Conas a oibríonn imeall eochair (TTL) i Redis?

Question

Accepted Answer

Ligeann Redis do eochracha **a fhéad go huachtar** tar éis am socraithe (**TTL** — Time To Live) — agus ansin a scriosadh. Is bunúsach don taisce (sonraí seanda a fhéad), seisiúin, agus aon sonraí sealadach, agus is ceann de na gné is minice a úsáidtear i Redis.

## Imeall a shocrú

```bash
SET session:abc "data" EX 3600    # expire in 3600 seconds (1 hour)
SET key "value" PX 5000            # expire in 5000 milliseconds

# or set expiry on an existing key
SET cache:user:1 "..."
EXPIRE cache:user:1 300           # expire in 300 seconds
EXPIREAT key 1735689600           # expire at a specific Unix timestamp
```

Socraíonn tú TTL nuair a bhíonn eochar á chruthú (`EX`/`PX`) nó cuir ceann i bhfeidhm níos déanaí (`EXPIRE`). Tar éis don TTL a fháil, scriosann Redis an eochar go huachtar.

## Seiceáil agus TTL a bhainistiú

```bash
TTL session:abc        # => seconds remaining (e.g. 3540), -1 if no expiry, -2 if gone
PTTL session:abc       # remaining time in milliseconds
PERSIST session:abc    # remove the expiry → make the key permanent
```

Taispeáineann `TTL` an t-am atá fágtha (`-1` = níl aon imeall socraithe, `-2` = níl an eochar ann); bhaineann `PERSIST` an imeall.

## Conas a fhéann Redis eochracha

```text
Redis uses a combination of:
  LAZY expiration → when a key is accessed, if expired, it's deleted then
  ACTIVE expiration → a background process periodically samples and removes expired keys
→ So expired keys are cleaned up even if never accessed again (memory is reclaimed).
```

## Úsáidí coitianta TTL

```text
✓ CACHING — cache data with a TTL so it auto-refreshes (stale data expires)
✓ SESSIONS — sessions expire after inactivity (set/refresh TTL on activity)
✓ RATE LIMITING — a counter with a TTL (e.g. "max 100 requests per minute")
✓ TEMPORARY data — OTPs, tokens, locks that should auto-clean
→ TTL means Redis manages cleanup automatically — no manual deletion needed.
```

## Cén fáth a theastaíonn sé

Is ceann de na gné is thábhachtaí agus is minice a úsáidtear i Redis an imeall eochair (TTL), mar sin tá sé riachtanach tuiscint a bheith agat ar an bhealach a oibríonn sé chun Redis a úsáid go héifeachtach, go háirithe do thaisc.

Is bunúsach an cumas lárnach — eochracha **a scriosadh go huachtar tar éis am socraithe** — do na cásanna is coitianta a úsáidtear Redis: **taisce** (iontrálacha taisce le TTL ionas go féadann sonraí seanda imeall go huachtar agus athnuachaint — an croílár de bharúil gach straitéis taisce), **seisiúin** (a fhéad tar éis easláinte, TTL a athnuachaint ar ghníomhaíochtaí), **socrú ráta** (frithchóir le TTL a chur i bhfeidhm