REDIS · MEDIUM CODEX

Redis Interview Questions — Medium

Real-world mechanisms, state management, edge cases, performance trade-offs, and practical coding.

15 Theory Questions5 Free Model Answers

THEORY QUESTIONS & SOLUTIONSShowing 5 of 15 questions

Q1Explain Redis persistence trade-offs: AOF fsync policies and rewrite mechanics.

appendfsync always/everysec/no — durability vs latency spectrum; everysec default losing ≤1s on crash.
AOF rewrite forks child replaying current state into compact new file; copy-on-write memory spike during rewrite is the ops surprise (plan headroom ~2x dataset).
auto-aof-rewrite-percentage triggers sizing; monitor aof_last_bgrewrite_status.

Q2How does replication work and what can be lost on failover?

Asynchronous by default: replicas stream replication feed; WAIT command provides sync acknowledgment option (latency cost).
Failover loses unreplicated writes — min-replicas-to-write guards accepting writes without healthy replicas.
Replicas are read-only serve reads; read-replica staleness visible via master_repl_offset deltas.

Q3What is Sentinel vs Cluster — decision boundaries?

Sentinel: HA for single-master topology (monitoring, automatic failover, client discovery). Data fits one node; scaling = vertical + read replicas.
Cluster: automatic sharding across 16384 hash slots; multi-key ops require same slot (hash tags {}); resharding online. Choose Sentinel for simplicity till dataset/QPS demands sharding; Cluster adds client complexity everywhere.

Q4How do hash slots and hash tags work in Redis Cluster?

CRC16(key) mod 16384 assigns slots; MOVED/ASK redirects guide clients.
Hash tags {user42}.profile and {user42}.cart co-locate related keys enabling MGET/Lua across them.
Resharding migrates slots live via CLUSTER SETSLOT migrating/importing + MIGRATE COPY batches.

Q5What is the atomicity model of Lua scripting (EVAL)?

Script executes atomically mid-command-stream — perfect check-and-set sequences (rate limiter, token bucket).
Constraints: no external reads besides KEYS/ARGV passed (cluster-safe requirement), deterministic scripts only (no random/time without effects flags), script timeout kills blocking.
SCRIPT LOAD + EVALSHA avoids resending bodies; NOSCRIPT fallback handling standard in clients.

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