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How does caching work end-to-end (where, what, when) — cache-aside vs write-through?

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01

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End-to-end caching: layers, what to cache, when to evict, and write strategies with TTL and stampede guards.

cachingcache-asidewrite-throughcdnrediseviction
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How Does Caching Work End-to-End? Where, What, and When (Cache-Aside vs. Write-Through)

Target Audience: Junior & Senior Software Engineers preparing for System Design Interviews — assumes basic familiarity with the idea of a cache. Difficulty: Medium

This doc builds on the <a href="https://www.interviewp

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04

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Cache-aside vs write-through: behavior after a write
Run Playground
class Store {
  constructor() {
    this.db = {};
    this.cache = {};
  }
}

// Cache-aside: write goes straight to the DB; cache entry is invalidated, not refreshed.
function cacheAsideWrite(store, key, value) {
  store.db[key] = value;
  delete store.cache[key]; // invalidate -- do NOT repopulate here
}

function cacheAsideRead(store, key) {
  if (key in store.cache) {
    return ['HIT', store.cache[key]];
  }
  const value = store.db[key];
  store.cache[key] = value; // populate on miss
  return ['MISS', value];
}

// Write-through: write updates the cache and the DB together, synchronously.
function writeThroughWrite(store, key, value) {
  store.db[key] = value;
  store.cache[key] = value; // cache updated in the same step as the write
}

function writeThroughRead(store, key) {
  if (key in store.cache) {
    return ['HIT', store.cache[key]];
  }
  const value = store.db[key];
  store.cache[key] = value;
  return ['MISS', value];
}

function printResult(result) {
  console.log(`('${result[0]}', '${result[1]}')`);
}

const s1 = new Store();
cacheAsideWrite(s1, 'x', 'v1');
printResult(cacheAsideRead(s1, 'x')); // MISS -- invalidated, not refreshed
printResult(cacheAsideRead(s1, 'x')); // HIT -- warmed by the previous read

const s2 = new Store();
writeThroughWrite(s2, 'x', 'v1');
printResult(writeThroughRead(s2, 'x')); // HIT -- already warm from the write
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