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// Pseudocode inside appendEndTxnMarker() or where checkProducerEpoch is called:

short current = updatedEntry.producerEpoch();
short marker = markerProducerEpoch;          
// Check if TransactionVersion field is available (version 2+)
int txnVersion = (request.version >= 2) ? request.transactionVersion() : 1;  

if (txnVersion >= 2) {
    // TV2: coordinator bumps epoch before marker; duplicates carry old epoch.
    // Accept only strictly greater epoch.
    if (marker <= current) {
        throw new InvalidProducerEpochException("Reject late/dup TV2 marker: " +
            "markerEpoch=" + marker + " <= currentEpoch=" + current);
    }
} else {
    // Legacy behavior
    if (marker < current) {
        throw new InvalidProducerEpochException("Marker epoch < current.");
    }
}