If you read at Consistency of at least quorum, you are guaranteed that
at least one of the nodes has the latest data, and so you get the right
data. If you read with less than quorum it would be possible for all the
nodes that respond to have stale data.
On 05/10/2012 09:46 PM, Carpenter, Curt wrote:
Hi all, newbie here. Be gentle.
From
http://www.datastax.com/docs/1.0/cluster_architecture/about_client_requests:
"Thus, the coordinator first contacts the replicas specified by the
consistency level. The coordinator will send these requests to the
replicas that are currently responding most promptly. The nodes
contacted will respond with the requested data; if multiple nodes are
contacted, the rows from each replica are compared in memory to see if
they are consistent. If they are not, then the replica that has the
most recent data (based on the timestamp) is used by the coordinator
to forward the result back to the client.
To ensure that all replicas have the most recent version of
frequently-read data, the coordinator also contacts and compares the
data from all the remaining replicas that own the row in the
background, and if they are inconsistent, issues writes to the
out-of-date replicas to update the row to reflect the most recently
written values. This process is known as/read repair/. Read repair can
be configured per column family (using/read_repair_chance/
<http://www.datastax.com/docs/1.0/configuration/storage_configuration#read-repair-chance>),
and is enabled by default.
For example, in a cluster with a replication factor of 3, and a read
consistency level of QUORUM, 2 of the 3 replicas for the given row are
contacted to fulfill the read request. Supposing the contacted
replicas had different versions of the row, the replica with the most
recent version would return the requested data. In the background, the
third replica is checked for consistency with the first two, and if
needed, the most recent replica issues a write to the out-of-date
replicas."
Always returns the most recent? What if the most recent write is
corrupt? I thought the whole point of a quorum was that consistency is
verified /before/ the data is returned to the client. No?
Thanks,
Curt
