On Mon, Jan 3, 2011 at 2:58 AM, Emmanuel Lécharny <elecharny@apache.org> wrote:
On 1/3/11 1:38 AM, Alex Karasulu wrote:
On Mon, Jan 3, 2011 at 2:09 AM, Emmanuel Lécharny<elecharny@apache.org>wrote:

On 1/3/11 12:57 AM, Alex Karasulu wrote:

On Mon, Jan 3, 2011 at 1:27 AM, Emmanuel Lecharny<elecharny@gmail.com
wrote:
 Hi,

 SNIP

 (I still have in mind to add an optional computation of the entries when
an
AP or a Subentry are modified, to avoid a postponed evaluation).


 Could you elaborate on this? I did not quite understand what you mean by
"an
optional computation of the entries".

We have three options here :
- the current trunk implementation modifies the impacted entries
immediately when a Subentry is added/removed/modified (using the
SubtreeSpecification). It's costly, but only when we add/remove/modify a
subentry.
- the current branch I'm working on is using a differed computation, ie the
entry relation to subentries is compted the first time the entry is accessed
(either during an addition or a modification, or when read). That means the
first read of an entry will imply a write on disk, the next read will be as
fast as a normal read. OTOH, the first read of an entry is always costly, as
we have to read the entry from the disk (unless it's in cache).
- the third option, if we don't want to impact users when adding a subentry
when the server is running, is to do as it's done in trunk, ie update all
the entries when adding a subentry. But this would be an option that can be
activated on the fly (by modifying th server configuration, or by sending a
control with the subentry operation).

I suggest we go for option #2 atm, assuming that implementing #3 is easy
and won't imply a huge refactoring, as the mechanisms used to update the
entries is already implemented.


It's up to you but IMO I don't think this option of delaying updates with
subentry changes is really worth the complexity and it also introduces other
serious issues. I wanted to express this thought but you seemed very
interested in this direction so I let it be.
In fact, the complexity is equivalent. You still have to update all the entries, which is pretty trivial. The only difference is that when you grab the entry, you have to check if it has a reference on a subentry's UUID and the same sequence number than its parent AP. But if we want to spare this extra processing, then option #3 can be triggered.

OTOH, computing all the entries while we process a subentry addition/removal is quite complex, as we have no way to correctly handle a server shutdown occurring in the middle of such an operation. One more thing : as the operation will be costly, it's unlikely to be atomic.

Just as a quick idea of what I was thinking. Sometimes a search with the
right parameters pursuant to a subentry alteration affecting the selected
region may trigger the entire area to be computed anyway making the lazy
computation effectively the same thing as eager computation. But this time
the computation effort is felt on a search operation. This will make our
performance metrics tests even harder to interpret down the line as well.
This is why I suggested in one previous mail I sent two weeks ago that if the admin does not want to face such impact, it's easy to do a full search and get the full base updated immediately.

Furthermore don't we want to know if a subentry altering operation succeeds
when the administrator performs it? It might be nice to have the operation
block as well so the admin knows when the operation completes so he can let
users back on the system.
There is no need to block with option #2, as the admin will know that the operation has succeeded as soon as he get the response.

My point is he does not know that all the entries impacted by the admin op have been massaged with #2. With #1 you know everything has been updated when the operation returns even if it takes a long time. 
 
We then have all the needed informations stored in the server to process any other operation leveraging the added Subentry :
- the AP is present
- the seqNumbers are updated in the AP
- the subentry is present
- the subentry caches are updated

Also when we  get local transactions implemented
in the server subentry alterations should be tied to a single atomic
operation. If something fails for some reason or another down the line while
making the updates don't you want to know immediately and roll back?
Someting clearly straightforward with option #2, way more complicated if you have thousands of entries to rollback with option #1. In fact, I don't want to think what impact it could have with millions of entries being parts of a subtreeSpecification...

Keep in mind that with the differed computation, knowing if the entry is associated with a subentry - or not- is just a matter of comparing the seqNumbers the entry has (or not) with it's AP sesqNumber, and if it's older, then check if the entry is part of one subtree, and update its references to the subentry. All those operations are done in memory, and don't require any disk access, except to store the updated attributes. And even so, if we don't write back to the disk this updated entry, it doesn't matter. If the server brutally stops, we simply will recompute those elements the next time.

All in all, what bothers me the most with option #1 is the failure recovery : we don't have any mechanism to restart the processing in the middle of it. This is not an issue with #2.


Yes I must admit that this is a huge advantage with #2 and extremely compelling. 

What's giving me an icky feeling inside is that #2 is making a read operation induce writes (although DSA driven maintenance operations) and hence causing us to consider one offs with the way a change log works. It's also something to be considered for replication as a change to be ignored.  

My whole religious issue with approach #2 is that it's essential an optimization for an administrative operation that is forcing us to settle for this path because of a lack of solid atomicity based on local transactions in the server. We're shifting many ideals we all believe in if I am not mistaken to optimize a seldom performed operation that usually occurs during outage times.

--
Alex Karasulu
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