manuzhang wrote > read quorum doesn't mean we read newest values from a quorum number of > replicas but to ensure we read at least one newest value as long as write > quorum succeeded beforehand and W+R > N.
I beg to differ here. Any read/write, by definition of quorum, should have at least n/2 + 1 replicas that agree on that read/write value. Responding to the user with a newer value, even if the write creating the new value hasn't completed cannot guarantee any read consistency > 1. Hiller, Dean wrote >> Kind of an interesting question >> >> I think you are saying if a client read resolved only the two nodes as >> said in Aaron's email back to the client and read -repair was kicked off >> because of the inconsistent values and the write did not complete yet and >> I guess you would have two nodes go down to lose the value right after >> the >> read, and before write was finished such that the client read a value >> that >> was never stored in the database. The odds of two nodes going out are >> pretty slim though. >> Thanks, >> Dean Bingo! I do understand that the odds of a quorum nodes going down are low and that any subsequent read would achieve a quorum. However, I'm wondering what would be the right thing to do here, given that the client has particularly asked for a certain consistency on the read and cassandra returns a value that doesn't have the consistency. The heart of the problem here is that the coordinator responds to a client request "assuming" that the consistency has been achieved the moment is issues a row repair with the super-set of the resolved value; without receiving acknowledgement on the success of a repair from the replicas for a given consistency constraint. In order to adhere to the given consistency specification, the row repair (due to consistent reads) should repeat the read after issuing a "consistency repair" to ensure if the consistency is met. Like Manu mentioned, this could of course lead to a number of repeat reads if the writes arrive quickly - until the read gets timed out. However, note that we would still be honoring the consistency constraint for that read. -- View this message in context: http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/What-does-ReadRepair-exactly-do-tp7583261p7583400.html Sent from the cassandra-u...@incubator.apache.org mailing list archive at Nabble.com.
