What microservice patterns do you know?

• Service Discovery: Allows services to find each other. Implemented through a centralized registry (e.g., ZooKeeper, Consul, etcd).
• API Gateway: A single entry point for clients, routing requests to the appropriate services. Provides authentication, authorization, load balancing.
• Circuit Breaker: Prevents cascading failures by temporarily disabling calls to failing services.
• Bulkhead: Isolates resources of different services to prevent mutual impact during failures.
• Saga: Manages distributed transactions, ensuring data consistency across multiple services through a sequence of local transactions and compensating actions.
• Event Sourcing: Stores all data changes as a sequence of events, providing a complete history of states.
• CQRS: Separates read and write logic into different models, optimizing performance for each operation type.
• Database per Service: Each service has its own database to ensure independence and decentralization.
• Strangler Fig: Gradually replaces a monolithic application with microservices by redirecting parts of the traffic.

Example implementation of Circuit Breaker in PHP (pseudo-code):

<?php
// Circuit Breaker class
class CircuitBreaker
{
    private $threshold; // Failure threshold
    private $timeout; // Time the breaker remains open
    private $failures = 0; // Current failure count
    private $lastFailureTime = 0; // Last failure time
    private $state = 'CLOSED'; // Current state: CLOSED, OPEN, HALF_OPEN

    public function __construct(int $threshold, int $timeout)
    {
        $this->threshold = $threshold;
        $this->timeout = $timeout;
    }

    public function execute(callable $operation)
    {
        switch ($this->state) {
            case 'OPEN':
                if (time() - $this->lastFailureTime > $this->timeout) {
                    $this->state = 'HALF_OPEN';
                    // Attempt to execute in HALF_OPEN state
                    try {
                        $result = $operation();
                        $this->reset(); // Success, reset
                        return $result;
                    } catch (\Exception $e) {
                        $this->recordFailure(); // Failure in HALF_OPEN, go back to OPEN
                        throw $e;
                    }
                } else {
                    throw new \Exception("Circuit breaker is open"); // Breaker is open
                }
                break;
            case 'HALF_OPEN':
                // Logic for HALF_OPEN already above
                break;
            case 'CLOSED':
            default:
                try {
                    $result = $operation();
                    $this->reset(); // Success, reset
                    return $result;
                } catch (\Exception $e) {
                    $this->recordFailure(); // Failure, increment counter
                    throw $e;
                }
                break;
        }
    }

    private function recordFailure(): void
    {
        $this->failures++;
        $this->lastFailureTime = time();
        if ($this->failures >= $this->threshold) {
            $this->state = 'OPEN'; // Transition to OPEN on threshold
        }
    }

    private function reset(): void
    {
        $this->failures = 0;
        $this->lastFailureTime = 0;
        $this->state = 'CLOSED'; // Transition to CLOSED on success
    }
}

// Usage
$breaker = new CircuitBreaker(3, 60); // Threshold 3 failures, timeout 60 seconds

try {
    $data = $breaker->execute(function() {
        // External service call code
        if (rand(0, 1) == 0) { // Simulate failure
             throw new \Exception("Service unavailable");
        }
        return ['status' => 'ok'];
    });
    echo "Success: " . json_encode($data) . "\n";
} catch (\Exception $e) {
    echo "Failure: " . $e->getMessage() . "\n";
}