When executing a LWT such as INSERT .... IF NOT EXISTS, the Java driver may throw a CASWriteUnknownException. It's unclear (a) what phase failed (b) if the LWT transaction has completely failed or was partially successful and caused a mutation and (b) what the appropriate behavior of the client should be to handle this exception.
Cassanda's Paxos has four phases:
- Prepare/Promise
- Read/Results
- Propose/Accept
- Commit/Acknowledge
Which are implemented as:
| Phase | Implemention Method | Failure Exceptions |
|---|---|---|
| Prepare/Promise | StorageProxy.beginAndRepairPaxos() | WriteTimeoutException, WriteFailureException |
| Read/Results | ? | ? |
| Propose/Accept | StorageProxy.proposePaxos() | WriteTimeoutException, CasWriteUnknownResultException |
| Commit/Acknowledge | StorageProxy.commitPaxos() | WriteTimeoutException |
The Java Driver 4.x documentation is silent on the CASWriteUnknownException, but the driver code has comment on the exception class which is terse and self-referential using the same words:
/**
* The result of a CAS operation is in an unknown state.
*
*...
*/
public class CASWriteUnknownException...
The Native Protocol v5 Spec has a bit more information but is also not very clear on the state of the transaction, and raises other questions about what may or may not be completed:
0x1700 CAS_WRITE_UNKNOWN: An exception occured due to contended Compare And Set write/update. The CAS operation was only partially completed and the operation may or may not get completed by the contending CAS write or SERIAL/LOCAL_SERIAL read.
Finally, looking at the Apache Cassandra server source code, the exception appears to only be thrown during the 3rd propose phase of Paxos:
/**
* Propose the {@param proposal} accoding to the {@param replicaPlan}.
* When {@param backoffIfPartial} is true, the proposer backs off when seeing the proposal being accepted by some but not a quorum.
* The result of the cooresponding CAS in uncertain as the accepted proposal may or may not be spread to other nodes in later rounds.
*/
private static boolean proposePaxos(Commit proposal, ReplicaPlan.ForPaxosWrite replicaPlan, boolean backoffIfPartial, long queryStartNanoTime)
throws WriteTimeoutException, CasWriteUnknownResultException
If that is the case -- that it occurred before the commit phase, is there any need to worry that partial commits happened? WriteTimeout is the only other exception thrown by propose.
