Redis 持久化与恢复：备份能不能真的用起来

import java.util.*;

public class Main {
  record Value(String text, long expiresAt) {
    Value {
      Objects.requireNonNull(text);
      if (expiresAt < -1) throw new IllegalArgumentException("expiry");
    }
  }
  record Event(long sequence, String key, Value value) {} // null 值明确表示删除事件
  static Map<String, Value> restore(long base, long last, Map<String, Value> snapshot,
      List<Event> events, long now) {
    if (base < 0 || last < base || now < 0) throw new IllegalArgumentException("range");
    Map<String, Value> state = new HashMap<>(snapshot); // 验证失败不污染输入快照
    long current = base;
    for (Event event : events) {
      Objects.requireNonNull(event.key());
      if (event.sequence() != Math.addExact(current, 1))
        throw new IllegalArgumentException("gap or duplicate");
      current = event.sequence();
      if (event.value() == null) state.remove(event.key());
      else state.put(event.key(), event.value()); // 先覆盖旧值，不能跳过已到期的新值
    }
    if (current != last) throw new IllegalArgumentException("manifest mismatch");
    state.values().removeIf(v -> v.expiresAt() != -1 && v.expiresAt() <= now);
    return Map.copyOf(state); // 返回恢复时仍有效的独立状态
  }
  static Value forever(String s) { return new Value(s, -1); }
  static void check(boolean ok) { if (!ok) throw new AssertionError(); }
  static void bad(Runnable action) {
    try { action.run(); } catch (IllegalArgumentException expected) { return; }
    throw new AssertionError("expected rejection");
  }
  public static void main(String[] args) {
    Map<String, Value> base = Map.of("k", forever("old"));
    check(restore(5,5,base,List.of(),100).get("k").text().equals("old"));
    check(restore(5,6,base,List.of(new Event(6,"k",new Value("new",90))),100).isEmpty());
    check(restore(5,6,base,List.of(new Event(6,"k",null)),100).isEmpty());
    check(restore(5,6,base,List.of(new Event(6,"x",new Value("live",101))),100).size()==2);
    check(restore(5,6,base,List.of(new Event(6,"k",new Value("edge",100))),100).isEmpty());
    bad(() -> restore(5,7,base,List.of(new Event(7,"x",forever("v"))),100));
    bad(() -> restore(5,7,base,List.of(new Event(6,"x",forever("v"))),100));
    check(base.get("k").text().equals("old")); // 重放成功或失败都不改变原始快照
    System.out.println("8 recovery checks passed");
  }
}