3 ms·

I think you can make Cassandra "C" by using quorum reads and writes. Also they support some SERIAL consistency level, which uses PAXOS underneath, and supposedl

by crypto5 10y ago

I think you can make Cassandra "C" by using quorum reads and writes. Also they support some SERIAL consistency level, which uses PAXOS underneath, and supposedly achieves consistency as well.

im_down_w_otp 10y ago
Quorum won't solve consistency problems like this unless you can also guarantee a serialized reader/writer for any given piece of data. The best you can do by pinning queries to primary replicas and enforcing R + W > N is RYOW consistency, and that depends on the aforementioned serialization point.
- pkolaczk 10y ago
  Quorum reads + writes and applying non-destructive updates only are enough to get linearizabile consistency in Cassandra, even if clocks are not perfectly synchronized.
  - im_down_w_otp 10y ago
    If you're applying non-destructive updates then you don't even need quorum to achieve consistency. You're just writing immutable data.
    - pkolaczk 10y ago
      If you're applying non-destructive updates without quorum, you would not get monotonic read consistency, because stale reads would be possible. You could write a row, immediately read it back from another replica and find out the row is not there, because that replica didn't get it yet. You'd have only eventual consistency.
mdani 10y ago
The IF statement uses Paxos as underlying implementation and still it may result into an inconsistency if you are using two separate CQL statements. Cassandra does not provide begin ... end construction where everything is atomically committed or rolled back.
- pkolaczk 10y ago
  You can use Cassandra LWT as a building block for multi-partition transactions, in a similar way like CAS atomic operations are used to build locks, mutexes and monitors and then monitors are used to implement ACID transactions in RDBMS. However, noone says it would be easy or performant, therefore this is probably not a good idea.
  - mdani 10y ago
    Right. Cassandra crawled at 20 TPS when we sent IF cql statements. One thing to note is that Cassandra txns can never be rolled back - they can only be committed/ retrried and so on.

by crypto5 10y ago

I think you can make Cassandra "C" by using quorum reads and writes. Also they support some SERIAL consistency level, which uses PAXOS underneath, and supposedly achieves consistency as well.

im_down_w_otp 10y ago

Quorum won't solve consistency problems like this unless you can also guarantee a serialized reader/writer for any given piece of data. The best you can do by pinning queries to primary replicas and enforcing R + W > N is RYOW consistency, and that depends on the aforementioned serialization point.

pkolaczk 10y ago
Quorum reads + writes and applying non-destructive updates only are enough to get linearizabile consistency in Cassandra, even if clocks are not perfectly synchronized.
- im_down_w_otp 10y ago
  If you're applying non-destructive updates then you don't even need quorum to achieve consistency. You're just writing immutable data.
  - pkolaczk 10y ago
    If you're applying non-destructive updates without quorum, you would not get monotonic read consistency, because stale reads would be possible. You could write a row, immediately read it back from another replica and find out the row is not there, because that replica didn't get it yet. You'd have only eventual consistency.

mdani 10y ago

The IF statement uses Paxos as underlying implementation and still it may result into an inconsistency if you are using two separate CQL statements. Cassandra does not provide begin ... end construction where everything is atomically committed or rolled back.

pkolaczk 10y ago
You can use Cassandra LWT as a building block for multi-partition transactions, in a similar way like CAS atomic operations are used to build locks, mutexes and monitors and then monitors are used to implement ACID transactions in RDBMS. However, noone says it would be easy or performant, therefore this is probably not a good idea.
- mdani 10y ago
  Right. Cassandra crawled at 20 TPS when we sent IF cql statements. One thing to note is that Cassandra txns can never be rolled back - they can only be committed/ retrried and so on.