After reading Cassandra source code, I will try to answer myself. It's kind of good exercise :)
>1/ Will I have an error because DC2 does not have any copy of the data ? I've not been able to find how endpoints are determined for the read request, but I guess endpoints are just coming from the current datacenter. >2/ Will Cassandra try to get the data from DC1 if nothing is found in DC2 ? Probably no since 1/ >3/ In case of partial replication to DC2, will I see sometimes errors about servers not holding the data in DC2 ? It seems to depend on RR. If read_repair_chance is set to 1 (default value), RR happens all the time : the answer is no. In case read_repair_chance is below 1, it seems CL.ONE will fail if the single read request fails. >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 ? It seems to depend on RR as in 3/ Are the answers right ? - Pierre -----Original Message----- From: Pierre Chalamet [mailto:pie...@chalamet.net] Sent: Wednesday, September 14, 2011 3:33 PM To: user@cassandra.apache.org Subject: Get CL ONE / NTS 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
