[jira] [Created] (CASSANDRA-21455) Unable to catch up TCM Log from peer with gaps in log sequence

["Sam Tunnicliffe (Jira)" <ji...@apache.org>]

Sam Tunnicliffe created CASSANDRA-21455:
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             Summary: Unable to catch up TCM Log from peer with gaps in log sequence
                 Key: CASSANDRA-21455
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-21455
             Project: Apache Cassandra
          Issue Type: Improvement
          Components: Transactional Cluster Metadata
            Reporter: Sam Tunnicliffe
            Assignee: Jon Meredith


The triggering scenario for this is a non-CMS peer receiving a snapshot when catching up from a peer/CMS node, leaving a gap near the tail of its local metadata log. If that node then subsequently receives catchup requests itself the heuristic that prioritises sending recent log entries over recent snapshots is counterproductive. 

A node receives log entries with epochs A, B, misses entries C, D, E and catches up from a peer, receiving snapshot at epoch F
Later, when another node asks this peer for its log since epoch B:
 1. listSnapshotsSince(B) returns exactly 1 snapshot (at epoch F)
 2. Since snapshotEpochs.size() <= 1, the code tries getEntries(B) - gets empty (or entries only up to B with nothing after)
 3. Empty entries are vacuously isContinuous() → returns LogState(null, []) without the snapshot
 4. The requesting node is stuck - it keeps getting empty responses and never advances past the gap

Proposed fix is in LogReader.getLogState. After confirming entries are continuous, check if there's a snapshot beyond the latest entry. If so, fall through to serve the snapshot instead.{quote}



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