Eleza async models: callbacks, promises, async/await, na event loop.

Question

IT Interview Practice · Accepted Answer

**Hizi ni njia zinazozidi kuwa nzuri za kueleza kazi ya non-blocking kwenye thread moja, zote zikiendeshwa chini na event loop inayoendesha callbacks zilizo tayari moja baada ya nyingine.** Zinasimamia *lini* code inaendelea tena, siyo threads nyingi.

## Event loop

Loop ya single-threaded hudumisha queue ya callbacks zilizo tayari. Wakati operation ya async (timer, I/O) inakamilika, callback yake inawekwa kwenye queue; loop inachukua inayofuata na kuiendesha hadi mwisho, kisha inarudia. Hakuna kinachozuia (preempt) callback inayoendeshwa, hivyo kazi ndefu ya CPU inazuia kila kitu.

```text
[event loop] → run callback → I/O finishes → enqueue callback → run next → ...
```

## Mitindo mitatu

```js
// 1. Callbacks — nesting gets ugly ("callback hell")
readFile('a', (e, a) => readFile('b', (e, b) => done(a, b)));

// 2. Promises — chainable, flat error handling
readFile('a').then(a => readFile('b')).then(b => ...).catch(err => ...);

// 3. async/await — sequential-looking, still non-blocking
async function run() {
  const a = await readFile('a');   // yields to the loop while waiting
  const b = await readFile('b');
  return done(a, b);
}
```

`await` inasimamisha function na kurudisha udhibiti kwa event loop; wakati promise inaposuluhika (resolve), utekelezaji unaendelea pale ulipoishia. Ni syntactic sugar juu ya promises — event loop ile ile chini.

## Kwa nini ni muhimu

Model hii inaendesha Node.js, browsers, na Python asyncio: concurrency kubwa ya I/O kwenye thread moja bila utata wa locks. Kikwazo ni lazima kamwe usizuie loop — call ya synchronous nzito ya CPU au blocking husimamisha kila task inayosubiri. Kuelewa kwamba async ≠ parallel ni kitofautishi cha kawaida cha usaili.

Event loop

Mitindo mitatu

Kwa nini ni muhimu

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