LWT != Last Write Wins. They are totally different.
LWTs give you (assuming you also read at SERIAL) “atomic consistency”, meaning you are able to perform operations atomically and in isolation. That’s the safety blanket everyone wants but is extremely expensive, especially in Cassandra. The lightweight part, btw, may be a little optimistic, especially if a key is under contention. With regard to the “last write” part you’re asking about - w/ LWT Cassandra provides the timestamp and manages it as part of the ballot, and it always is increasing. See org.apache.cassandra.service.ClientState#getTimestampForPaxos. From the code:
* Returns a timestamp suitable for paxos given the timestamp of the last known commit (or in progress update).
* Paxos ensures that the timestamp it uses for commits respects the serial order of those commits. It does so
* by having each replica reject any proposal whose timestamp is not strictly greater than the last proposal it
* accepted. So in practice, which timestamp we use for a given proposal doesn't affect correctness but it does
* affect the chance of making progress (if we pick a timestamp lower than what has been proposed before, our
* new proposal will just get rejected).
Effectively paxos removes the ability to use custom timestamps and addresses clock variance by rejecting ballots with timestamps less than what was last seen. You can learn more by reading through the other comments and code in that file.
Last write wins is a free for all that guarantees you *nothing* except the timestamp is used as a tiebreaker. Here we acknowledge things like the speed of light as being a real problem that isn’t going away anytime soon. This problem is sometimes addressed with event sourcing rather than mutating in place.
Hope this helps.
@Justin I read this article http://www.datastax.com/dev/blog/lightweight-transactions-in-cassandra-2-0
. And it clearly says Linearizable consistency can be achieved with LWT's. so should I assume the Linearizability
in the context of the above article is possible with LWT's and synchronization of clocks through ntpd ? because LWT's also follow Last Write Wins. isn't it? Also another question does most of the production clusters do setup ntpd? If so what is the time it takes to sync? any idea
If you require the best precision you can get, setting up a pair of
stratum 1 ntpd masters in each data center location with a GPS modules
is not terribly complex. Low latency and jitter on servers you manage.
140ms is a long way away network-wise, and I would suggest that was a
poor choice of upstream (probably stratum 2 or 3) source.
As Jonathan mentioned, there's no guarantee from Cassandra, but if you
need as close as you can get, you'll probably need to do it yourself.
On 02/09/2017 06:47 PM, Kant Kodali wrote:
> Hi Justin,
> There are bunch of issues w.r.t to synchronization of clocks when we
> used ntpd. Also the time it took to sync the clocks was approx 140ms
> (don't quote me on it though because it is reported by our devops :)
> we have multiple clients (for example bunch of micro services are
> reading from Cassandra) I am not sure how one can achieve
> Linearizability by setting timestamps on the clients ? since there is no
> total ordering across multiple clients.
> On Thu, Feb 9, 2017 at 4:16 PM, Justin Cameron <justin@xxxxxxxxxxxxxxx
> <mailto:justin@xxxxxxxxxxxxxxx>> wrote:
> Hi Kant,
> Clock synchronization is important - you should ensure that ntpd is
> properly configured on all nodes. If your particular use case is
> especially sensitive to out-of-order mutations it is possible to set
> timestamps on the client side using the
> drivers. https://docs.datastax.com/en/developer/java-driver/3.1/manual/query_timestamps/
> We use our own NTP cluster to reduce clock drift as much as
> possible, but public NTP servers are good enough for most
> uses. https://www.instaclustr.com/blog/2015/11/05/apache-cassandra-synchronization/
> On Thu, 9 Feb 2017 at 16:09 Kant Kodali <kant@xxxxxxxxxxxx
> <mailto:kant@xxxxxxxxxxxx>> wrote:
> How does Cassandra achieve Linearizability with “Last write
> wins” (conflict resolution methods based on time-of-day clocks) ?
> Relying on synchronized clocks are almost certainly
> non-linearizable, because clock timestamps cannot be guaranteed
> to be consistent with actual event ordering due to clock skew.
> isn't it?
> Justin Cameron
> Senior Software Engineer | Instaclustr
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