On Mon, Dec 13, 2010 at 12:41 PM, Emmanuel Lecharny <firstname.lastname@example.org>
I'm resuming the APs implementation. I have had a new idea this morning about the best way to handle them.
Currently, we had devised about too possible options
1) Processing the entries while adding a new AP
The idea is that when we add a new AP (or delete an existing one, or modifying an existing one), we evaluate all the underlying entries and update each one which is selected by the associated SubtreeSpecification by adding a reference to its associated AP in the entry.
This is done once, then when a user grab an entry, we don't need to evaluate it again, as this work has already been done, so we accept to pay the price once, but it's free for every operation after this initial update.
The price to pay is pretty expensive, if we consider a huge base with hundred of thousands entries, as we have to update many, and as an update is slow (we currently can process 600 per second of such updates - depending on the server we are running on, of course -, so for 1 000 000 entries it might take up to 30 minutes ).
2) Processing the evaluation on the fly
The second idea is to evaluate the entry when a user requests it, to avoid paying the price for the initial update.
The big problem with this approach is that if we have many APs SubtreeSpecifications to evaluate, this evaluation is not free, plus it has to occur *everytime* a user access the entry. That could slow down the server a lot (probably double the time it takes to return an entry).
So far, we have decider to go for option #1, simply because adding an AP is an administrative task, which does not occur very often. We all realize that it has drawbacks, but it's far better than option #2 anyway.
Now, I'd like to propose a third option, which might be acceptable, considering that we will pay a small price compared to both option #1 and #2.
3) Processing on the fly and store the result
The idea is to process the evaluation when someone fetches an entry, but to store the evaluation result in the entry. If the fetched entry has already been evaluated, then no need to process it again.
The direct benefit will be that we won't have this huge initial update required by option #1, and the entry evaluation wil be just a matter to check if an AT has been set instead of fully evalute the entry as in option #2.
This sounds like an excellent compromise.
Is there a chance though of triggering a large set of these updates on for example a search making this approach in practice perform close to approach #1? Let me explain:
Usually directories containing millions of entries have most of their entries under a small number of containers like ou=users for example. Directories today are flatter than in the past where there was more depth and hierarchy.
Using a search filter without many constraints (AVAs) under a congested base with one or sub tree scope may trigger the need to evaluate a large set of entries.
To think this through let me just for the record go through the way search interacts with the administrative model subsystem. I won't consider base/object scoped searches.
(1) On the way into the server a search may have some evaluations done on
(2) The search call returns a Cursor for the search request parameters positioned
at the first candidate to return. This may have actually processed several
candidates that do not comply with the filter. During candidate processing
administrative aspects will be considered, so subtree specification evaluation
(3) There after calls to get the next candidate and so on repeat the steps in #2
until the Cursor is exhausted or search limits if in effect halt the search.
In step #2 evaluations checking entries for inclusion in administrative areas defined by subtree specifications, occur only after the filter's constraints have been applied and the entry in question passes those assertions.
So a search can only trigger updates on entries satisfying it's filter. Also this will be shorted by search limits as well for standard users (non-admin). If the filter is not very restrictive for example (objectClass=*) then this is bad but search limits should constrain the number of updates performed.
So only a subset of the updates will take place. It does sound good going through the entire thought process.
How will it works ? Simple : when an AP is added,
What about when the AP is modified and deleted ?
a Timestamp (TS) is updated.
Is this a global timestamp used for this purpose system wide?
This TS will be incremented every time we add a new AP.
Again also when we update or delete an AP (implies subentries as well)?
We don't update any entry.
Then when a user fetch some entries, for every one of the selected entries, we check if it has a TS AT present.
I presume this timestamp attribute is separate from the modifyTimestamp or createTimestamp right?
So there's a special timestamp in the entry and on somewhere in the server to do these TS comparisons?
If not, then we evaluate the entry against the AP it depends upon, and when done, we add the last TS into the entry, save it and return (or not) the entry to the user.
If the entry has a TS AT, then we have two cases
- The TS equals the last TS : the entry has already been evaluated, and we can return it (or not, depending on the evaluation result)
- The TS is below, so we reevaluate the entry.
Now, how will we distinguish between an entry evaluating to true or false ? Easy : if the entry is selected by a SubtreeSpecification, we will have a ref to the associated AP in it, otherwise no.
So you have to update the timestamp and the ref back to the AP containing the subentry selecting it right?
I do think that this third option has great advantages, and not only from the user or admin POV : the implementation will be greatly improved, as we don't have to deal with all the cases when handling an AP addition/modification/deletion.
It's still somewhat hazy.
One last thing : one may object that the on the fly evaluation is not acceptable, as it will cost a lot for the first access to an entry. True.
I'd think it's very acceptable but does the gain outweigh the complexity is my question and I still don't fully understand your approach.
This is a really complex part of the server and I'm hoping we can find ways to simplify it further.
At some point you had a great idea of using indirection via UUID instead of a direct reference with DN back to the AP. Are you still thinking about this approach. I need to dig up that email you originally had sent to the dev list. It was a great idea.
But I claim that it's the administrator to deal with this problem, and then its not anymore a user issue. When the admin add an new AP, he also can fetch all the entries below the added AP, and they will be updated on the fly. The users won't be impacted anymore.
The idea is great but I'm worried about how much more complexity it may introduce on top of something that already needs simplification but I guess experimentation will really tell you that.
The best thing to do is get a clear idea of your approach and try implementing it in a branch after some discussion to help address all the issues.