Yes. Digest includes following: {name, value, timestamp, flags(deleted,
expired, etc.)}On Fri, Jul 25, 2014 at 2:33 PM, DuyHai Doan <doanduy...@gmail.com> wrote: > Thanks Mark for the very detailed explanation. > > However what's about timestamp checking ? You're saying that the > coordinator checks for the digest of data (cell value) from both nodes but > if the cell name have different timestamp would it still request a full > data read to the node having the most recent time ? > > > On Fri, Jul 25, 2014 at 11:25 PM, Mark Reddy <mark.re...@boxever.com> > wrote: > >> Hi Brian, >> >> A read request will be handled in the following manner: >> >> Once the coordinator receives a read request it will firstly determine >> the replicas responsible for the data. From there those replicas are sorted >> by "proximity" to the coordinator. The closest node as determined by >> proximity sorting will be sent a command to perform an actual data read >> i.e. return the data to the coordinator >> >> If you have a Replication Factor (RF) of 3 and are reading at CL.QUORUM, >> one additional node will be sent a digest query. A digest query is like a >> read query except that instead of the receiving node actually returning the >> data, it only returns a digest (hash) of the would-be data. The reason for >> this is to discover whether the two nodes contacted agree on what the >> current data is, without sending the data over the network. Obviously for >> large data sets this is an effective bandwidth saver. >> >> Back on the coordinator node if the data and the digest match the data is >> returned to the client. If the data and digest do not match, a full data >> read is performed against the contacted replicas in order to guarantee that >> the most recent data is returned. >> >> Asynchronously in the background, the third replica is checked for >> consistency with the first two, and if needed, a read repair is initiated >> for that node. >> >> >> Mark >> >> >> >> On Fri, Jul 25, 2014 at 9:12 PM, Brian Tarbox <briantar...@gmail.com> >> wrote: >> >>> We're considering a C* setup with very large columns and I have a >>> question about the details of read. >>> >>> I understand that a read request gets handled by the coordinator which >>> sends read requests to <quorum> of the nodes holding replicas of the data, >>> and once <quorum> nodes have replied with consistent data it is returned to >>> the client. >>> >>> My understanding is that each of the nodes actually sends the full data >>> being requested to the coordinator (which in the case of very large columns >>> would involve lots of network traffic). Is that right? >>> >>> The alternative (which I don't think is the case but I've been asked to >>> verify) is that the replicas first send meta-data to the coordinator which >>> then asks one replica to send the actual data. Again, I don't think this >>> is the case but was asked to confirm. >>> >>> Thanks. >>> >>> -- >>> http://about.me/BrianTarbox >>> >> >> >
