Menene cache stampede kuma ta nada shi yadda?

Question

Accepted Answer

A **cache stampede** (ko dogpile) yana faruwa lokacin da maɓuɓɓugar cache key ya ɗan karewa kuma **kimantse bukatun daidaita da yawa sun rasa shi a lokaci gida**, dukansu suna faɗowa zuwa asali don sake ƙididdige aji gida a lokaci gida. Asali (DB ko hidima) ana bugi ta allurar da ba za ta iya sarrafa ba, wani lokaci na kawo da tsarin.

## Dalilin

```text
t=0   hot key expires
t=0+  1000 requests arrive → all miss → all query the DB for the SAME value
      → DB overwhelmed; latency spikes; failures cascade
```

Dasi ina **lokaci gida daidaita** na babbar key. Warewa ita ce a tabbatar da cewa cacakken (ko kadan) sake ƙididdige ya faruwa, ko bukatun ba duka sun karewa a lokaci gida.

## Warewawa

- **Kullin / single-flight** — farkon rasa ya karɓa kulle kuma ya sake ƙididdige; kowa sauran yana jira wannan sakamako gida a wajen bugi asali.
- **Stale-while-revalidate** — bugi aji kaɗan da tsoho nan da nan a yayin aiki na baki gida yana sake sabuntawa. Babu bukatun da ya kulle akan asali.
- **Sake ƙididdige da wuri / yuwuwa** — sabunta key *kafin* ya karewa, da yuwuwar da ta ƙaru yayin karewa ta kusurawa, saboda bukatun bata sa'i ƙa sake gina shi kafin stampede.
- **TTLs da girgija** — ƙari jita-jita ga TTLs don kullin ba duka suna karewa a lokaci gida.
- **Haɗin bukatun** — daidaita bukatun daidaita daidaita a waje gida gida.

## Single-flight pseudo-code

```text
function get(key):
  val = cache.get(key)
  if val is present: return val            // fast path: a hit

if lock.acquire(key, ttl=5s):            // only ONE request wins the lock
    try:
      val = recompute()                    // hit the origin exactly once
      cache.set(key, val, ttl)
    finally:
      lock.release(key)
    return val
  else:
    wait_briefly(); return get(key)        // others wait, then read the filled cache
```

Kullin yana tabbatar da gaida gida daidai; kowa sauran bukatun daidaita yana jira sannan ya karanta sabuwar aji.

## Me yasa ya matuwa

Stampede ya canza karewa na cache na yau-a-yau zuwa kakaita: cache wacce ta kare DB dinku a gaɓa ta tsaya a gaɓa, kuma zirga-zirgar jiya ya fadi akan asali a lokaci gida. Sanin dalilin (daidaita karewa na babbar keys) da warewawa (single-flight, stale-while-revalidate, girgija) yana da mahimmanci don kiyaye caches abin tsinkayi a ƙarƙar aiki.