Eleza locks, mutexes, critical sections, na semaphores.

Question

IT Interview Practice · Accepted Answer

**Mutex ni lock inayohakikisha mutual exclusion — thread moja tu inaweza kuishikilia na kuingia katika critical section iliyolindwa kwa wakati mmoja.** Semaphore hupanua hili kuruhusu hadi washikaji N kwa wakati mmoja.

## Jinsi inavyofanya kazi

- **Critical section**: eneo la code linalogusa shared state na ambalo halipaswi kuendeshwa kwa wakati mmoja.
- **Mutex (mutual exclusion)**: chukua (acquire) kabla ya sehemu, achilia (release) baada. Thread ya pili inazuiwa hadi iwe huru. Kwa kawaida inamilikiwa na thread iliyoichukua (ni hiyo tu inayoweza kuachilia).
- **Semaphore**: inashikilia counter ya permits N. `acquire()` inapunguza (inazuia ikiwa 0), `release()` inaongeza. Mutex kimsingi ni semaphore yenye N=1 lakini yenye semantics za umiliki.

```python
lock = threading.Lock()
def transfer(a, b, amt):
    with lock:                 # critical section — one thread at a time
        a.balance -= amt
        b.balance += amt

# Semaphore: cap concurrent DB connections at 10
sem = threading.Semaphore(10)
def query():
    with sem:                  # up to 10 threads run this concurrently
        db.execute(...)
```

## Makosa ya kawaida

- **Kusahau kuachilia (release)** → block ya kudumu. Tumia `with`/RAII/`defer` ili release iwe otomatiki hata wakati wa exceptions.
- **Kushikilia lock kwa muda mrefu mno** huua throughput; weka critical sections ndogo.
- **Lock granularity**: lock moja pana ni rahisi lakini inafanya kila kitu kuwa serial; locks nyingi finyu zinaongezeka (scale) vizuri zaidi lakini zina hatari ya deadlock.

## Kwa nini ni muhimu

Hivi ni vifaa vya kila siku vya kulinda shared state. Mutexes hufanya ufikiaji kuwa serial ili kuzuia races; semaphores hubana resource pool (connections, permits, rate limits). Kuchagua primitive sahihi na kuweka critical sections fupi ndiyo tofauti kati ya sahihi-na-haraka na sahihi-lakini-polepole.

Jinsi inavyofanya kazi

Makosa ya kawaida

Kwa nini ni muhimu

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