[infinispan-dev] tx optimizations

[Manik Surtani]

On 6 Apr 2009, at 15:26, Mircea Markus wrote:

> Manik Surtani wrote:
>>
>> On 3 Apr 2009, at 20:33, Mircea Markus wrote:
>>
>>> Hi,
>>>
>>> Here are two optimizations that can be implemented in our 2PC model:
>>> 1) if there are only two members int the cluster use an 1PC (or if  
>>> you only replicate to one buddy, like in buddy replication). If  
>>> the 1st phase fails remotely, then also rollback locally. This  
>>> would reduce one network roundtrip.
>>
>> Interesting.  I assume with BR you mean DIST where a key is mapped  
>> on to 1 other peer - Infinispan won't support BR as in JBC.  ;-)
>>
>> While this is an interesting thought, it does raise the potential  
>> for race conditions - since this decision will have to be taken in  
>> the TxInterceptor in the beforeCompletion phase of a transaction,  
>> and by the time the call gets to the interceptor for replication,  
>> the topology may have changed such that you need to replicate to 2  
>> instead of 1 other peer.  Which would mean a 2PC again.  So it does  
>> need some thought.
> Good point with concurrency. Even so, this is a valid optimization  
> and I think worths thinking about it.

Yes, of course.  I just pointed out one of the challenges that came to  
mind.

>>> 2) when asked to prepare, a participant might return a value  
>>> indicating that no changes were made (read-only participant), so  
>>> this one won't need an commit message, so less roundtrip.
>>
>> No, prepares only contain modifications.   Read commands don't get  
>> added to a prepare,
> I know that :)
>> and if a prepare doesn't contain any writes, it isn't broadcast  
>> economising on the network call.
> e.g. a bunch of remove() operations on keys that does not exist  
> would cause an "read only" participant. There are other ops that  
> might not modify the remote node, e.g. putIfAbsent, replace(k,v)

Yes, but we need to be sure the behaviour of all of these commands are  
the same cluster-wide.  And they may not be.  E.g., a remove() may be  
a no-op on one node due to eviction, but on the neighbour it may  
actually remove something.  Same with pIA() and replace().

Cheers
--
Manik Surtani
manik at jboss.org
Lead, JBoss Cache
http://www.jbosscache.org