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
Glob: **/Resilience/**/*.php
Glob: **/CircuitBreaker/**/*.php
Grep: "CircuitBreaker|RateLimiter|Retry" --glob "**/*.php"
Grep: "CURLOPT_TIMEOUT|timeout|setTimeout" --glob "**/Http/**/*.php"
Grep: "retry|backoff|exponential" --glob "**/*.php"
Grep: "RateLimiter|throttle|TokenBucket" --glob "**/*.php"
Grep: "Semaphore|Bulkhead|maxConcurrent" --glob "**/*.php"
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 detailsreferences/retry-patterns.md — Retry strategies and backoff algorithmsreferences/rate-limiting.md — Rate limiting algorithms and configurationsreferences/bulkhead.md — Bulkhead isolation patternsreferences/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