[infinispan-dev] Write skew check implementation

[Manik Surtani]

Pedro,

The write skew check is only really supported for non-distributed cases at the moment.  As you can tell, it uses object references ("pointers") to compare whether the value has changed and then throw an exception if the write skew check is enabled.  As such, this scheme makes no sense in a distributed sense.

One approach to enable write skews in clustered mode would be to use vector clocks as entry versions.  This could then be sent to a caller when a value is read, and subsequently, when written, can be passed back to the owner as a part of a commit message.  The write skew check can then be made on the vector clock which will be able to detect concurrent updates from elsewhere in the cluster.

Cheers
Manik


On 13 Jun 2011, at 18:05, Pedro Ruivo wrote:

> Hi,
> 
> I am progressing with my implementation of total-order-based commit 
> protocol for the partial replication mode, and I would be grateful if 
> you could help me understanding how the write skew check is implemented 
> in ISPN.
> 
> Assume the scenario in which a node, say n1, that owns a data item d 
> receives a prepare message for a transaction, say T, originated on 
> different replica n2.
> 
> Of course write skew check is enabled.
> 
> After n1 acquiring the lock on d, it should check whether the value of d 
> read during T's execution is still the most recently committed one.
> 
> I may be missing something here, but looking at [1] (lines 160 to 168) 
> and [2] at copyForUpdate method, n1 is reading twice the value of d that 
> is locally stored (once in line 160([1]) to create the 
> RepeatableReadEntry and a second time in line 52([2]) to do the write 
> skew check).
> 
> I would have expected that the node that originated the transaction 
> would send the values that it read (before writing them) in piggyback 
> with the prepare message, in order to allow the "owners" to perform the 
> write skew.
> 
> Thanks
> 
> Pedro
> 
> [1] 
> https://github.com/infinispan/infinispan/blob/master/core/src/main/java/org/infinispan/container/EntryFactoryImpl.java 
> 
> [2] 
> https://github.com/infinispan/infinispan/blob/master/core/src/main/java/org/infinispan/container/entries/RepeatableReadEntry.java
> 
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> 
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--
Manik Surtani
manik at jboss.org
twitter.com/maniksurtani

Lead, Infinispan
http://www.infinispan.org