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From Sriram Srinivasan <sri...@malhar.net>
Subject "CommitLog" semantics
Date Mon, 28 Jun 2010 09:44:38 GMT
Hiya.

The architecture internals document and various presentations (such as http://www.slideshare.net/jhammerb/data-presentations-cassandra-sigmod)

  say something like the following:

   ... a row mutation is first written to the commit log before being  
propagated to the memtable ...

This gives the impression of traditional commitlog semantics found in  
databases and transaction managers, where a sync'd write is a  
confirmed state change.

However, this is NOT the semantics of the code (db/Table.apply), which  
merely queues up a log request to be flushed, then directly proceeds  
to update the memtable. Which means it is possible for a client to  
receive a confirmation (even in a quorum case) before the data has  
been written to disk.

Am I wrong?

--sriram



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