Exercise 1: Visitors Keep Seeing an Old Version of an Edited Post — Possible Solution ==================================================================== WHAT'S MOST LIKELY WRONG ------------------------------ The caching setup is failing to invalidate (clear or regenerate) the cached version of that specific post when it was edited - the caching system correctly served the cached page, but never noticed the underlying content had changed and needed a fresh copy generated. WHY THIS MATCHES THIS CHAPTER'S OWN NAMED PROBLEM ------------------------------ Per this chapter's own warn-box, "a cached page has to be regenerated or cleared the moment its underlying content actually changes - a new comment, an edited post... A caching setup that doesn't correctly invalidate stale pages will confidently serve outdated content indefinitely, with no visible error anywhere." This scenario is exactly that failure mode in action: the post was genuinely edited, but the cached HTML from before the edit is still what's being served to visitors, with no error or warning anywhere indicating anything is wrong. WHY THIS PRODUCES "HOURS" OF STALE CONTENT SPECIFICALLY ------------------------------ Per this chapter, page caching works by storing "the fully-rendered HTML output of a page after its first generation, then [serving] that static copy directly for subsequent requests." Without proper invalidation, that stored copy simply continues being served indefinitely - there's no built-in expiration forcing a refresh on its own within any particular timeframe, so the stale version can persist for however long the caching mechanism's own configuration allows, which easily explains a multi-hour gap. WHY THIS IS DESCRIBED AS A "CLASSIC HARD PROBLEM" ------------------------------ Correctly detecting every situation where cached content needs refreshing (an edit, a new comment, a changed setting that affects page output) and clearing exactly the right cached pages - no more, no less - is a genuinely difficult problem to get completely right, which is why this chapter specifically calls it out as a classic, recurring difficulty rather than a simple, easily-solved detail. WHAT THE FIX WOULD LOOK LIKE ------------------------------ The caching plugin or system needs to be configured (or, if misconfigured, corrected) to properly detect post edits and clear or regenerate the cached version of that specific page immediately when the edit is saved, rather than leaving the stale cached copy in place indefinitely. WHY THIS WORKS AS AN ANSWER ------------------------------ It identifies the specific failure (missing cache invalidation) using this chapter's own explicit language, explains why the resulting stale content has no obvious error to signal the problem, and connects the severity/duration of the issue to this chapter's own framing of invalidation as a genuinely hard, easy-to-get-wrong problem.