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The "ArchitectureInternals" page has been changed by PeterSchuller. The comment on this change is: Avoid StorageProxy becoming links. http://wiki.apache.org/cassandra/ArchitectureInternals?action=diff&rev1=22&rev2=23 -------------------------------------------------- = Read path = * !StorageProxy gets the endpoints (nodes) responsible for replicas of the keys from the !ReplicationStrategy * This may be a !SliceFromReadCommand, a !SliceByNamesReadCommand, or a !RangeSliceReadCommand, depending - * StorageProxy filters the endpoints to contain only those that are currently up/alive + * !StorageProxy filters the endpoints to contain only those that are currently up/alive - * StorageProxy then sorts, by asking the endpoint snitch, the responsible nodes by "proximity". + * !StorageProxy then sorts, by asking the endpoint snitch, the responsible nodes by "proximity". * The definition of "proximity" is up to the endpoint snitch * With a SimpleSnitch, proximity directly corresponds to proximity on the token ring. * With the NetworkTopologySnitch, endpoints that are in the same rack are always considered "closer" than those that are not. Failing that, endpoints in the same data center are always considered "closer" than those that are not. * The DynamicSnitch, typically enabled in the configuration, wraps whatever underlying snitch (such as SimpleSnitch and NetworkTopologySnitch) so as to dynamically adjust the perceived "closeness" of endpoints based on their recent performance. This is in an effort to try to avoid routing traffic to endpoints that are slow to respond. - * StorageProxy then arranges for messages to be sent to nodes as required: + * !StorageProxy then arranges for messages to be sent to nodes as required: * The closest node (as determined by proximity sorting as described above) will be sent a command to perform an actual data read (i.e., return data to the co-ordinating node). * As required by consistency level, additional nodes may be sent digest commands, asking them to perform the read locally but send back the digest only. For example, at replication factor 3 a read at consistency level QUORUM would require one digest read in additional to the data read sent to the closest node. (See ReadCallback, instantiated by StorageProxy) * If read repair is enabled (probabilistically if read repair chance is somewhere between 0% and 100%), remaining nodes responsible for the row will be sent messages to compute the digest of the response. (Again, see ReadCallback, instantiated by StorageProxy)
