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From Emmanuel Lecharny <elecha...@gmail.com>
Subject Re: Heads up
Date Sat, 21 Jan 2012 00:48:17 GMT
Just FYI, I will continue to play around the BTree I've pushed in my 
branch. It's funny, and it helps me to understand the concepts without 
having to go deep into books and existing algorithm. Call it an 
exercise, this is exactly what it is.

As far as I can tell, it also seems that JDBM has a few sub-optimal 
methods : to insert N nodes into a BTree, JDBM do 10% more comparison 
than I do in my implementation. We can probably shave those extra 10% 
from JDBM code later (I tried, but it had some bad side effect when done 
brutally).

Otherwise, I do think that if your modified JDBM already implements the 
MVCC Btree, then we should use it in trunk.

I'm also wondering if there is any added GC in the modified JDBM code ? 
Just because if we don't have one, the JDBM files will grow very quickly.


On 1/20/12 10:07 PM, Selcuk AYA wrote:
> On Fri, Jan 20, 2012 at 9:47 AM, Emmanuel Lecharny<elecharny@gmail.com>  wrote:
>> Hi guys,
>>
>> I was a bit silent last week and this week, let me update you about what I
>> was working on.
>>
>> First of all, I have had to deal with some familly issues, which ate half of
>> my time.
>>
>> Regarding the Txn branch I was working on until last week, I stopped because
>> I was not able to fix the code without a serious help from Selcuk. As he is
>> busy, I preferred to wait for him to be available again, instead of bullying
>> into the code and break it seriously. I believe that there are some
>> improvement since the moment I started to work on the branch, but it's not
>> working fully yet.
>>
>> So I switched to something we wanted to do a long time ago : designing a new
>> version of JDBM. JDBM is a BTree implementation, with locks to protect
>> concurrent access. The idea was to implement a MVCC solution on top of a
>> BTree :
>> - each search can be done concurrently with any other operation, because it
>> asks for a specific existing revision from the btree
>> - each modification is done on a new revision
>> - two modifications can't be done at the same time (so modifications are
>> queued and executed one after the other)
>
> Exactly what you are saying here is already implemented in JDBM thanks
> to  the latest changes. How is what you are implementing is different?
>
>> The consequence is that searches will be very fast. It comes to a price
>> though : we keep a track on every revision, until it's not used anymore.
>> This is done by copying every modified pages when applying some
>> modification.
>>
>> As of today, the addition operation and the find operation is working just
>> fine. I conducted some benchmark on additions, and it seems that the system
>> is pretty decent.
>>
>> A *lot* remains to be done :
>> - deletion must be implemented
>> - browsing the tree is not yet implemented
>> - it's all in memory atm
>> - we must add some semaphore for concurrent modifications
>> - a GC must be added to discard unused pages
>>
>> But most of all, as it's a in-memory btree atm, I must add the disk layer.
>> It will be based on Memory Mapped files.
>>
>> Once those preliminary works will be done, the idea is to use this
>> implementation to replace JDBM. That would make the server consistent, and
>> we may then use it without the in-memory txn layer.
>>
>> Not to say that this txn layer is useless; using a MVCC btree based backend
>> is *not* enough : we have no way to guarantee the atomicity of move
>> operation across partitions.
> Txn layer enables us to group multiple atomic atomic changes and
> consistent search into a single atomic unit. For example, when you
> change an entry, change to the entry and indices happen as an atomic
> unit. Do you have code to provide this at your new implementation?
>
>> This work has been done in my sandbox, where you can follow the work in
>> progress :
>> http://svn.apache.org/viewvc/directory/sandbox/elecharny/shared-mvbt
>>
>> At the same time, thanks to Pierre-Arnaud, a first milstone of Studio 2.0
>> has been released, and it exposed some nasty bugs in the LDAP API. Which is
>> actually a good thing : we can fix them !
>>
>> So keep tuned, a lot of new things are coming soon !
>>
>> --
>> Regards,
>> Cordialement,
>> Emmanuel L├ęcharny
>> www.iktek.com
>>


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


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