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From Michael Dürig <mdue...@apache.org>
Subject Re: [jr3] clustering
Date Thu, 01 Mar 2012 13:50:41 GMT
Hi Marcel,

Nice to see new ideas wrt. to clustering. I quickly skimmed over the HP 
paper and think that approach sounds promising. Having a flexible 
partitioning scheme which also supports dynamic repartioning (i.e. 
additions of server, migration of nodes) seems a better approach to me 
than fixed partitioning by path as we discussed earlier.

An open question though is how replication would fit into this picture. 
There is some mention in the paper about backup nodes for fail-over. Not 
sure if that is what we are aiming for or whether we want to go beyond 
that.

The paper assumes network connections to be reliable (i.e. no messages 
altered, dropped or duplicated). However there is no mention on how the 
system would recover from a partitioned network. That is, how it would 
recover when some links go down and come up later. However, since it 
uses 2 phase commit, I think it would basically inherit that behaviour 
which means cluster nodes could become blocked (See [1] proposition 7.1 
and 7.2).

OTOH the combination of optimistic locking during the transaction itself 
and pessimistic locking only for the commit itself will probably result 
in very good write throughput. Even more so since probably in many cases 
there is only a single node involved in the transaction such that a 
simple commit suffices.

More comments see inline below.

[1] http://research.microsoft.com/en-us/people/philbe/ccontrol.aspx

On 1.3.12 11:05, Marcel Reutegger wrote:

[...]
>
> so, I was thinking of something similar as described in this
> paper [1] or similar [2]. since a B-tree is basically an ordered
> list of items we'd have to linearize the JCR or MK hierarchy. I'm
> not sure whether a depth or breadth first traversal is
> better suited. maybe there even exists a more sophisticated
> space filling curve, which is a combination of both. linearizing
> the hierarchy on a B-tree should give some since locality for
> nodes that are hierarchically close and probability is high that
> they are requested in succession.

Node types may give hints here. As long as they are not recursive (i.e. 
nt:hierarchy) node types usually define "things that belong together".

[...]

> Open questions:
>
> how does MVCC fit into this? multiple revisions of the same
> JCR/MK node could be stored on a B-tree node. whenever
> an update happens the garbage collection could kick in an
> purge outdated revisions. providing a consistent journal across
> all servers is not clear to me right now.

I think MVCC is not a problem as such. To the contrary, since it is 
append only it should even be less problematic. IMO garbage collection 
is an entirely different story and we shouldn't worry too much about it 
until we have a good working model for clustering itself.

Wrt. the journal: isn't that just the list of versions of the root node? 
This should be for free then. But I think I'm missing something here...

>
> How does backup work? this is quite tricky because it is
> difficult to get a consistent snapshot of the distributed
> tree.

MVCC should make that easy: just make a backup of the head revision at 
that time.

Michael

>
> Regards
>   Marcel
>
>
> [0] https://docs.google.com/presentation/pub?id=131sVx5s58jAKE2FSVBfUZVQSl1W820_syyzLYRHGH6E&start=false&loop=false&delayms=3000#slide=id.g4272a65_0_39
> [1] http://www.hpl.hp.com/techreports/2007/HPL-2007-193.pdf
> [2] http://research.microsoft.com/en-us/people/aguilera/distributed-btree-vldb2008.pdf
>

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