Hi Graham,
On 21/02/14 07:54, graham sanderson wrote:
Note also; that reading at ONE there will be no read repair, since the
coordinator does not know that another replica has stale data (remember at ONE,
basically only one node is asked for the answer).
I don't think this is right. My understanding is that while only one node will
be sent a direct read request, all other replicas will (not on every query - it
depends on the value of read_repair_chance) get a background read repair
request. You can test this experimentally using cqlsh and turning tracing on:
issue a read request many times. Most of the time you will see that the
coordinator sends a message to one node, but from time to time (depending on
read_repair_chance) you will see it sending messages to many nodes.
Best wishes, Duncan.
In practice for our use cases, we always write at LOCAL_QUORUM (failing the whole
update if that doesn’t work - stale data is OK if >1 node is down), and we read
at LOCAL_QUORUM, but (because stale data is better than no data), we will fall
back per read request to LOCAL_ONE if we detect that there were insufficient nodes
- this lets us cope with 2 down nodes in a 3 replica environment (or more if the
nodes are not consecutive in the ring).
On Feb 20, 2014, at 11:21 PM, Drew Kutcharian <d...@venarc.com> wrote:
Hi Guys,
I wanted to get some clarification on what happens when you write and read at
consistency level 1. Say I have a keyspace with replication factor of 3 and a
table which will contain write-once/read-only wide rows. If I write at
consistency level 1 and the write happens on node A and I read back at
consistency level 1 from another node other than A, say B, will C* return “not
found” or will it trigger a read-repair before responding? In addition, what’s
the best consistency level for reading/writing write-once/read-only wide rows?
Thanks,
Drew
