This issue happens in distributedLogReplay mode when recovering multiple puts of the same key + version(timestamp). After replay, the value is nondeterministic of the key
The original concern situation raised from Elliott Neil Clark:
For all edits the rowkey is the same.
There's a log with: [ A (ts = 0), B (ts = 0) ]
Replay the first half of the log.
A user puts in C (ts = 0)
Memstore has to flush
A new Hfile will be created with [ C, A ] and MaxSequenceId = C's seqid.
Replay the rest of the Log.
The issue will happen in similar situation like Put(key, t=T) in WAL1 and Put(key,t=T) in WAL2
a) During replay, we pass original wal sequence number of each edit to the receiving RS
b) In receiving RS, we store negative original sequence number of wal edits into mvcc field of KVs of wal edits
c) Add handling of negative MVCC in KVScannerComparator and KVComparator
d) In receiving RS, write original sequence number into an optional field of wal file for chained RS failure situation
e) When opening a region, we add a safety bumper(a large number) in order for the new sequence number of a newly opened region not to collide with old sequence numbers.
In the future, when we stores sequence number along with KVs, we can adjust the above solution a little bit by avoiding to overload MVCC field.
a) disallow writes during recovery
b) during replay, we pass original wal sequence ids
c) hold flush till all wals of a recovering region are replayed. Memstore should hold because we only recover unflushed wal edits. For edits with same key + version, whichever with larger sequence Id wins.
a) During replay, we pass original wal sequence ids
b) for each wal edit, we store each edit's original sequence id along with its key.
c) during scanning, we use the original sequence id if it's present otherwise its store file sequence Id
d) compaction can just leave put with max sequence id
Please let me know if you have better ideas.