IPA IP 2024 Question 67
A question IPA Japan published in 令和6年度 ITパスポート試験 公開問題 問67, translated into English. ITPEC is based on the same exams.
Same question on ITPEC: Apr 2025 Q16
There is a web system that is composed of two (2) web servers and one (1) database server as shown in the figure. When the availability of each Web server is 0.8 and the availability of the Database server is 0.9, which of the following is the availability of this system rounded off to two (2) decimal places? Here, if at least one (1) of the two (2) Web servers are operating and the Database server is operating, it is considered that the system is operating. Also, a fault occurs randomly on each server.
Explanation
0.86 — reduce the two Web servers in parallel to one block, then multiply by the Database server in series.
- •One Web server fails: 1 − 0.8 = 0.2
- •Both Web servers fail: 0.2 × 0.2 = 0.04
- •Web tier works (at least one server up): 1 − 0.04 = 0.96
- •Whole system, Web tier and Database server both needed: 0.96 × 0.9 = 0.864, rounded 0.86
Answer (c)
Why not others:
- •(a) 0.04 — the probability that both Web servers are down, a failure rate of one tier, not the availability
- •(b) 0.58 — 0.8 × 0.8 × 0.9 = 0.576: treats all three servers as in series, as if both Web servers had to be running
- •(d) 0.96 — the availability of the Web tier alone, forgetting to multiply by the Database server
Key rule: Series: multiply the availabilities; parallel: 1 − the product of the unavailabilities. Reduce each parallel group to one block first.
System Component · AI-written, may contain errors
This question comes from the past questions IPA Japan publishes, translated into English by ITPEC Practice. ITPEC Practice is an independent study tool and is not affiliated with IPA or ITPEC. See the original paper (PDF) or Report an issue.