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advanced Phase 79 · Scaling Fundamentals

Redis Scaling

Redis scaling strategies including Redis Cluster, Redis Sentinel, memory management, and eviction policies

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Redis Cluster Architecture

Redis Cluster Topology

┌─────────────┐  ┌─────────────┐  ┌─────────────┐
│  Primary 1  │  │  Primary 2  │  │  Primary 3  │
│  Slots 0-   │  │  Slots 5461-│  │  Slots 10923│
│  5460       │  │  10922      │  │  16383      │
└──────┬──────┘  └──────┬──────┘  └──────┬──────┘
       │                │                │
┌──────┴──────┐  ┌──────┴──────┐  ┌──────┴──────┐
│  Replica 1  │  │  Replica 2  │  │  Replica 3  │
└─────────────┘  └─────────────┘  └─────────────┘

Key Distribution

16384 slots distributed across primary nodes:
- Primary 1: slots 0-5460
- Primary 2: slots 5461-10922
- Primary 3: slots 10923-16383

Each key hashed: HASH = CRC16(key) MOD 16384
Key routes to responsible primary node

Magento Redis Cluster Config

// app/etc/env.php
'cache' => [
    'frontend' => [
        'default' => [
            'backend' => 'Magento\Framework\Cache\Backend\Redis',
            'backend_options' => [
                'server' => 'redis-cluster.example.com',
                'port' => '6379',
                'database' => '0',
                'compress_data' => '1',
                'password' => ''
            ]
        ],
        'page_cache' => [
            'backend' => 'Magento\Framework\Cache\Backend\Redis',
            'backend_options' => [
                'server' => 'redis-cluster.example.com',
                'port' => '6379',
                'database' => '1',
                'compress_data' => '0'
            ]
        ]
    ]
],

Redis Sentinel for High Availability

Sentinel Architecture

┌─────────────┐     ┌─────────────┐
│  Sentinel 1 │────▶│  Master     │
└─────────────┘     └─────────────┘
┌─────────────┐     ┌─────────────┐
│  Sentinel 2 │────▶│  Slave 1    │
└─────────────┘     └─────────────┘
┌─────────────┐     ┌─────────────┐
│  Sentinel 3 │────▶│  Slave 2    │
└─────────────┘     └─────────────┘

Sentinels monitor master, auto-failover on failure

Sentinel Configuration

# sentinel.conf
port 26379
sentinel monitor magento-master 10.0.1.10 6379 2
sentinel down-after-milliseconds magento-master 5000
sentinel failover-timeout magento-master 60000
sentinel parallel-syncs magento-master 1

Magento Sentinel Integration

// Use Sentinel-aware connection
'session' => [
    'save' => 'redis',
    'redis' => [
        'host' => 'sentinel://sentinel1:26379',
        'sentinel_master' => 'magento-master',
        'timeout' => '2.5',
        'database' => '0'
    ]
],

Memory Management

Redis Memory Configuration

# redis.conf
maxmemory 8gb
maxmemory-policy allkeys-lru

# Memory usage monitoring
redis-cli info memory

# Key metrics:
# used_memory: 6.2GB
# used_memory_rss: 6.8GB
# mem_fragmentation_ratio: 1.09
# evicted_keys: 15234

Memory Usage by Data Type

Data Type    | Bytes/Key | Magento Use Case
─────────────|───────────|──────────────────
String       | 56 + len  | Cache, sessions
Hash         | 80 + len  | Product data
List         | 64 + len  | Queue messages
Set          | 80 + len  | Tag sets
ZSet         | 80 + len  | Scored data

Memory Optimization

# Check memory usage per database
redis-cli INFO keyspace
redis-cli DBSIZE

# Find memory-hungry keys
redis-cli --bigkeys
redis-cli --memkeys

# Compress large values
# Use zstd compression for cache values
redis-cli CONFIG SET compress-data yes

Eviction Policies

Eviction Policy Comparison

Policy          | Description                    | Use Case
────────────────|────────────────────────────────|──────────────
noeviction      | Return errors when full         | Critical data
allkeys-lru     | LRU across all keys             | General cache
volatile-lru    | LRU only on keys with TTL       | Mixed workloads
allkeys-lfu     | LFU across all keys             | Hot data caching
volatile-lfu    | LFU only on keys with TTL       | Mixed with priorities
allkeys-random  | Random eviction                 | Uniform access
volatile-random | Random on keys with TTL         | Mixed workloads
volatile-ttl    | Evict closest to expire         | Time-sensitive data

Magento Eviction Config

// Set eviction policy
'session' => [
    'redis' => [
        'maxmemory_policy' => 'allkeys-lru',
        'compression_threshold' => '2048'
    ]
],

// Monitor evictions
// redis-cli INFO stats | grep evicted_keys

Key Expiration Strategy

Cache TTL Hierarchy:
- Page cache: 86400s (24h)
- Block cache: 3600s (1h)
- Config cache: 7200s (2h)
- Session: 14400s (4h)
- Full page cache: 86400s (24h)

Quiz

1. How many hash slots does Redis Cluster use?

Question 1 options

2. What is Redis Sentinel's primary function?

Question 2 options

3. Which eviction policy is best for general caching?

Question 3 options

Flashcards

Question

Redis Cluster slots?

Click to reveal answer

Answer

16384 hash slots distributed across primary nodes

Question

Redis Sentinel purpose?

Click to reveal answer

Answer

High availability with automatic failover on master failure

Question

Best eviction policy for caching?

Click to reveal answer

Answer

allkeys-lru: evicts least recently used across all keys

Question

Memory monitoring command?

Click to reveal answer

Answer

redis-cli info memory and redis-cli --bigkeys

Revision Notes

Key Takeaways

  • 1. Redis Cluster distributes data across 16384 hash slots
  • 2. Redis Sentinel provides automatic failover for high availability
  • 3. Monitor memory usage and evictions with redis-cli
  • 4. allkeys-lru is best eviction policy for general caching
  • 5. Compress large cache values to save memory

Interview Tips

  • • Explain Redis Cluster key distribution and slot management
  • • Compare Redis Sentinel vs Redis Cluster use cases
  • • Discuss memory optimization and eviction strategies

Cheat Sheet

Redis Scaling:
  Cluster: 16384 slots, data sharding
  Sentinel: HA with auto-failover

Memory:
  maxmemory + maxmemory-policy
  Monitor: info memory, --bigkeys

Eviction:
  allkeys-lru: General cache
  volatile-lru: Mixed workloads
  noeviction: Critical data

Magento Config:
  cache → Redis cluster
  sessions → Redis/Sentinel