What is Inter-service Communication?

How one microservice talks to another to:

• Fetch data

• Trigger actions

• Share events

There are two main approaches:

1. Synchronous (REST/HTTP)

2. Asynchronous (Messaging – Kafka/RabbitMQ)

🔹 1️⃣ REST (Synchronous Communication)

How it works

• Service A calls Service B using HTTP

• Service A waits for response

📌 Example:

Characteristics

• Request/response model

• Blocking call

• Tight runtime dependency

Pros

✅ Simple to implement
✅ Easy to debug
✅ Immediate response
✅ Good for CRUD operations

Cons

❌ Service A fails if Service B is down
❌ Higher latency under load
❌ Causes cascading failures

When to use REST?

✔ Real-time queries
✔ Simple data fetch
✔ Low-latency operations

🔹 2️⃣ Messaging (Asynchronous Communication)

How it works

• Service publishes a message/event

• Consumer processes it later

• No direct dependency

📌 Example:

Characteristics

• Event-driven

• Non-blocking

• Loosely coupled

Pros

✅ High scalability
✅ Fault tolerance
✅ Better performance under load
✅ No cascading failures

Cons

❌ Eventual consistency
❌ Harder debugging
❌ Message ordering complexity

When to use Messaging?

✔ Notifications
✔ Payments
✔ Auditing
✔ Event-driven workflows

🔹 REST vs Messaging (Quick Comparison)

🔹 Real-World Example (Best Practice)

🛒 E-commerce App

• REST → Get order details

• Kafka → Order placed → Payment → Email → Inventory update