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From Edward Ribeiro <>
Subject Re: Facebook messaging and choice of HBase over Cassandra - what can we learn?
Date Sun, 21 Nov 2010 22:06:11 GMT
> Also I believe saying HBASE is consistent is not true. This can happen:
> Write to region server. -> Region Server acknowledges client-> write
> to WAL -> region server fails = write lost
> I wonder how facebook will reconcile that. :)

Are you sure about that? Client writes to WAL before ack user?

According to these posts[1][2], "if writing the record to the WAL fails the
whole operation must be considered a failure.", so it would be nonsense
acknowledge clients before writing the lifeline. I hope any cloudera guy
explain this...

Besides that, you know that WAL is written to HDFS that takes care of
replication and fault tolerance, right? Of course, even so, there's a
"window of inconsistency" before the HLog is flushed to disk, but I don't
think you can dismiss this as not consistent. At most, you may classify it
as "eventual consistent". :)


E. Ribeiro

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