public long calcModEntryId(long delta) {
    long oplogKeyId = recoverModEntryId + delta;
    if (logger.isTraceEnabled(LogMarker.PERSIST_RECOVERY_VERBOSE)) {
      logger.trace(LogMarker.PERSIST_RECOVERY_VERBOSE,
          "calcModEntryId delta={} recoverModEntryId={}  oplogKeyId={}", delta,
          recoverModEntryId, oplogKeyId);
    }
    recoverModEntryId = oplogKeyId;
    if (oplogKeyId > recoverModEntryIdHWM) {
      recoverModEntryIdHWM = oplogKeyId; // fixes bug 41301
    }
    return oplogKeyId;
  }
Given a delta calculate the OplogEntryId for a MOD_ENTRY.
geode-core/src/main/java/org/apache/geode/internal/cache/Oplog.java