Microservice - Communication

Sure bro, I’ll explain it in a simple and clear way, like we're just talking casually.
Let’s go step-by-step.

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⭐ How Microservices Communicate With Each Other (Detailed Explanation)

Microservices usually don’t run as one big system.
They are separate small services, running in different servers/containers, and they need to talk to each other.

Communication happens in two ways:

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🔵 1. Synchronous Communication

(Real-time, blocking — “I ask now, you answer now”)

Example:
Service A → waits → Service B responds → then A continues.

Common Protocols/Methods:

a) REST API (HTTP calls)

• Most common

• Simple

• Uses HTTP + JSON

• Example:

  • Order Service calls Payment Service’s API:
    POST /payment/process

b) gRPC

• Faster than REST

• Uses binary protocol

• Uses HTTP/2

• Best for internal communication

• Requires .proto files

c) GraphQL

• Not very common between services, but possible

• One service queries another for custom data

✔ Advantages

• Simple and easy

• Widely supported

❌ Disadvantages

• Service A is dependent on Service B

• If B is slow/down → A also becomes slow/down

• High latency

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🔵 2. Asynchronous Communication

(Not real-time — “I send a message, you process whenever you want”)

Service A → sends message → doesn’t wait → continues
Service B → processes message later

This uses message brokers.

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🔥 Common Messaging Systems

a) Apache Kafka

• Distributed message streaming platform

• High performance

• Used for large-scale event-driven systems

• Services publish events → others subscribe

b) RabbitMQ

• Message queue

• Uses AMQP protocol

• Reliable, supports acknowledgement

• Good for task queues

c) AWS SQS / SNS

• Cloud-based

• Serverless

• Very reliable and scalable

d) Azure Service Bus / Google Pub-Sub

• Cloud message brokers

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✔ Where Asynchronous is usually used?

• Sending emails

• Notifications

• Payment confirmation

• Event-driven architectures

Example:

Order Service → sends event to Kafka → Payment Service receives → Inventory Service receives
All services remain loosely coupled.

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⭐ Types of Communication Patterns

1. Request–Response (Sync)

A → B → Response → A continues

2. Fire-and-Forget (Async)

A → sends message → doesn't care about response

3. Event-Driven

A → publishes event
Many services → subscribe

4. Publish–Subscribe (Pub/Sub)

One message → multiple listeners
(Kafka, SNS, etc.)

5. Message Queue

Messages are stored in queue → consumers process them
(RabbitMQ, SQS)

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⭐ Service Discovery

Since microservices run on multiple servers with dynamic IPs:

Systems used:

• Eureka (Netflix)

• Consul

• Zookeeper

• Kubernetes Service Discovery

They help one service find another automatically.

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⭐ API Gateway

All external users talk to one entry point (API Gateway)

It handles:

• Routing

• Authentication

• Load balancing

• Rate limiting

• Logging

Examples:

• Kong

• NGINX

• AWS API Gateway

• Zuul

• Istio Ingress in Kubernetes

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⭐ Circuit Breaker Pattern

If Service B is slow/failed:

Service A uses circuit breaker to:

• Stop calling B temporarily

• Return fallback response

• Avoid cascading failures

Libraries:

• Netflix Hystrix

• Resilience4j

• Istio retry policies

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⭐ Diagram (Simple)

        ┌──────────────┐       Sync REST       ┌──────────────┐
        │ Order Service ├──────────────────────► Payment Serv. │
        └───────▲──────┘                        └──────▲──────┘
                │ Async Event (Kafka)                  │
                └──────────────────────────────────────┘

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⭐ Summary (In Style You Prefer 😄)

Bro, microservices talk to each other mainly in two styles:

🔹 Sync (REST, gRPC)

Like calling your friend on phone and waiting for reply.

🔹 Async (Kafka, RabbitMQ)

Like sending a WhatsApp message — reply comes later.

Microservices also need:

• API Gateway

• Service Discovery

• Circuit Breakers

• Load Balancers

All of these ensure a scalable, fault-tolerant, loosely coupled system.

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If you want, I can also explain:
✔ With real project example
✔ With code (Java + Spring Boot)
✔ With diagrams
✔ When to use sync vs async

Just tell me, bro!