Microservices # Fault Tolerance

 Here is a clear, simple, interview-ready explanation of Fault Tolerance, especially useful for Java, Microservices, Cloud, and System Design interviews.

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⭐ What is Fault Tolerance?

Fault tolerance means a system continues to operate properly even when some part of it fails.

In other words:

➡️ System should not crash
➡️ System should keep giving correct output
➡️ Failures should be handled gracefully

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⭐ Real-Life Example

✔ Airplanes:
If one engine fails, the plane continues flying using the other.

✔ Microservices:
If one service fails, the API Gateway reroutes traffic to another instance.

✔ Netflix:
If a server fails, users never notice because traffic shifts automatically.

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⭐ Where We Use Fault Tolerance in Software

1️⃣ Microservices

• Retry mechanism

• Circuit breaker (Hystrix, Resilience4j)

• Load balancing

• Fallback service

2️⃣ Cloud Systems (AWS/Azure/GCP)

• Auto-scaling

• Multi-AZ deployment

• Rolling updates

• Health checks

3️⃣ Databases

• Replication

• Failover nodes

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⭐ Common Fault Tolerance Techniques

✔ 1. Retry Logic

If a request fails due to network issues, try again automatically.

✔ 2. Circuit Breaker

Stop calling a failing service temporarily to avoid system crash.

✔ 3. Fallback Response

Return a default response when real service is down.

✔ 4. Load Balancing

Distribute requests across multiple servers.

✔ 5. Redundancy

Backup servers → if one fails, another takes over.

✔ 6. Failover

Automatic switch to a healthy instance when one instance fails.

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⭐ Fault Tolerance Example Using Resilience4j (Java)

@CircuitBreaker(name = "paymentService", fallbackMethod = "paymentFallback")
public String processPayment() {
    return restTemplate.getForObject("/pay", String.class);
}

public String paymentFallback(Exception ex) {
    return "Payment service is temporarily unavailable.";
}

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⭐ Fault Tolerance in System Design

A Highly Fault-Tolerant System Has:

• Multiple servers (replicas)

• Distributed architecture

• Auto-recovery

• Monitoring & alerts

• Stateless services

• Zero single-points-of-failure

Example diagram:

• Client
  ↓

• Load Balancer
  ↓

• Multiple App Servers (A, B, C)
  ↓

• Primary DB + Read Replica DB

If any one server goes down, system still works.

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⭐ Interview-Ready 2-Line Answer

Fault tolerance is the ability of a system to continue working even when some components fail.
Techniques include retries, circuit breakers, fallback, redundancy, failover, and load balancing.

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If you want, I can also explain:

👉 Fault Tolerance vs High Availability
👉 Fault Tolerance in Microservices architecture (diagram)
👉 Fault Tolerance in AWS (ELB, ASG, Multi-AZ)
👉 Fault Tolerance coding examples in Java

Just tell me!