Ask HN: How does Cassandra achieve strong consistency if there is no rollback?

3 pointsby olafA2 anni fa1 comments

Confused on how Cassandra is strongly consistent in the following scenario

Write of key1, value1 is requested with consistency level of QUORUM but only N replica responded success where N < QUORUM

What happen to those N nodes that just updated key1? Do they get rollback?

In cassandra documentation https://docs.datastax.com/en/cassandra-oss/3.0/cassandra/dml/dmlTransactionsDiffer.html

> if using a write consistency level of QUORUM with a replication factor of 3, Cassandra will replicate the write to all nodes in the cluster and wait for acknowledgement from two nodes. If the write fails on one of the nodes but succeeds on the other, Cassandra reports a failure to replicate the write on that node. However, the replicated write that succeeds on the other node is not automatically rolled back.

It mentions if some write failed and did not satisfied consistency level, coordinator will return failure, but the data will persist on nodes that have write succeeded

But this means strong consistency can never be achieved even if R + W > number of replica as official. documentation suggested

https://docs.datastax.com/en/cassandra-oss/3.0/cassandra/dml/dmlAboutDataConsistency.html

Consider the following situation

    replica number = 5
    consistency level write = 3
    consistency level read = 3

If a write is attempted , but one nodes succeeds , coordinator will return failure, but that one node will not rollback, so you need a consistency level of 5 in order to achieve strong consistency

The documentation has conflicting information

What am I getting wrong here?

1 comments

rustyrazorblade2 anni fa

It sounds like you're conflating strong consistency with ACID transactions. The strong consistency guarantee with QUORUM writes with Cassandra is only if the write is acknowledged as successful. If you write something successfully, you'll be able to read it. When a write fails the end state is undefined.

The definition of strong consistency does not require a rollback, nor does it define the behavior of the system during a write failure. Strong consistency makes no assertions regarding atomicity (all or nothing) or isolation.