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From Emmanuel Lecharny <elecha...@gmail.com>
Subject Some more thoughts about AP handling
Date Sat, 15 Jan 2011 11:01:22 GMT
Hi guys,

I'd like to share some thoughts I have had yesterday and this morning 
while I was lying in my bed (one of the best place to find solutions to 
problems with the shower).

AP support suppose you deal with some constraints, as explicited in 
various RFCs :
- we must support SAP and IAP, otherwise it's too complicated to manage 
ACIs when they are overlapping rights
- Subentries must be referenced in selected entries as a DN
- we must cover at least 3 administrative role : ACI, 
Collective-Attributes, Subschema, two of those aspects being described 
in RFCs
- every operation must be atomic, ie either execute *fully* or be 
aborted *fully*. We can't have a partial execution.

In order to correctly implement APs, we don't have many possible 
solutions. Let me list them again :
#0 don't support AP at all. Well, it's not really an option :)
#1 evaluate entries on the fly when we access them, all the time
#2 evaluate entries on the fly the first time we access them, and store 
the result
#3 evaluate entries when the AP is modified, and persist the state

All the 3 last approaches have pros and cons. I have ruled out option 
#0, so let's analyze the three others

#1 pros :
- atomicity is easy to enforce : we always do one single modification 
when managing APs
- it's easy to implement, as we just have to handle subentries in the 
search and lookup methods
- we don't have any operation that could impact the whole set of entries
#1 cons :
- we will have to pay the price of the entry evaluation for every single 
read we do on any entry

#2 pros :
- atomicity is somehow easier to enforce, but we still have some case 
where we have to update more than one entry when managing APs
- potentially let the system in a consistent state if it crashes
#2 cons :
- very complex to implement
- there are some cases (like for the rename/move operations) where we 
either decide to impact a set of entries for nothing, or we have to 
immediately update a set of entries, defeating one of the [pros]
- the first reader pay the price

#3 pros :
- the time it costs to update the entries is paid only once
- update only the set of impacted entries
#3 cons :
- atomicity is not enforced, if the server crashes in the middle of an 
operation, it's unstable
- it can take forever to update a huge base, assuming that we have to 
update all the entries, and it make take a lot of time at 1000 entries 
written per second
- implementation is complex, but less than for #2

At this point, I'm facing some big issue... If I want the rename/move 
operations implemented for option #2, it will imply either that we might 
impact many more entries than necessary, as I have to tell the parent AP 
that it's not up to date, *or* I have to adopt the #3 solution and 
update immediately the children entries...

Ok, let's face the facts : maybe option #1 is the only possible solution 
we have atm, from different aspects :
- delay : implementing option #1 is probably a matter of one week
- atomicity : it's the only solution that guarantees it

The price to pay is a slower server for search operations, as each entry 
will be evaluated again and again.

I don't know about you, but (running away from such things like "I feel 
in my guts", "I believe that", etc) from the technical point of view and 
the schedule point of view, I think that what matters right now are the 
following points :
- we need to get the AP working fully, following the specifications and 
constraints
- we need to get a 2.0-M1 out to be able to move to the next step
- we need to implement a Transaction Manager in 2.0-M1 in order to 
implement a better strategy (ie #2 or #3)

Really the problem now, and this is what I was trying to workaround with 
option #2, is the atomicity constraint. We need a transaction manager 
badly, and we need it for 2.0, no matter what, before we can implement 
either #2 or #3. And once we get this feature added in the server, then 
I do think that #3 is the best possible option.

I'll discuss the TM (Transaction Manager) in another thread, I just want 
to know what's your opinion about this aspect.

so, wdyt ?

-- 
Regards,
Cordialement,
Emmanuel L├ęcharny
www.iktek.com


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