Redis Interview Questions — Hard

• Single I/O thread processes command queue atomically (I/O threads parallelize networking in Redis 6+, execution remains serialized) → per-command O(N) costs stall EVERYTHING.
• Latency breakers to audit: KEYS/Large DEL/SLOW Lua/Big ZADD batches - SLOWLOG + LATENCY HISTORY diagnose.
• Client-side pipelining multiplies throughput without atomicity changes; MULTI differs (queued atomicity).

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• RESP3 adds out-of-band push messages separate from reply ordering - foundation of CLIENT TRACKING invalidations.
• Tracking modes: exact keys / prefix broadcast; redirect option lets a dedicated tracking connection serve others.
• Failure modes: invalidation floods overwhelming slow clients; namespace design + batching mitigates.

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• Subjective down (sdown) per sentinel timeout → objective down (odown quorum) → leader sentinel election (raft-ish epoch bump) → replica promotion selection (priority, offset freshness) → reconfiguration propagation.
• Split-brain guards: min-replicas-to-write + min-replicas-max-lag cause old-master writes to REJECT during partition, bounding loss window.
• Client responsibility: re-query sentinels on role errors; idempotent operations assumed across lost-write windows.

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• Nodes gossip cluster bus (port+10000) sharing slot ownership epochs; config epoch resolves conflicts.
• Migration: SETSLOT MIGRATING on source → IMPORTING on target → loop MIGRATE keys (blocking source ops per key) → SETSLOT NODE finalized; ASK redirection during transit honored by smart clients.
• Safety: MIGRATE is atomic per key (dump+restore+del), but application multi-key assumptions break mid-reshard - hash tags mandatory discipline.

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Reality: streams give at-least-once. Exactly-once illusion assembled:

• Dedup store: processed-id SETNX within same transaction as effect (Lua combining effect+mark atomically when effect target is redis itself).
• External-effect dedup: transactional outbox pattern - stream entry written inside DB transaction via CDC relay instead of direct XADD.
• PEL reclaim (XAUTOCLAIM) with min-idle-time prevents zombie ownership; delivery counters route poison entries to dead-letter stream.

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