Distributed Cache (Redis / Memcached cluster) — Detailed

flowchart TB
  subgraph App[App Tier]
    A1[App pod]
    A2[App pod]
    AN[App pod]
    SDK([Smart client<br/>consistent hash<br/>or slot map])
  end

  subgraph Cluster[Cache Cluster]
    direction TB
    subgraph Shard1
      P1[Primary 1]
      R1a[Replica 1a]
    end
    subgraph Shard2
      P2[Primary 2]
      R2a[Replica 2a]
    end
    subgraph ShardN
      PN[Primary N]
      RNa[Replica Na]
    end
    SLOT[Slot map<br/>16384 slots → shards]
    SENT[Sentinel / Cluster bus<br/>failover]
  end

  subgraph Engines[Data structures]
    KV[String GET/SET]
    LIST[List - LPUSH BRPOP]
    HASH[Hash - HSET]
    SET[Set / ZSET]
    STREAM[[Stream - XADD XREAD]]
    HLL[HyperLogLog]
    GEO[Geo - GEOADD]
    PSUB[[Pub/Sub]]
    LUA[Lua scripts<br/>atomic]
  end

  subgraph Patterns[Access patterns]
    CA[Cache-aside]
    RT[Read-through]
    WT[Write-through]
    WB[Write-back]
    LOCK[Distributed lock<br/>SET NX EX]
    QUEUE[[Job queue]]
    RL[Rate limiter]
  end

  subgraph Pitfalls[Pitfalls]
    STMP[Stampede / dogpile]
    BIG[Big keys / hot keys]
    EVICT[Eviction storm]
    NET[Latency spikes]
  end

  subgraph Mit[Mitigations]
    SF[Single-flight / lock]
    JIT[Jittered TTL]
    SPLIT[Split big key into chunks]
    HOTS[Hot key replication / split]
    LRU[allkeys-lru / lfu]
    PIPE[Pipelining / batching]
  end

  subgraph Persist[Optional Persistence]
    RDB[RDB snapshot]
    AOF[AOF append log]
  end

  A1 --> SDK
  A2 --> SDK
  AN --> SDK
  SDK -. slot mod 16384 .-> SLOT
  SLOT --> Shard1
  SLOT --> Shard2
  SLOT --> ShardN
  P1 -.repl.-> R1a
  P2 -.repl.-> R2a
  PN -.repl.-> RNa
  SENT -.health.-> Cluster
  SENT -. promote .-> R1a
  Engines --- Cluster
  Patterns --- App
  STMP -. fix .-> SF
  STMP -. fix .-> JIT
  BIG -. fix .-> SPLIT
  BIG -. fix .-> HOTS
  EVICT -. fix .-> LRU
  NET -. fix .-> PIPE
  Cluster --> Persist

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class SDK client;
    class SENT edge;
    class A1,A2,AN,P1,R1a,P2,R2a,PN,RNa,SLOT,KV,LIST,HASH,SET,HLL,GEO,LUA,CA,RT,WT,WB,LOCK,RL,STMP,BIG,EVICT,NET,SF,JIT,SPLIT,HOTS,LRU,PIPE,RDB,AOF service;
    class STREAM,PSUB,QUEUE queue;

Sharding

• Redis Cluster: 16384 slots, slot = CRC16(key) mod 16384.
• Memcached: client-side consistent hashing (Ketama).
• Resharding: SLOT MIGRATE moves slots online.

Replication

• Async leader-follower; replica promotion via Sentinel / Cluster bus quorum.
• Optional waitfor sync confirm before ack.

Eviction

• maxmemory-policy: allkeys-lru, allkeys-lfu, volatile-lru, noeviction.
• TinyLFU / W-TinyLFU (Caffeine) for app-local L1.

Common usages

• Object cache, session store, leaderboard (ZSET), rate limiter, distributed lock, event stream (Stream), pub/sub bus, geo index.

Pitfalls

• Big key: 10 MB ZSET blocks the event loop. Split or move to DB.
• Hot key: shard cannot scale on one key. Replicate or pre-compute at app.
• Cache stampede: single-flight via Redis LOCK or async refresh.

Glossary & fundamentals

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag	Concept	What it is	Page
HLD	Sharding	horizontal partitioning across nodes	database-sharding
HLD	Consistent hashing	key placement with minimal remap	consistent-hashing
HLD	Cache strategies	cache-aside, read/write-through, eviction	caching-strategies
HLD	Pub/Sub & message brokers	topics, consumer groups, delivery semantics	pub-sub-pattern
HLD	Raft / Paxos consensus	replicated state machine via majority quorum	consensus-raft-paxos
HLD	Leader/follower replication	sync/semi-sync/async replication, failover	replication-leader-follower
HLD	Probabilistic data structures	Bloom, HLL, Count-Min, MinHash, t-digest	probabilistic-data-structures
HLD	Idempotency & retries	safe re-execution, backoff + jitter	idempotency-retries
LLD	Concurrency primitives	mutex, semaphore, RW lock, atomic, CAS	concurrency-primitives