In-Memory Database

Ownership and transitions

Version selection precedes expiration testing. Choose the latest processed version at or before the requested time, then evaluate that version alone; an expired replacement cannot resurrect an older value. Deletion appends a tombstone. Equal timestamps select the last processed write. Append is amortized O(1), history lookup can use binary search O(log H), and scans visit known fields and sort names.

Java boundaries

Use the supplied nested records as immutable value carriers. A record is only shallowly immutable: copy every retained mutable input and every outward list/map, including lists inside records. Absence is null; validation failures use IllegalArgumentException, missing-key purchase errors use NoSuchElementException, and invalid transaction boundaries use IllegalStateException. Java primitive types already exclude booleans from numeric inputs. Numeric values use long where signatures say long and all computations must fit signed 64-bit values. Calls in these service exercises are serialized.

First choose the latest version, then evaluate its lifetime. Expiration must not search backward for an older replacement:

import java.util.*;

class Example {
    public static void main(String[] args) {
        record Version(long writtenAt, String value, Long expiresAt) {}
        Version older = new Version(2, "old", null);
        Version latest = new Version(5, "new", 8L);
        long queryTime = 8;
        boolean visible = latest.expiresAt() == null || queryTime < latest.expiresAt();
        assert !visible;
        assert older.value().equals("old");
    }
}

The older value still exists as history, but the correct current answer is absence. A tombstone has the same no-fallback effect. Equal-timestamp history lookup must choose the last processed version.

Design boundary

Validate TTL and lookback before advancing accepted time; otherwise a rejected future request can accidentally prohibit later valid calls. Retention policies in a real temporal store must preserve versions needed by promised historical reads, even after their current values expire.