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From graham sanderson <gra...@vast.com>
Subject Re: Consistency Level One Question
Date Fri, 21 Feb 2014 14:15:25 GMT
My bad; should have checked the code:

    /**
     * This function executes local and remote reads, and blocks for the results:
     *
     * 1. Get the replica locations, sorted by response time according to the snitch
     * 2. Send a data request to the closest replica, and digest requests to either
     *    a) all the replicas, if read repair is enabled
     *    b) the closest R-1 replicas, where R is the number required to satisfy the ConsistencyLevel
     * 3. Wait for a response from R replicas
     * 4. If the digests (if any) match the data return the data
     * 5. else carry out read repair by getting data from all the nodes.
     */

On Feb 21, 2014, at 3:10 AM, Duncan Sands <duncan.sands@gmail.com> wrote:

> 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 <drew@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
>>> 
>> 
> 


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