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From Emmanuel Lecharny <>
Subject Re: Heads up
Date Sat, 21 Jan 2012 00:32:10 GMT
On 1/20/12 10:07 PM, Selcuk AYA wrote:
> On Fri, Jan 20, 2012 at 9:47 AM, Emmanuel Lecharny<>  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?

Damn it... I had to dig back into last august mails to realize that...

Call it a complete duplication of effort. The sad thing is that I didn't 
start from the JDBM code in the branch, otherwise I would have 
immediately realize that it was already implemented, but from the trunk 

We have had a quick convo last week with Alex, and I told him that I was 
going to play around this MVCC-btree idea a bit, waiting for you to come 
back, and it seems none of us remembered that it was already a done work.

>> 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?
No. In my mind, and this is what I tried to explain, the best way to 
offer this grouped changes atomicity is to use the txn layer you are 
working on.

Do you have any idea about when you'll be back with us ? I'm afraid that 
I'm losing my time on things you either have already done, or you can do 
better than me, and I'm musing around trying to get those things done 
when I might have spent my time in other -less urgent- area...

Emmanuel L├ęcharny

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