stability-patterns-knowledge

Stability Patterns Knowledge Base

Quick reference for resilience and fault tolerance patterns in PHP applications.

Core Patterns Overview

┌─────────────────────────────────────────────────────────────────────────────┐
│                        STABILITY PATTERNS                                    │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│   ┌────────────────────────────────────────────────────────────────────┐    │
│   │                      REQUEST FLOW                                   │    │
│   │                                                                     │    │
│   │   Client  ──▶  Rate Limiter  ──▶  Circuit Breaker  ──▶  Service   │    │
│   │      │              │                   │                  │        │    │
│   │      │         Throttle            Monitor State       Actual      │    │
│   │      │         requests            Open/Closed         work        │    │
│   │      │              │                   │                  │        │    │
│   │      │              ▼                   ▼                  ▼        │    │
│   │      │         ┌────────┐          ┌────────┐         ┌────────┐   │    │
│   │      │         │Bulkhead│          │ Retry  │         │Timeout │   │    │
│   │      │         │Isolate │          │Pattern │         │Control │   │    │
│   │      │         └────────┘          └────────┘         └────────┘   │    │
│   └────────────────────────────────────────────────────────────────────┘    │
│                                                                              │
├─────────────────────────────────────────────────────────────────────────────┤
│   Pattern          │ Purpose                    │ Protects Against           │
│   ─────────────────┼────────────────────────────┼─────────────────────────── │
│   Rate Limiter     │ Throttle request rate      │ DDoS, overload, abuse     │
│   Circuit Breaker  │ Fail fast on failures      │ Cascading failures        │
│   Retry            │ Retry transient failures   │ Temporary outages         │
│   Bulkhead         │ Isolate resources          │ Resource exhaustion       │
│   Timeout          │ Limit wait time            │ Slow dependencies         │
└─────────────────────────────────────────────────────────────────────────────┘

Pattern Relationships

┌─────────────────────────────────────────────────────────────────────────────┐
│                    PATTERN INTERACTION                                       │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│                         ┌────────────────────┐                               │
│                         │   Rate Limiter     │                               │
│                         │  (Entry Point)     │                               │
│                         └─────────┬──────────┘                               │
│                                   │                                          │
│                                   ▼                                          │
│                         ┌────────────────────┐                               │
│                         │     Bulkhead       │                               │
│                         │ (Resource Limits)  │                               │
│                         └─────────┬──────────┘                               │
│                                   │                                          │
│           ┌───────────────────────┼───────────────────────┐                  │
│           │                       │                       │                  │
│           ▼                       ▼                       ▼                  │
│   ┌──────────────┐       ┌──────────────┐        ┌──────────────┐           │
│   │   Service A  │       │   Service B  │        │   Service C  │           │
│   │  ┌────────┐  │       │  ┌────────┐  │        │  ┌────────┐  │           │
│   │  │Circuit │  │       │  │Circuit │  │        │  │Circuit │  │           │
│   │  │Breaker │  │       │  │Breaker │  │        │  │Breaker │  │           │
│   │  └───┬────┘  │       │  └───┬────┘  │        │  └───┬────┘  │           │
│   │      │       │       │      │       │        │      │       │           │
│   │  ┌───▼────┐  │       │  ┌───▼────┐  │        │  ┌───▼────┐  │           │
│   │  │ Retry  │  │       │  │ Retry  │  │        │  │ Retry  │  │           │
│   │  │Pattern │  │       │  │Pattern │  │        │  │Pattern │  │           │
│   │  └────────┘  │       │  └────────┘  │        │  └────────┘  │           │
│   └──────────────┘       └──────────────┘        └──────────────┘           │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

Quick Reference

Circuit Breaker States

State	Behavior	Transitions To
Closed	Requests pass through, failures counted	Open (on threshold)
Open	Requests fail fast, no calls to service	Half-Open (after timeout)
Half-Open	Limited requests allowed for testing	Closed (on success) / Open (on failure)

Retry Backoff Strategies

Strategy	Formula	Use Case
Fixed	delay	Simple cases, known recovery time
Linear	delay * attempt	Gradual increase
Exponential	delay * 2^(attempt-1)	Unknown recovery, default choice
Exponential + Jitter	exponential ± random	High concurrency, prevents thundering herd

Rate Limiter Algorithms

Algorithm	Precision	Memory	Burst Handling
Token Bucket	Medium	Low	Allows bursts
Sliding Window	High	Medium	Smooth limiting
Fixed Window	Low	Low	Edge bursts
Leaky Bucket	High	Low	No bursts

Bulkhead Types

Type	Isolation	Use Case
Semaphore	Thread/request count	Single-process apps
Thread Pool	Dedicated threads	CPU-bound work
Queue-based	Request queue	Async processing
Distributed	Redis/shared state	Multi-instance apps

PHP Implementation Patterns

Circuit Breaker with PSR Clock

<?php

declare(strict_types=1);

namespace Infrastructure\Resilience;

use Psr\Clock\ClockInterface;

final class CircuitBreaker
{
    private CircuitState $state = CircuitState::Closed;
    private int $failures = 0;
    private ?\DateTimeImmutable $openedAt = null;

    public function __construct(
        private readonly string $name,
        private readonly int $failureThreshold,
        private readonly int $openTimeoutSeconds,
        private readonly ClockInterface $clock
    ) {}

    public function execute(callable $operation, ?callable $fallback = null): mixed
    {
        if (!$this->isAvailable()) {
            return $fallback ? $fallback() : throw new CircuitOpenException($this->name);
        }

        try {
            $result = $operation();
            $this->recordSuccess();
            return $result;
        } catch (\Throwable $e) {
            $this->recordFailure();
            throw $e;
        }
    }

    private function isAvailable(): bool
    {
        if ($this->state === CircuitState::Closed) return true;
        if ($this->state === CircuitState::Open) {
            if ($this->hasTimeoutElapsed()) {
                $this->state = CircuitState::HalfOpen;
                return true;
            }
            return false;
        }
        return true;
    }
}

Retry with Exponential Backoff

<?php

declare(strict_types=1);

namespace Infrastructure\Resilience;

final readonly class RetryExecutor
{
    public function execute(
        callable $operation,
        int $maxAttempts = 3,
        int $baseDelayMs = 100
    ): mixed {
        $attempt = 0;
        $lastException = null;

        while ($attempt < $maxAttempts) {
            try {
                return $operation();
            } catch (\Throwable $e) {
                $lastException = $e;
                $attempt++;

                if ($attempt < $maxAttempts) {
                    $delay = $baseDelayMs * (2 ** ($attempt - 1));
                    $jitter = random_int(0, (int)($delay * 0.3));
                    usleep(($delay + $jitter) * 1000);
                }
            }
        }

        throw $lastException;
    }
}

Token Bucket Rate Limiter

<?php

declare(strict_types=1);

namespace Infrastructure\Resilience;

final class TokenBucketRateLimiter
{
    private float $tokens;
    private int $lastRefill;

    public function __construct(
        private readonly int $capacity,
        private readonly float $refillRate,
        private readonly \Redis $redis,
        private readonly string $key
    ) {
        $this->tokens = $capacity;
        $this->lastRefill = time();
    }

    public function attempt(): bool
    {
        $this->refill();

        if ($this->tokens >= 1) {
            $this->tokens--;
            return true;
        }

        return false;
    }

    private function refill(): void
    {
        $now = time();
        $elapsed = $now - $this->lastRefill;
        $this->tokens = min(
            $this->capacity,
            $this->tokens + ($elapsed * $this->refillRate)
        );
        $this->lastRefill = $now;
    }
}

Common Violations Quick Reference

Violation	Where to Look	Severity
No timeout on external calls	HTTP clients, DB queries	Critical
Retry without backoff	Retry implementations	Warning
No circuit breaker on external services	API clients, adapters	Critical
Unbounded connection pools	Database, HTTP pools	Warning
No fallback strategy	Circuit breaker usage	Warning
Retry non-idempotent operations	Command handlers	Critical
Rate limiting only in-memory	Multi-instance apps	Warning
No jitter in retry	High-concurrency systems	Warning

Detection Patterns

# Find resilience implementations
Glob: **/Resilience/**/*.php
Glob: **/CircuitBreaker/**/*.php
Grep: "CircuitBreaker|RateLimiter|Retry" --glob "**/*.php"

# Check for proper timeout usage
Grep: "CURLOPT_TIMEOUT|timeout|setTimeout" --glob "**/Http/**/*.php"

# Detect retry patterns
Grep: "retry|backoff|exponential" --glob "**/*.php"

# Find rate limiting
Grep: "RateLimiter|throttle|TokenBucket" --glob "**/*.php"

# Check for bulkhead patterns
Grep: "Semaphore|Bulkhead|maxConcurrent" --glob "**/*.php"

# Detect missing patterns
Grep: "->request\(|curl_exec|file_get_contents" --glob "**/Infrastructure/**/*.php"

Configuration Guidelines

Circuit Breaker Settings

Service Type	Failure Threshold	Open Timeout	Success Threshold
Critical API	3-5	30-60s	3-5
Background Job	5-10	60-120s	2-3
Internal Service	3-5	15-30s	2-3
Database	2-3	10-20s	1-2

Retry Configuration

Operation Type	Max Attempts	Base Delay	Max Delay
HTTP API Call	3	100ms	10s
Database Query	3	50ms	5s
Message Queue	5	1s	60s
File Operation	2	10ms	100ms

Rate Limiter Settings

Endpoint Type	Rate	Window	Burst
Public API	100/min	1 min	20
Authenticated API	1000/min	1 min	100
Admin API	10000/min	1 min	1000
Webhook	60/min	1 min	10

Integration Points

┌─────────────────────────────────────────────────────────────────────────────┐
│                    INFRASTRUCTURE LAYER                                      │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│   src/Infrastructure/                                                        │
│   ├── Resilience/                                                           │
│   │   ├── CircuitBreaker/                                                   │
│   │   │   ├── CircuitBreaker.php                                           │
│   │   │   ├── CircuitBreakerConfig.php                                     │
│   │   │   ├── CircuitBreakerRegistry.php                                   │
│   │   │   └── CircuitState.php                                             │
│   │   ├── Retry/                                                            │
│   │   │   ├── RetryExecutor.php                                            │
│   │   │   ├── RetryPolicy.php                                              │
│   │   │   └── BackoffStrategy.php                                          │
│   │   ├── RateLimiter/                                                      │
│   │   │   ├── RateLimiterInterface.php                                     │
│   │   │   ├── TokenBucketRateLimiter.php                                   │
│   │   │   └── SlidingWindowRateLimiter.php                                 │
│   │   └── Bulkhead/                                                         │
│   │       ├── BulkheadInterface.php                                        │
│   │       ├── SemaphoreBulkhead.php                                        │
│   │       └── BulkheadRegistry.php                                         │
│   │                                                                         │
│   ├── Http/                                                                  │
│   │   ├── ResilientHttpClient.php    ◀── Uses CircuitBreaker + Retry       │
│   │   └── Middleware/                                                       │
│   │       └── RateLimitMiddleware.php                                       │
│   │                                                                         │
│   └── Payment/                                                               │
│       └── PaymentGatewayAdapter.php  ◀── Uses CircuitBreaker + Bulkhead    │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

Advanced Resilience Patterns

Backpressure Mechanisms

Strategy	How It Works	PHP Example
Drop	Discard excess requests	Rate limiter returning 429
Buffer	Queue up to limit	RabbitMQ with max-length
Throttle	Slow down producer	Sleep between batch items
Reject	Refuse, signal overload	HTTP 503 Service Unavailable
Scale	Add more consumers	Auto-scaling worker pool

Graceful Degradation Levels

Level	Mode	What Happens
0	Full	All features active
1	Non-Critical Off	Recommendations, analytics disabled
2	Read-Only	Writes disabled, reads from cache
3	Static Fallback	Serve cached/static content only
4	Maintenance	System unavailable page

Adaptive Retry Jitter Algorithms

Algorithm	Formula	Best For
Full Jitter	random(0, base * 2^attempt)	High concurrency
Equal Jitter	half + random(0, half)	Balanced spread
Decorrelated	random(base, prev_delay * 3)	Sequential retries

Health Check Types

Check	Endpoint	Purpose
Liveness	/health/live	Process running (restart if fails)
Readiness	/health/ready	Can accept traffic (remove from LB)
Startup	/health/startup	Has initialized

Chaos Engineering Principles

Practice	Description
Steady state hypothesis	Define normal behavior metrics
Vary real-world events	Inject realistic failures
Run in production	Test real behavior
Minimize blast radius	Limit failure scope
Automate experiments	Continuous chaos testing

References

For detailed information, load these reference files:

• references/circuit-breaker.md — Circuit Breaker implementation details
• references/retry-patterns.md — Retry strategies and backoff algorithms
• references/rate-limiting.md — Rate limiting algorithms and configurations
• references/bulkhead.md — Bulkhead isolation patterns
• references/advanced-resilience.md — Backpressure mechanisms, graceful degradation, adaptive retry with jitter, chaos engineering, health-based routing, fallback strategies

Assets

• assets/report-template.md — Structured audit report template