REDIS · EASY CODEX

Redis Interview Questions — Easy

Q: What is TTL / key expiration?

EXPIRE key seconds or SET with EX option; TTL auto-deletes keys — the backbone of caching semantics. TTL returns remaining seconds (-1 no expiry, -2 missing); PERSIST removes expiry. Expiration is laz

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THEORY QUESTIONS & SOLUTIONSShowing 5 of 15 questions

Q1What is Redis and what is it used for?

In-memory data structure store (key-value with rich types) usable as cache, message broker, and lightweight database.
Common uses: caching, sessions, rate limiting, leaderboards, queues, pub/sub, distributed locks.
Single-threaded command execution (mostly) → atomic per-command, predictable latency; persistence optional (RDB/AOF).

Q2What data types does Redis provide?

STRING (counters, tokens), LIST (queues via LPUSH/BRPOP), SET (unique members, set ops), HASH (objects), ZSET (sorted sets — leaderboards/ranges).
Plus specialized: Streams (event logs/consumer groups), Bitmaps (presence flags), HyperLogLog (cardinality estimates), Geospatial. Choosing the right type unlocks atomic server-side operations instead of client-side read-modify-write.

Q3What is TTL / key expiration?

EXPIRE key seconds or SET with EX option; TTL auto-deletes keys — the backbone of caching semantics.
TTL returns remaining seconds (-1 no expiry, -2 missing); PERSIST removes expiry.
Expiration is lazy (checked on access) + active sampling cycle — expired-but-uncached keys may linger briefly in memory.

Q4What is the difference between RDB and AOF persistence?

RDB: point-in-time binary snapshots on interval — compact, fast restarts, risk of losing recent writes.
AOF: append-only log of every write — configurable fsync policy (always/everysec/no), better durability, larger files + rewrite compaction.
Hybrid (aof-use-rdb-preamble default modern): fast load + durability. Choose per loss-tolerance budget.

Q5What is a cache hit/miss and how do you compute ratio?

Hit = served from redis; miss = fell through to origin DB.
Ratio = hits/(hits+misses) via INFO stats keyspace metrics.
Low ratios signal wrong TTLs, poor key design, or genuinely uncachable data — investigate before scaling hardware.

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