What types of interactions between microservices exist?

• Synchronous interaction (Request/Response): ServiceB waits for a response from ServiceA before continuing execution.

  • HTTP/REST
  • gRPC

• Asynchronous interaction (Event-driven): ServiceA sends a message or event to a message broker, and ServiceB subscribes to these events and processes them independently.

  • Message brokers (Kafka, RabbitMQ, ActiveMQ)
  • Message queues (SQS, Azure Service Bus)

• Mixed interaction: A combination of synchronous and asynchronous approaches. For example, synchronous requests for data retrieval and asynchronous events for change notifications.

Comparison table:

Type of interaction	Advantages	Disadvantages
Synchronous	Implementation simplicity, predictable data flow	High coupling, blocking, cascading failures
Asynchronous	Low coupling, scalability, resilience	Debugging complexity, eventual consistency, state management

Example of asynchronous interaction (pseudo-code):

<?php

// Service A (Publisher)
class OrderService
{
    private MessageBroker $broker;

    public function __construct(MessageBroker $broker)
    {
        $this->broker = $broker;
    }

    public function placeOrder(array $orderData): void
    {
        // Order processing...
        echo "Order placed.";

        // Publish event
        $this->broker->publish('order.placed', json_encode($orderData));
    }
}

// Service B (Subscriber)
class ShippingService
{
    private MessageBroker $broker;

    public function __construct(MessageBroker $broker)
    {
        $this->broker = $broker;
        $this->broker->subscribe('order.placed', [$this, 'handleOrderPlaced']);
    }

    public function handleOrderPlaced(string $eventPayload): void
    {
        $orderData = json_decode($eventPayload, true);
        // Handle event: initiate delivery
        echo "Shipping order for customer {$orderData['customer_id']}.";
    }
}

// Usage example (simplified)
// $broker = new RabbitMQBroker(); // Or another broker
// $orderService = new OrderService($broker);
// $shippingService = new ShippingService($broker);
//
// $orderService->placeOrder(['customer_id' => 123, 'items' => [...]]);