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From shankarpnsn <shankarp...@gmail.com>
Subject Re: What does ReadRepair exactly do?
Date Tue, 23 Oct 2012 22:10:38 GMT
Hello, 

This conversation precisely targets a question that I had been having for a
while - would be grateful if you someone cloud clarify it a little further: 

Considering the case of a "repair" created due to a consistency constraint
(first case in the discussion above), would the following interpretation be
correct ?

1. A digest mismatch exception is raised even if one among the many
responses (even if consistency is met on an out-of-date value, say by virtue
of timestamp).
2. A read is initiated by the callback to fetch data from all replicas
3. Resolve() is invoked to find the deltas for each replica that was out of
date. 
4. ReadRepair is scheduled to the above replicas. 
5. Perform a normal read and check if this meets the consistency
constraints. Mismatches would trigger a repair again. 

Assuming the above is true, would the mutations in step 4 and the read in
step 5 happen in parallel ? In other words, would the time taken by the read
correction be the round trip between the coordinator and its farthest
replica that meets the consistency constraint.  

Thanks,
Shankar



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