A brief description of the write path of an update.
Cassandra treats each new row as an upsert: if the new row has the same primary key as that of an existing row, Cassandra processes it as an update to the existing row.
During a write , Cassandra adds each new row to the database without checking on whether a duplicate record exists. This policy makes it possible that many versions of the same row may exist in the database. For more details about writes, see The write path to compaction.
Periodically, the rows stored in memory are streamed to disk into structures called SSTables. At certain intervals, Cassandra compacts smaller SSTables into larger SSTables. If Cassandra encounters two or more versions of the same row during this process, Cassandra only writes the most recent version to the new SSTable. After compaction, Cassandra drops the original SSTables, deleting the outdated rows.
Most Cassandra installations store replicas of each row on two or more nodes. Each node performs compaction independently. This means that even though out-of-date versions of a row have been dropped from one node, they may still exist on another node.
This is why Cassandra performs another round of comparisons during a read process. When a client requests data with a particular primary key, Cassandra retrieves many versions of the row from one or more replicas. The version with the most recent timestamp is the only one returned to the client ("last-write-wins").