incubator-cassandra-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Philip O'Toole <>
Subject QUORUM writes, QUORUM reads -- and eventual consistency
Date Sat, 25 Aug 2012 04:55:09 GMT
Hello -- perhaps someone could provide me some clarification about this.


"If consistency is top priority, you can ensure that a read will always reflect the most recent
write by using the following formula:

(nodes_written + nodes_read) > replication_factor"

But consider this. Say I have a replication factor of 3. I request a QUORUM write, and it
fails because the write only reaches 1 node. Perhaps there is a temporary partition in my
cluster. Now, asynchronously, a different reader performs a QUORUM read of the same cluster
and just before it issues the read, the partition is resolved. The quorum read is satisfied
by the two nodes that have *not* received the latest write (yet). Doesn't this mean that the
read does not "reflect the most recent write"? I realise this is very unlikely to happen in
practise, but I want to be sure I understand all this.

Perhaps the documentation would be more correct if the statement read as "...reflect the most
recent SUCCESSFUL write..."?



Philip O'Toole
Senior Developer
Loggly, Inc.
San Francisco, CA

View raw message