Cassandra write consistency level ALL clarification

Question

According to Datastax documentation for Cassandra :

"If the coordinator cannot write to enough replicas to meet the requested consistency level, it throws an Unavailable Exception and does not perform any writes."

Does this mean that while the write is in process, the data updated by the write will not be available to read requests? I mean it is possible that 4/5 nodes have successfully sent a SUCCESS to the coordinator, meaning that their data have been updated. But the 5th one is yet to do the write. Now if a read request comes in and goes to one of these 4 nodes, it will still show the old data until the coordinator recieves a confirmation from the 5th node and marks the new data valid?

Answer 1

If the coordinator knows that it cannot possibly achieve consistency before it attempts the write, then it will fail the request immediately before doing the write. (This is described in the quote given)

However, if the coordinator believes that there are enough nodes to achieve its configured consistency level at the time of the attempt, it will start to send its data to its peers. If one of the peers does not return a success, the request will fail and you will get into a state where the nodes that fail have the old data and the ones that passed have the new data.

If a read requests comes in, it will show the data it finds on the nodes it reaches no matter if it is old or new.

Let us take your example to demonstrate.

If you have 5 nodes and you have replication 3. This will mean that 3 of those 5 nodes will have the write that you have sent. However, one of the three nodes returned a failure to the coordinator. Now if you read with consistency level ALL. You will read all three nodes and will always get the new write (Latest timestamp always wins).

However, if you read with consistency level ONE, there is a 1/3 chance you will get the old value.