cassandra-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Ryan King <r...@twitter.com>
Subject Re: Propose new ConsistencyLevel.ALL_AVAIL for reads
Date Thu, 16 Jun 2011 16:05:10 GMT
On Thu, Jun 16, 2011 at 8:18 AM, AJ <aj@dude.podzone.net> wrote:
> Good morning all.
>
> Hypothetical Setup:
> 1 data center
> RF = 3
> Total nodes > 3
>
> Problem:
> Suppose I need maximum consistency for one critical operation; thus I
> specify CL = ALL for reads.  However, this will fail if only 1 replica
> endpoint is down.  I don't see why this fail is necessary all of the time
> since the data could have been updated since the node became unavailable and
> it's data is old anyways.  If only one node goes down and it has the key I
> need, then the app is not 100% available and it could take some time making
> the node available again.
>
> Proposal:
> If all of the *available* replica nodes answer the read operation and the
> latest value timestamp is clearly AFTER the time the down node became
> unavailable, then this situation can meet the requirements for *near* 100%
> consistency since the value in the down node would be outdated anyway.
>  Clearly, the value was updated some time *after* the node went down or
> unavailable.  This way, you can have max availability when using read with
> CL.ALL... or something CL close in meaning to ALL.
>
> I say "near" 100% consistency to leave room for some situation where the
> unavailable node was only unavailable to the coordinating node for some
> reason such as a network issue and thus still received an update by some
> other route after it "appeared" unavailable to the current coordinating
> node.  In a situation like this, there is a chance the read will still not
> return the latest value.  So, this will not be truly 100% consistent which
> CL.ALL guarantees.  However, I think this logic could justify a new
> consistency level slightly lower than ALL, such as ALL_AVAIL.
>
> What do you think?  Is my logic correct?  Is there a conflict with the
> architecture or base principles?  This fits with the tunable consistency
> principle for sure.

I don't think this buys you anything that you can't get with quorum
reads and writes.

-ryan

Mime
View raw message