What a Cache Remembers
Written by Studio AM.
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When a browser requests the same image repeatedly, downloading it from a distant server every time wastes time and network capacity. A cache keeps a temporary local copy. On the next request, the browser may reuse that copy and display the image faster.
Speed creates a freshness problem. The original image can change while the cache still holds the old version. Systems attach expiration times, version labels, or validation information so a cache can decide whether to reuse a copy or ask the source about an update.
Caches exist at several levels: inside browsers, operating systems, servers, and content-delivery networks. A copy closer to the requester reduces repeated work and delay. It also means that clearing one cache may not remove every stored layer.
A stale copy has passed its freshness limit. It may still match the original, so validation can check whether reuse is appropriate. Good cache design balances speed, storage, network use, and the cost of being temporarily out of date.
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Question 1 of 4
What is the main idea of the passage?
The answer is D: A cache speeds repeated requests by storing nearby copies, but refresh rules are needed to prevent outdated results.
The passage develops the performance benefit, freshness problem, multiple levels, and design tradeoff.
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Question 2 of 4
Why might a cached image differ from the source?
The answer is A: The source changed, but the cache still holds the old copy.
The second paragraph describes this delay after the source changes.
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Question 3 of 4
What does “stale” mean in the final paragraph?
The answer is B: past its permitted freshness period
The paragraph defines a stale copy by its expired freshness limit, even if its content still matches the source.
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Question 4 of 4
What can tell a cache when to check again?
The answer is C: an expiration time
The second paragraph lists expiration time among the freshness mechanisms.
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