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From Drew Kutcharian <>
Subject Re: Consistency Level One Question
Date Fri, 21 Feb 2014 17:22:33 GMT
Thanks, this clears things up. 

> On Feb 21, 2014, at 6:47 AM, Edward Capriolo <> wrote:
> When you write at one, as soon as one node acknowledges the write the ack is returned
to the client. This means if you quickly read from aome other node
> 1)you may get the result because by the time the read is processed the data may be on
that node
> 2)the node you read from may proxy the request to the node woth the data or not
> 3)you may get a column not found because the read might hit a node where the data does
not exist yet.
> Generally even at level one the replication is fast. I have done an experiment on what
you are asking. read from another as soon as client gets  an ack. Most of the time
the data is replicated by the time the second requeat is received. However "most of the time"
is not a guarentee. If the nodes are geographically separate who is to say if the firat request
and the second route around the internet a different way and the second action arrives on
a node before the first. That is eventual consistency for you.
> On Friday, February 21, 2014, graham sanderson <> wrote:
> > 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 <> 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 <> 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
> >>>>
> >>>
> >>
> >
> >
> -- 
> Sorry this was sent from mobile. Will do less grammar and spell check than usual.

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