[infinispan-dev] Write skew check implementation

[Pedro Ruivo]

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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