الـ Cache invalidation والـ Stampede: لما الـ key الشهير يموت

import { redis, db } from "./lib.js";

await redis.flushall();

async function getTop() {
  const hit = await redis.get("top");
  if (hit) return JSON.parse(hit);
  const value = await db.topCourses();
  await redis.set("top", JSON.stringify(value), "EX", 60);
  return value;
}

// الـ key مات، و100 request وصلوا في نفس الوقت
await Promise.all(Array.from({ length: 100 }, getTop));
console.log("db calls:", db.calls);
redis.disconnect();

$ node main.js
db calls: 100

import { redis, db } from "./lib.js";

await redis.flushall();

// single-flight: لو فيه request بيجيب نفس الـ key، استنى نتيجته بدل ما تعمل واحد تاني
function makeInstance() {
  const inflight = new Map();
  return async function getTop() {
    const hit = await redis.get("top");
    if (hit) return JSON.parse(hit);
    if (!inflight.has("top")) {
      inflight.set(
        "top",
        db.topCourses().then(async (v) => {
          await redis.set("top", JSON.stringify(v), "EX", 60);
          inflight.delete("top");
          return v;
        }),
      );
    }
    return inflight.get("top");
  };
}

const one = makeInstance();
await Promise.all(Array.from({ length: 100 }, one));
console.log("instance واحد، db calls:", db.calls);

// بس في الإنتاج عندك instances كتير، وكل واحد عنده Map لوحده
db.calls = 0;
await redis.del("top");
const instances = Array.from({ length: 5 }, makeInstance);
await Promise.all(instances.flatMap((get) => Array.from({ length: 20 }, get)));
console.log("5 instances، db calls:", db.calls);
redis.disconnect();

$ node main.js
instance واحد، db calls: 1
5 instances، db calls: 5

import { redis, db, sleep } from "./lib.js";

await redis.flushall();

// stale-while-revalidate: القيمة فيها «فرشة لحد إمتى»، والـ key نفسه عايش أطول
async function getTop() {
  const raw = await redis.get("top");
  if (raw) {
    const { value, freshUntil } = JSON.parse(raw);
    if (Date.now() > freshUntil) {
      const lock = await redis.set("top:refresh", "1", "NX", "PX", 5000);
      if (lock) refresh(); // في الخلفية، من غير await
      return { value, state: "stale" };
    }
    return { value, state: "fresh" };
  }
  return { value: await refresh(), state: "miss" };
}
async function refresh() {
  const value = await db.topCourses();
  await redis.set("top", JSON.stringify({ value, freshUntil: Date.now() + 300 }), "EX", 60);
  await redis.del("top:refresh");
  return value;
}

console.log((await getTop()).state);
console.log((await getTop()).state);
await sleep(350); // الـ fresh خلص
const burst = await Promise.all(Array.from({ length: 50 }, getTop));
console.log("burst:", [...new Set(burst.map((b) => b.state))], "| db calls:", db.calls);
await sleep(250);
console.log((await getTop()).state, "| db calls:", db.calls);
redis.disconnect();

$ node main.js
miss
fresh
burst: [ 'stale' ] | db calls: 2
fresh | db calls: 2

import { redis, db, sleep } from "./lib.js";

await redis.flushall();


async function courseKey(name) {
  const v = (await redis.get("courses:version")) ?? "0";
  return `courses:v${v}:${name}`;
}

async function getCached(key, ttl, loader) {
  const hit = await redis.get(key);
  if (hit) return JSON.parse(hit);
  if (await redis.set(`${key}:lock`, "1", "NX", "PX", 2000)) {
    const value = await loader();
    await redis.set(key, JSON.stringify(value), "EX", ttl);
    await redis.del(`${key}:lock`);
    return value;
  }
  for (let i = 0; i < 40; i++) {
    await sleep(25);
    const v = await redis.get(key);
    if (v) return JSON.parse(v);
  }
  return loader();
}

const invalidateCourses = () => redis.incr("courses:version");

const request = async () => getCached(await courseKey("top"), 60, () => db.topCourses());
await Promise.all(Array.from({ length: 100 }, request));
console.log("burst 1، db calls:", db.calls);
await invalidateCourses();
await Promise.all(Array.from({ length: 20 }, request));
console.log("بعد invalidate، db calls:", db.calls);
// SCAN بدل KEYS: بيلف على دفعات من غير ما يوقّف Redis
const [, keys] = await redis.scan("0", "MATCH", "courses:*", "COUNT", "100");
console.log(keys.sort());
redis.disconnect();

$ node main.js
burst 1، db calls: 1
بعد invalidate، db calls: 2
[ 'courses:v0:top', 'courses:v1:top', 'courses:version' ]