Petr Spacek wrote:
>> I would like to get opinions from 389 gurus to following problem.
>>
>> I have an application (DNS server), which needs to read content of whole one
>> sub-tree (cn=dns, dc=test) and keep it synchronized.
>>
>> The work flow is:
>> 1) Application (DNS server) starts
>> 2) Application reads all existing data out from the sub-tree
>> 3) Application does /something/ with the existing data and starts replying
>> to application clients
>> 4) Sub-tree has to be kept in sync with LDAP server, i.e. updates from LDAP
>> server should be incrementally applied to the 'state' inside the application
>>
>>
>> The problem with persistent search is that it doesn't offer any reliable
>> 'signal' that step (2) ended. The search is just running for infinite time
>> and I can't find any signal that all existing entries were read already and
>> now the application will get only Entry Change Notifications.
>>
>>
>> Basically, I'm looking for something like LDAP syncRepl in refreshAndPersist
>> mode with no cookie (RFC 4533 section 1.3.2 and section 3.4).
>>
>>
>> I know that Entry Change Notification from persistent search contains bit
>> field which denotes if the entry was added/modified/deleted/nothing (i.e.
>> not modified, just read). Unfortunately, this bit field can't be used for
>> *reliable* detection that all existing entries were read.
>>
>>
>> Could this 'hack' work reliably?
>> 1) Start persistent search (in separate application thread), but suspend
>> result processing.
>> 2) In another application thread, do the normal sub-tree search on the same
>> sub-tree. Normal search will be started *after* the persistent search.
>> 3) Process all results from normal search first
>> 4) Do /something application specific/
>> 5) Start processing updates from persistent search
>>
>> In my application I can cope with duplicates, when 'normal' search returned
>> entry cn=xyz and the persistent search returned the same entry cn=xyz again.
>
> Could you use entryUSN?  For example - keep searching until the entryUSN in
> the entry is the same as the global entryUSN, then fallback to persistent search?

Could you elaborate it a bit more, please? I'm not sure if I understood.
What exactly 'global' entryUSN means?
Do you mean 'lastUSN' value on particular server?
Can it work on server where modification are scarce? (Note that I do sub-tree 
search on subset of the whole database.)

I considered normal search followed by persistent search with entryUSN filter, 
but IMHO it will not work with entry deletion.

For example:
1) Start normal search and request entryUSN attribute (among others)
2) Process all results from search and compute max(entryUSN)
3) Start persistent search with filter (entryUSN > computedMaxValue)

I can see the race condition if an entry is deleted between steps (2) and (3).

That is exactly what I tried to solve with 'parallel' searches, i.e. 
effectively avoid any time gap between steps (2) and (3).


Of course, I could read entryUSN during normal and persistent search and then 
skip all results from persistent search with entryUSN < computedMaxValue. Is 
that what you meant?


>> I can see another option:
>> To implement 389 plugin which will provide (very partial) support for RFC
>> 4533. The idea is to implement only state-less pieces (no cookies) and
>> return some error when client attempts to use a cookie.
>
> This would also likely use entryUSN for the cookie, internallly.
Yes, that was also my idea, but I don't want to implement the 'state-full 
part' of the RFC in all it's complexity. Now I'm interested only in detection 
that all existing entries were read :-)

>> Could somebody judge how difficult it can be? From my (naive) point of view
>> are state-less parts of RFC 4533 only 'persistent search encapsulated in
>> another LDAP controls'.

Thank you very much for your time.

-- 
Petr^2 Spacek