問72
頻出度★☆☆
2台の PC から一つのファイルを並行して更新した。ファイル中のデータ n に対する処理が①～④の順に行われたとき，データ n は最後にどの値になるか。ここで，データ n の初期値は10であった。 PC-A 【処理内容】 ① データ n を読み込み，5を引く。 ③ 処理①の結果を，データ n に書き込む。 PC-B 【処理内容】 ② データ n を読み込み，7を足す。 ④ 処理②の結果を，データ n に書き込む。
令和2年度 ITパスポート試験 問72
テクノロジ系データベース › トランザクション処理
This question's explanation and your answers so far.
解説
問 72
Incorrectア
5 is the value right after ③ (PC-A's write). If ④ occurred before ③, or if PC-A wrote last, 5 would remain; but in this question's order ④ is last, so 5 is overwritten by ④'s value 17 and does not remain as the final value.
Incorrectイ
10 is the initial value unchanged. This assumes that neither write takes effect, but in fact writes occur in both ③ and ④, and at least the result of the last write ④ becomes the final value, so it does not stay 10.
Incorrectウ
12 is the value of 10−5＋7, obtainable only under serial execution where locking (mutual exclusion) works correctly and one side finishes “read → compute → write” before the other reads the latest value. But in this question both PCs read the same initial value 10, so a lost update occurs and the result is not 12.
- エ
正解 The initial value of data n is 10, and the operations run in the order ①→②→③→④. In ①, PC-A reads n (=10), subtracts 5, and keeps the result 5 in its own memory. In ②, PC-B reads n (=10), adds 7, and keeps the result 17 in its own memory. At this point both PCs have read the same original value, 10. In ③, PC-A writes its result 5 to the file, making n=5; in ④, PC-B writes its result 17, making n=17. The last write is ④, so the final value is “17” (エ). The 5 written by PC-A is overwritten and lost by the write in ④ (loss of update = lost update).
ここがポイント
- Without locking (mutual exclusion), when multiple operations update the same data in parallel, a later write overwrites an earlier one, causing a lost update, and the final value is the result of the last operation to write.
- To maintain transaction isolation (the I = Isolation of the ACID properties), locking is required; without it, an inconsistent result arises as in this question.
用語