# 인터페이스가 적는 계약
  /**
   * Records a completed migration, only if the fence is still the current one.
   *
   * <p>Conditioned on the fence because the runner that wrote the batches may no longer be the
   * runner that owns the lease. A ledger entry from a superseded runner says a migration completed
   * when the work it describes was overwritten by the runner that replaced it.
   *
   * @param fence the acquisition token from {@link MongoMigrationLock#fence()}
   * @throws dev.caskeleton.adapter.outbound.mongo.api.error.MongoOperationRejectedException when a
   *     newer acquisition exists
   */
  void recordApplied(

# 구현이 하는 일
    requireCurrentFence(fence, "ledger entry for " + migrationId.value());
    ledger.insertOne(
        new Document(MIGRATION_ID, migrationId.value())
            .append("checksum", checksum.value())
            .append("operator", operator)
            .append("appliedAt", Date.from(appliedAt))
            .append("fence", fence));

#   그 구현이 부르는 검사와 그 검사의 javadoc
  /**
   * Refuses a write from an unfenced lease.
   *
   * <p>An unfenced lease cannot prove the writer is still the owner, and the whole point of the
   * ledger is that it says what actually happened.
   */
  private static void requireCurrentFence(long fence, String what) {
    if (fence == MongoMigrationLock.UNFENCED) {
      throw MongoOperationRejectedException.of(
          "migration.fence",
          "refusing to write the "
              + what

# 같은 파일의 saveCheckpoint 는 저장된 펜스를 조건으로 건다
    long replaced =
        checkpoints
            .replaceOne(
                Filters.and(
                    Filters.eq(MIGRATION_ID, checkpoint.migrationId().value()),
                    Filters.or(Filters.exists("fence", false), Filters.lte("fence", fence))),
                document,
                new ReplaceOptions().upsert(false))
            .getMatchedCount();

#   그리고 중복 키를 잡아 번역한다. 그렇게 된 이유가 주석에 있다
    // Nor was the refusal itself reachable. With `upsert(true)`, a superseded write matches nothing
    // and MongoDB attempts an insert, which the unique index on migrationId rejects — so a
    // superseded runner got a driver-level duplicate-key error instead of the sentence written for
    // it, and the branch meant to produce that sentence was dead.
    try {
      checkpoints.insertOne(document);
    } catch (MongoWriteException duplicate) {
      if (duplicate.getError().getCategory() != ErrorCategory.DUPLICATE_KEY) {
        throw duplicate;
      }
      // Either the checkpoint was already there and the fence filter excluded it, or another runner
      // inserted between the replace and this insert. Both are the same fact about this runner.
      throw MongoOperationRejectedException.of(
          "migration.fence",
          "a newer migration runner owns the lease; this runner's checkpoint write was refused");