Apa iku cache stampede lan kepiye carane nyegah?

Question

Accepted Answer

A **cache stampede** (utawa dogpile) kedadi nalika kunci cache panas kadaluarsa lan **akeh request concurrent ilang ing saiki**, kabeh mudhun menyang origin kanggo nglakokake nilai sing padha bebarengan. Origin (DB utawa layanan) kena pipet saka ledakan sing ora bisa ditangani, kadhang-kadhang mateni sistem.

## Panyebab

```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
```

Pemicu yaiku **kadaluarsa simultane** saka kunci populer. Perbaikan yaiku njamin mung siji (utawa sawetara) rekomputasi kedadi, utawa request ora kabeh kadaluarsa bebarengan.

## Perbaikan

- **Locking / single-flight** — miss pertama njupuk kunci lan nglakokake; kabeh liyane ngenteni hasil siji iku tinimbang kabeh tekan origin.
- **Stale-while-revalidate** — layani nilai rada kadaluarsa luwih dhisik nalika siji tugas latar mburi nyegarake. Ora ana request sing kuncen ing origin.
- **Rekomputasi awal / probabilistik** — segarake kunci *sebelum* kadaluarsa, kanthi kemungkinan sing mundhak nalika kadaluarsa cedhak, dadi siji request untung ngbangun maneh sadurunge stampede.
- **Jittered TTLs** — nambah kasawongan ing TTLs supaya kunci ora kabeh kadaluarsa ing cakrepan padha.
- **Request coalescing** — nutup request concurrent padha dadi siji panggilan origin.

## Pseudo-kode single-flight

```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
```

Kunci njamin pancalik siji panggilan origin; saben request concurrent liyane ngenteni banjur maca nilai cached sing anyar.

## Kepiye carane penting

A stampede ngabari kadaluarsa cache rutin dadi mati: cache sing nglindhungi DBmu mendadak mandheg, lan lalu lengkap tekan origin bebarengan. Ngerti panyebab (kadaluarsa simultane saka kunci panas) lan perbaikan (single-flight, stale-while-revalidate, jitter) minangka penting kanggo njaga cache bisa dipercaya duwur kargo nyata.