How does Scylla LWT Differ from Apache Cassandra ?

Scylla is making an effort to be compatible with Cassandra, down to the level of limitations of the implementation. How is it different?

  • Scylla most commonly uses fewer rounds than Cassandra to complete a lightweight transaction. While Cassandra issues a separate read query to fetch the old record, scylla piggybacks the read result on the response to the prepare round.

  • Scylla will automatically use synchronous commit log write mode for all lightweight transaction writes. Before a lightweight transaction completes, scylla will ensure that the data in it has hit the device. This is done in all commitlog_sync modes.

  • Conditional statements return a result set, and unlike Cassandra, Scylla result set metadata doesn’t change from execution to execution: Scylla always returns the old version of the row, regardless of whether the condition is true or not. This ensures conditional statements work well with prepared statements.

  • For batch statement, the returned result set contains an old row for every conditional statement in the batch, in statement order. Cassandra returns results in clustering key order.

  • Unlike Cassandra, Scylla uses per-core data partitioning, so the RPC that is done to perform a transaction talks directly to the right core on a peer replica, avoiding the concurrency overhead. This is, of course, true, if Scylla’s own shard-aware driver is used - otherwise we add an extra hop to the right core at the coordinator node.

  • Scylla does not store hints for lightweight transaction writes, since this is redundant as all such writes are already present in system.paxos table.

More on Lightweight Transactions (LWT)