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From Pierre-Arnaud Marcelot ...@marcelot.net>
Subject Re: Update about subtree problems
Date Tue, 20 Jul 2010 07:04:28 GMT
Hi Emmanuel,

As we've seen together, that's not an easy subject and there's no real solution, just relative
trade-offs.

I really believe that a good discussion on the ML is necessary in order to be sure about what
type of operation is to be preferred in those kind of situations.

A community-wide consensus is really important and will help a lot simplifying the decisions
in the future.

Regards,
Pierre-Arnaud


On 20 juil. 2010, at 01:34, Emmanuel Lecharny wrote:

> Hi guys,
> 
> just an update about the current situation.
> 
> We discussed on this ML about options regarding how to handle subtrees in the most efficient
way in the server. We currently have 3 possibilities :
> 
> - the first one, which is partially implemented in trunk, consist on static references
to subentries in all the entries selected by the subtreeSpecifications defined in each subentry.
> 
> - the second one suppose that we compute when needed if an entry is a selected entry,
each time we access this entry
> 
> - the third solution, which has not been discussed already, would be to have an index
which stores the list of all the selected entries associated with each subentry.
> 
> Let's first think about the third solution (this is the one implemented by OpenDS, btw).
It's a costly one when you create a new subentry, or when you move one, as you have to update
the index with all the impacted entries. Plus it also has a cost as you have to hit the index
to get the list of subentries an entry is part of. Of course, we can use a cache, but I'm
not sure that it will save a lot of time. In fact, AFAICT, you pay the price twice : when
creating the subentry, and each time you access any entry.
> 
> Ok, so option 3 does not seems to be reasonable. To be frank, we discussed about t with
Pierre-Arnaud but not really considered this as a decent solution. We may be wrong though.
> 
> Regarding option 1 and 2, none is perfect.
> 
> Let's see the pros and cons for each of them
> 
> Solution 1 (update when manipulating the subentry)
> --------------------------------------------------
> pros :
> - *very* efficient operations on normal entries, as they already have a reference to
all the subentries that select them.
> - no extra cost when doing a rename, a modify, an add or a delete of a normal entry.
> - all the subentries are stored in a cache loaded at startup, no need to fetch an index
to get a subentry
> - evaluating if an entry is selected is done when the subentry is added or moved
> - move operations applied on normal entries are rare
> - any operation applied on a subentry are considered as admin operation, thus unfrequent
and scheduled. Downtime is under control.
> 
> cons :
> - *very* costly add/delete/move/rename operations applied on a subentry, as every selected
entries will have to be modified
> - *very* costly move operation of a normal entry if it moves from one AP to another one,
and when it has many children, as we have to update all the children twice : first to reove
the old subentries' reference, the second one to add the new references
> - Adding or removing a subentry is a non atomic operation, as we have first to update
the selected entries one by one, and then to add/delete the subentry
> 
> Solution 2 (evaluating when needed )
> ------------------------------------
> pros :
> - adding/removing/moving/renaming a subentry is a O(1) operation
> - moving normal entries selected by some subentries does not cost more than a standard
move
> - easy to implement
> 
> cons :
> - each time we access an entry (search, compare), we will have to evaluate the subtreeSpecifciation
for all the subentries this entry is dependent on
> - we will need another cache to lookup for the entry's related subentries (this cache
will work pretty much like the one we use for the NamingContexts)
> 
> 
> Basically, sol.1 and sol.2 are both painful, but there is no perfect solution anyway.
> 
> I thought about what is the best implementation all day long, and if yesterday I was
convinced that the solution 2 was better. But the evaluation cost done on each entry every
time a client does a search is quite high (let's say up to 20% of the search cost, for simple
SS). That will not slow down the server that much, as most of the time is consummed in the
network layer right now, but this won't last.
> 
> Now I think that the current solution is may be a not so bad compromise, assuming we
don't do a lot of move operation cross APs on normal entries.
> 
> What bugs me here is that last year, for the opposite reason, the DN was removed from
the entries stored in the master table, just t be able to do O(1) move operations. This is
quite paradoxal ! I still think that move operations are very rare, and that storing the DN
into the entries is a net gain for most of the operations, except for a move...
> 
> 
> Some side note :
> after having done some perf tests on the evaluator, and applied some improvement, I can
tell that depending on the number of subentries an entry is depending on, the cost of this
evaluation can goes up to 50% of the search itself cost - not counting the network layer -.
For instance, evaluating a subtreeSpecification with a min and a max, no chop, will be done
up to 1 000 000 times per second on a 3 level DN (this is all dependent on the DN size)
> 
> Last, not least : the current implementation is really incomplete. The Move, Rename and
MoveAndRename operations are not correctly handled, with many entries not being updated. I'm
going to fix them. I have also created a branch to play with subtree without breaking the
trunk. I'm not sure I will continue to work on this branch if a decision is  made to keep
the first solution.
> 
> 
> -- 
> Regards,
> Cordialement,
> Emmanuel L├ęcharny
> www.iktek.com
> 


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