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High watermark or reservation counter is incremented for each pending update, which even not guaranteed to succeed. Reservation counter is assigned during tx prepare phase.

/** Updates applied out of order. */
private SortedSet<Range> seq = new TreeSet<>();

This field is used for recording the updates that are applied out of order. This is possible because updates with higher counter could be applied to WAL before updates with lower counter, causing gaps in the update sequence.

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