What is the main idea of the passage?

A cache speeds repeated requests by storing nearby copies, but refresh rules are needed to prevent outdated results.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.)

Why might a cached image differ from the source?

The source changed after the local copy was stored and the cache has not refreshed it yet. (The second paragraph directly describes the old-copy period after an origin update.)

What does “stale” mean in the final paragraph?

outdated because a newer version exists (The sentence describes a remembered version that remains beyond its intended refresh point.)

What can tell a cache when to check again?

an expiration time (The second paragraph lists expiration time among the freshness mechanisms.)

What a Cache Remembers: Level 3 Reading Comprehension Questions

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 is not evidence that the original data vanished. It shows that a remembered version outlived the rule meant to refresh it. Good cache design balances speed, storage, network use, and the cost of being temporarily out of date.

Comprehension questions