Scalability Knowledge Base

Quick reference for scalability patterns, stateless design, PHP-FPM tuning, and capacity planning in PHP applications.

Vertical vs Horizontal Scaling

Aspect	Vertical (Scale Up)	Horizontal (Scale Out)
Approach	Bigger server (CPU, RAM)	More servers
Cost curve	Exponential (diminishing returns)	Linear (commodity hardware)
Downtime	Often required for upgrade	Zero-downtime rolling deploys
Limit	Hardware ceiling	Theoretically unlimited
Complexity	Low (single server)	High (distributed system)
Data consistency	Simple (single node)	Requires distributed coordination
Failure blast radius	Entire application	Single instance
PHP suitability	Quick win, limited ceiling	Natural fit (shared-nothing)

When to Use Each

Scenario	Strategy	Why
Early stage, simple app	Vertical	Cheapest, simplest
Read-heavy workload	Horizontal + read replicas	Distribute read load
Write-heavy workload	Horizontal + sharding	Distribute write load
Unpredictable traffic	Horizontal + auto-scaling	Elastic capacity
Legacy monolith	Vertical first, then decompose	Buys time for refactoring

Stateless vs Stateful: PHP Shared-Nothing Architecture

PHP is shared-nothing by design — each request starts with a clean process, no shared memory between requests. This is a natural advantage for horizontal scaling.

┌─────────────────────────────────────────────────────────────────────────┐
│                    SHARED-NOTHING ARCHITECTURE                          │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│   Load Balancer                                                         │
│       │                                                                 │
│   ┌───┼───────────────────┬─────────────────────┐                      │
│   │   │                   │                     │                      │
│   ▼   ▼                   ▼                     ▼                      │
│ ┌──────────┐        ┌──────────┐          ┌──────────┐                 │
│ │ PHP-FPM  │        │ PHP-FPM  │          │ PHP-FPM  │                 │
│ │ Worker 1 │        │ Worker 2 │          │ Worker N │                 │
│ │          │        │          │          │          │                 │
│ │ No shared│        │ No shared│          │ No shared│                 │
│ │ state    │        │ state    │          │ state    │                 │
│ └────┬─────┘        └────┬─────┘          └────┬─────┘                 │
│      │                   │                     │                       │
│      └───────────────────┼─────────────────────┘                       │
│                          │                                              │
│                          ▼                                              │
│                 ┌─────────────────┐                                     │
│                 │  External State  │                                    │
│                 │  Redis / DB      │                                    │
│                 └─────────────────┘                                     │
│                                                                         │
│   Key Principle: ANY request can be served by ANY worker.               │
│   State lives in external stores (Redis, DB), NOT in process memory.   │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

Stateless Checklist

Requirement	Stateless	Stateful (Problem)
Session data	Redis / JWT	`$_SESSION` with file storage
File uploads	Object storage (S3)	Local filesystem
Cache	Redis / Memcached	APCu (per-process)
Configuration	Env vars / config service	Local config files that vary per server
Scheduled jobs	Centralized scheduler	Local cron per server
WebSocket state	Redis pub/sub	In-memory connections

Session Management

File-Based Sessions (Problem)

Server A: session file → /tmp/sess_abc123
Server B: no session file → user logged out!

Sticky sessions (workaround) → couples user to server → defeats horizontal scaling

Redis Sessions (Solution)

<?php

declare(strict_types=1);

namespace Infrastructure\Session;

final readonly class RedisSessionConfig
{
    public function __construct(
        private string $redisHost,
        private int $redisPort = 6379,
        private string $redisPrefix = 'sess:',
        private int $ttlSeconds = 1800,
    ) {}

    public function configure(): void
    {
        ini_set('session.save_handler', 'redis');
        ini_set('session.save_path', sprintf(
            'tcp://%s:%d?prefix=%s&timeout=2',
            $this->redisHost,
            $this->redisPort,
            $this->redisPrefix,
        ));
        ini_set('session.gc_maxlifetime', (string) $this->ttlSeconds);
    }
}

JWT Stateless Alternative

<?php

declare(strict_types=1);

namespace Infrastructure\Auth;

final readonly class JwtTokenFactory
{
    public function __construct(
        private string $secretKey,
        private string $algorithm = 'HS256',
        private int $ttlSeconds = 3600,
    ) {}

    /**
     * @param array<string, mixed> $claims
     */
    public function create(string $userId, array $claims = []): string
    {
        $header = base64_encode(json_encode([
            'alg' => $this->algorithm,
            'typ' => 'JWT',
        ], JSON_THROW_ON_ERROR));

        $payload = base64_encode(json_encode(array_merge($claims, [
            'sub' => $userId,
            'iat' => time(),
            'exp' => time() + $this->ttlSeconds,
        ]), JSON_THROW_ON_ERROR));

        $signature = base64_encode(hash_hmac(
            'sha256',
            sprintf('%s.%s', $header, $payload),
            $this->secretKey,
            true,
        ));

        return sprintf('%s.%s.%s', $header, $payload, $signature);
    }
}

Connection Pooling

PHP creates a new database connection per request (shared-nothing). Without pooling, high-concurrency scenarios exhaust database connections.

Why PHP Needs External Poolers

Problem	Cause	Solution
Connection exhaustion	Each PHP-FPM worker opens own connection	pgbouncer / ProxySQL
Connection overhead	TCP handshake + auth per request	Persistent connections
Idle connections	Workers hold connections while waiting for I/O	External pooler reclaims idle
Max connections limit	PostgreSQL default 100, MySQL 151	Pooler multiplexes

Connection Pool Wrapper

<?php

declare(strict_types=1);

namespace Infrastructure\Database;

final readonly class ConnectionPoolConfig
{
    public function __construct(
        private string $host,
        private int $port,
        private string $database,
        private string $user,
        private string $password,
        private bool $persistent = true,
        private int $connectTimeoutSeconds = 5,
        private int $statementTimeoutMs = 30000,
    ) {}

    public function createPdo(): \PDO
    {
        $dsn = sprintf(
            'pgsql:host=%s;port=%d;dbname=%s',
            $this->host,
            $this->port,
            $this->database,
        );

        $options = [
            \PDO::ATTR_ERRMODE => \PDO::ERRMODE_EXCEPTION,
            \PDO::ATTR_DEFAULT_FETCH_MODE => \PDO::FETCH_ASSOC,
            \PDO::ATTR_EMULATE_PREPARES => false,
            \PDO::ATTR_PERSISTENT => $this->persistent,
            \PDO::ATTR_TIMEOUT => $this->connectTimeoutSeconds,
        ];

        $pdo = new \PDO($dsn, $this->user, $this->password, $options);
        $pdo->exec(sprintf(
            'SET statement_timeout = %d',
            $this->statementTimeoutMs,
        ));

        return $pdo;
    }
}

External Pooler Architecture

┌──────────────────────────────────────────────────────────────────┐
│                   CONNECTION POOLING                              │
├──────────────────────────────────────────────────────────────────┤
│                                                                   │
│   PHP-FPM Workers (200+)                                         │
│   ┌──────┐ ┌──────┐ ┌──────┐ ┌──────┐ ┌──────┐                 │
│   │  W1  │ │  W2  │ │  W3  │ │ ...  │ │ W200 │                 │
│   └──┬───┘ └──┬───┘ └──┬───┘ └──┬───┘ └──┬───┘                 │
│      │        │        │        │        │                       │
│      └────────┴────────┴────┬───┴────────┘                       │
│                             │                                    │
│                             ▼                                    │
│                    ┌─────────────────┐                           │
│                    │   pgbouncer /   │  Pool: 20-50 connections  │
│                    │   ProxySQL      │  Mode: transaction        │
│                    └────────┬────────┘                           │
│                             │                                    │
│                             ▼                                    │
│                    ┌─────────────────┐                           │
│                    │   PostgreSQL /  │  max_connections: 100     │
│                    │   MySQL         │                           │
│                    └─────────────────┘                           │
│                                                                   │
│   200 PHP workers share 20-50 DB connections via pooler          │
│                                                                   │
└──────────────────────────────────────────────────────────────────┘

Capacity Planning

Amdahl's Law

Speedup = 1 / ((1 - P) + P/N)

P = parallelizable fraction of workload
N = number of processors/instances

Example: If 90% of work is parallelizable (P=0.9), 10 servers:
  Speedup = 1 / ((1 - 0.9) + 0.9/10) = 1 / (0.1 + 0.09) = 5.26x

Lesson: Serial bottlenecks (DB writes, locks) limit scaling.

Little's Law

L = λ × W

L = average number of concurrent requests
λ = arrival rate (requests/second)
W = average response time (seconds)

Example: 500 req/s, 200ms avg response time:
  L = 500 × 0.2 = 100 concurrent requests needed

Lesson: To handle 500 req/s at 200ms, you need capacity for 100 concurrent requests.

Throughput Formula

Throughput = Workers / Avg_Response_Time

Example: 50 PHP-FPM workers, 100ms avg:
  Throughput = 50 / 0.1 = 500 req/s

To increase throughput:
  1. Add more workers (horizontal scaling)
  2. Reduce response time (optimization)
  3. Both

PHP-FPM Scaling

Worker Calculation Formula

pm.max_children = Available_Memory / Avg_Worker_Memory

Example:
  Server RAM: 4 GB
  OS + overhead: 512 MB
  Available: 3584 MB
  Avg PHP worker: 40 MB

  pm.max_children = 3584 / 40 = 89 workers

Process Manager Modes

Mode	pm.max_children	Workers	Use Case
`static`	Fixed pool size	Always running	Stable, predictable traffic
`dynamic`	Max pool size	Scale between min/max	General purpose, variable traffic
`ondemand`	Max pool size	Created per request, killed after idle	Low-traffic, memory-constrained

Recommended Settings

Setting	Static Mode	Dynamic Mode	Ondemand Mode
`pm`	`static`	`dynamic`	`ondemand`
`pm.max_children`	89	89	89
`pm.start_servers`	—	20	—
`pm.min_spare_servers`	—	10	—
`pm.max_spare_servers`	—	30	—
`pm.max_requests`	500	500	500
`pm.process_idle_timeout`	—	—	`10s`

OPcache Preloading (PHP 8.4)

; php.ini — OPcache settings for production
opcache.enable=1
opcache.memory_consumption=256
opcache.max_accelerated_files=20000
opcache.validate_timestamps=0
opcache.preload=/app/config/preload.php
opcache.preload_user=www-data
opcache.jit=1255
opcache.jit_buffer_size=128M

<?php

declare(strict_types=1);

namespace App\Config;

// preload.php — Preload hot classes into OPcache at FPM startup
// All preloaded classes are available without autoloading overhead

$classMap = [
    __DIR__ . '/../src/Domain/Entity/',
    __DIR__ . '/../src/Domain/ValueObject/',
    __DIR__ . '/../src/Application/UseCase/',
];

foreach ($classMap as $directory) {
    $iterator = new \RecursiveIteratorIterator(
        new \RecursiveDirectoryIterator($directory),
    );

    /** @var \SplFileInfo $file */
    foreach ($iterator as $file) {
        if ($file->isFile() && $file->getExtension() === 'php') {
            opcache_compile_file($file->getRealPath());
        }
    }
}

Quick Reference Tables

Scaling Decision Matrix

Signal	Action	Implementation
CPU > 80% sustained	Add instances or upgrade CPU	Horizontal + auto-scaling
Memory > 85%	Reduce worker count or add RAM	pm.max_children tuning
Response time > SLA	Profile + optimize or add capacity	APM + horizontal scaling
Connection pool exhausted	Add pooler or increase pool	pgbouncer / ProxySQL
Disk I/O bottleneck	Move to SSD, offload to object storage	Infrastructure change
Request queue growing	Add PHP-FPM workers	pm.max_children increase

Common Bottlenecks

Bottleneck	Symptom	Fix
Database queries	Slow response, high DB CPU	Query optimization, caching, read replicas
Session storage	Inconsistent sessions across servers	Redis sessions
File uploads	Disk I/O, storage limits	Object storage (S3)
External API calls	Timeout, high latency	Circuit breaker, async processing
PHP-FPM workers	502/504 errors, request queue	Increase pm.max_children
OPcache	Slow first requests after deploy	Preloading, warm-up scripts

Detection Patterns

# PHP-FPM configuration
Grep: "pm\.max_children|pm\.start_servers|pm\.min_spare|pm\.max_spare" --glob "**/php-fpm*.conf"
Grep: "pm\.max_children|pm\.start_servers" --glob "**/www.conf"
Grep: "pm\.max_requests|pm\.process_idle_timeout" --glob "**/php-fpm*.conf"

# OPcache settings
Grep: "opcache\." --glob "**/php.ini"
Grep: "opcache_compile_file|opcache_reset" --glob "**/*.php"
Grep: "opcache\.preload" --glob "**/php.ini"

# Session configuration
Grep: "session\.save_handler|session\.save_path" --glob "**/php.ini"
Grep: "session_start|SESSION" --glob "**/*.php"
Grep: "Redis.*session|session.*redis" --glob "**/*.php"

# Connection pooling
Grep: "PDO::ATTR_PERSISTENT|ATTR_PERSISTENT" --glob "**/*.php"
Grep: "pgbouncer|proxysql" --glob "**/docker-compose*.yml"

# Stateless violations
Grep: "file_put_contents|fwrite.*tmp" --glob "**/src/**/*.php"
Grep: "\\\$_SESSION" --glob "**/src/**/*.php"
Grep: "apc_store|apcu_store" --glob "**/src/**/*.php"

# Scaling indicators
Grep: "HORIZONTAL_SCALE|AUTO_SCALE|REPLICAS" --glob "**/.env*"
Grep: "replicas:|scale:" --glob "**/docker-compose*.yml"

References

For detailed information, load these reference files:

references/scaling-patterns.md — Horizontal scaling strategies, auto-scaling triggers, read replicas, write scaling, caching as scaling tool
references/php-specifics.md — PHP-FPM tuning, OPcache settings, shared-nothing architecture, persistent connections, external poolers, real-time alternatives

scalability-knowledge