Your current approach to Consistency opens the door to some inconsistent behavior.
> 1/ Will I have an error because DC2 does not have any copy of the data ? If you read from DC2 at CL ONE and the data is not replicated it will not be returned. > 2/ Will Cassandra try to get the data from DC1 if nothing is found in DC2 ? Not at CL ONE. If you used CL EACH QUORUM then the read will go to all the DC's. If DC2 is behind DC1 then you will get the data form DC1. > 3/ In case of partial replication to DC2, will I see sometimes errors about > servers not holding the data in DC2 ? Depending on the API call and the client, working at CL ONE, you will see either errors or missing data. > 4/ Does Get CL ONE failed as soon as the fastest server to answer tell it > does not have the data or does it waits until all servers tell they do not > have the data ? yes Consider using LOCAL QUORUM for write and read, will make things a bit more consistent but not add inter DC overhead into the request latency. Still possible to not get data in DC2 if it is totally disconnected from the DC1 write at LOCAL QUORUM and read at EACH QUORUM . Will so you can always read, requests in DC2 will fail if DC1 is not reachable. Hope that helps. ----------------- Aaron Morton Freelance Cassandra Developer @aaronmorton http://www.thelastpickle.com On 15/09/2011, at 1:33 AM, Pierre Chalamet wrote: > Hello, > > I have 2 datacenters. Cassandra is configured as follow: > - RackInferringSnitch > - NetworkTopologyStrategy for CF > - strategy_options: DC1:3 DC2:3 > > Data are written using CL LOCAL_QUORUM so data written from one datacenter > will eventually be replicated to the other datacenter. Data is always written > exactly once. > > On the other side, I'd like to improve the read path. I'm using actually the > CL ONE since data is only written once (ie: timestamp is more or less > meaningless in my case). > > This is where I have some doubts: if data is written on DC1 and tentatively > read from DC2 while the data is still not replicated or partially replicated > (for whatever good reason since replication is async), what is the behavior > of Get with CL ONE / NTS ? > 1/ Will I have an error because DC2 does not have any copy of the data ? > 2/ Will Cassandra try to get the data from DC1 if nothing is found in DC2 ? > 3/ In case of partial replication to DC2, will I see sometimes errors about > servers not holding the data in DC2 ? > 4/ Does Get CL ONE failed as soon as the fastest server to answer tell it > does not have the data or does it waits until all servers tell they do not > have the data ? > > Thanks a lot, > - Pierre
