Horizontal Scaling

Scaling Models

Vertical (Scale Up)          Horizontal (Scale Out)
┌──────────────┐              ┌──────┐ ┌──────┐
│  Bigger CPU  │              │Node 1│ │Node 2│
│  More RAM    │    vs        └──────┘ └──────┘
│  Faster Disk │              ┌──────┐ ┌──────┐
└──────────────┘              │Node 3│ │Node 4│
                              └──────┘ └──────┘

When to Scale Horizontally

• Traffic exceeds single node capacity
• Need zero-downtime deployments
• Require geographic distribution
• Need fault tolerance

// Magento web node configuration
// app/etc/env.php
'deployment' => [
    'default' => [
        'cron_run' => true,
        'content_region' => 'default'
    ]
],

Load Balancer Algorithms

Round Robin:     A→1, B→2, C→3, D→1, E→2...
Least Connect:   Route to server with fewest connections
IP Hash:         Hash(client IP) → server index
Weighted:        Server 1 (weight 3), Server 2 (weight 1)

NGINX Load Balancer Config

upstream magento_backend {
    least_conn;
    server web1.example.com:8080 weight=5;
    server web2.example.com:8080 weight=5;
    server web3.example.com:8080 backup;

    keepalive 32;
}

server {
    listen 443 ssl http2;
    server_name store.example.com;

    location / {
        proxy_pass http://magento_backend;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_connect_timeout 30s;
        proxy_read_timeout 60s;
    }
}

HAProxy Configuration

frontend magento_https
    bind *:443 ssl crt /etc/ssl/magento.pem
    default_backend magento_nodes

backend magento_nodes
    balance leastconn
    option httpchk GET /health_check.php
    server web1 10.0.1.10:8080 check inter 5s fall 3 rise 2
    server web2 10.0.1.11:8080 check inter 5s fall 3 rise 2
    server web3 10.0.1.12:8080 check backup

Shared Session Storage Options

┌─────────────┐     ┌──────────────┐
│  Web Node 1 │────▶│              │
└─────────────┘     │   Redis /    │
┌─────────────┐     │   Memcached  │
│  Web Node 2 │────▶│              │
└─────────────┘     │  (Shared)    │
┌─────────────┐     │              │
│  Web Node 3 │────▶│              │
└─────────────┘     └──────────────┘

Redis Session Configuration

// app/etc/env.php
'session' => [
    'save' => 'redis',
    'redis' => [
        'host' => 'redis-cluster.example.com',
        'port' => '6379',
        'password' => '',
        'timeout' => '2.5',
        'persistent_identifier' => '',
        'database' => '0',
        'compression_threshold' => '2048',
        'compression_library' => 'zstd',
        'log_level' => '1',
        'max_concurrency' => '6',
        'break_after_frontend' => '5',
        'break_after_adminhtml' => '30',
        'first_lifetime' => '600',
        'bot_first_lifetime' => '60',
        'disable_locking' => '0',
        'min_lifetime' => '60',
        'max_lifetime' => '2592000'
    ]
],

Memcached Session Configuration

'session' => [
    'save' => 'memcached',
    'save_path' => 'memcached-cluster:11211?persistent=1&weight=1&timeout=1&retry_interval=15'
],

Sticky Session Strategies

Without Sticky Sessions:           With Sticky Sessions:
Request 1 → Node A (session)      Request 1 → Node A (session)
Request 2 → Node B (MISS!)        Request 2 → Node A (HIT)
Request 3 → Node C (MISS!)        Request 3 → Node A (HIT)

NGINX Sticky Sessions

upstream magento_backend {
    sticky cookie srv_id expires=1h domain=.example.com path=/;
    server web1.example.com:8080;
    server web2.example.com:8080;
}

HAProxy Sticky Sessions

backend magento_nodes
    balance roundrobin
    cookie SERVERID insert indirect nocache
    server web1 10.0.1.10:8080 cookie web1
    server web2 10.0.1.11:8080 cookie web2

Trade-offs

Recommendation

• Use shared Redis/Memcached sessions over sticky sessions
• Sticky sessions are a workaround, not a solution
• Shared sessions enable true horizontal scaling