I do not agree here. I trade "consistency" (it's more data miss than consistency here) over performance in my case.
I'm okay to handle the popping of the Spanish inquisition in the current DC by triggering a new read with a stronger CL somewhere else (for example in other DCs). If the data is nowhere to be found or nothing is reachable, well, it's sad but true but it will be the end of the game. Fine. What I'm missing is a clear behavior for CL.ONE. I'm unsure about what nodes are used by ONE and how the filtering of missing data/error is done. I've landed in ReadCallback.java but error handling is out of my reach for the moment. Thanks, - Pierre From: aaron morton [mailto:aa...@thelastpickle.com] Sent: Thursday, September 15, 2011 12:27 AM To: user@cassandra.apache.org Subject: Re: Get CL ONE / NTS Are you advising CL.ONE does not worth the game when considering read performance ? Consistency is not performance, it's a whole new thing to tune in your application. If you have performance issues deal with those as performance issues, better code / data model / hard ware. By the way, I do not have consistency problem at all - data is only written once Nobody expects a consistency problem. It's chief weapon is surprise. Surprise and fear. It's two weapons are fear and surprise. And so forth http://www.youtube.com/watch?v=Ixgc_FGam3s If you write at LOCAL QUORUM in DC 1 and DC 2 is down at the start of the request, a hint will be stored in DC 1. Some time later when DC 2 comes back that hint will be sent to DC 2. If in the mean time you read from DC 2 at CL ONE you will not get that change. With Read Repair enabled it will repair in the background and you may get a different response on the next read (Am guessing here, cannot remember exactly how RR works cross DC) Cheers ----------------- Aaron Morton Freelance Cassandra Developer @aaronmorton http://www.thelastpickle.com On 15/09/2011, at 10:07 AM, Pierre Chalamet wrote: Thanks Aaron, didn't seen your answer before mine. I do agree for 2/ I might have read error. Good suggestion to use EACH_QUORUM - it could be a good trade off to read at this level if ONE fails. Maybe using LOCAL_QUORUM might be a good answer and will avoid headache after all. Are you advising CL.ONE does not worth the game when considering read performance ? By the way, I do not have consistency problem at all - data is only written once (and if more it is always the same data) and read several times across DC. I only have replication problems. That's why I'm more inclined to use CL.ONE for read if possible. Thanks, - Pierre -----Original Message----- From: aaron morton [mailto:aa...@thelastpickle.com] Sent: Wednesday, September 14, 2011 11:48 PM To: user@cassandra.apache.org; pie...@chalamet.net Subject: Re: Get CL ONE / NTS 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
