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🛡️ System Design: Reliability & Resiliency Guide

Designing for reliability and resiliency ensures your system stays available, consistent, and correct—even when components fail.

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📘 1. Definitions

• Reliability: The ability of a system to function correctly and consistently over time.
• Resiliency: The ability of a system to recover quickly from failures and continue operating.

💡 A reliable system minimizes downtime. A resilient system recovers gracefully.

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🧰 2. Core Strategies

✅ Redundancy

• Duplicate components to avoid single points of failure
• Examples:
  • Multiple app instances
  • DB replication
  • Multi-region deployment

⚖️ Load Balancing

• Distribute traffic to healthy services
• Automatically redirect if a node goes down

💬 Graceful Degradation

• Prioritize core functionality during partial failures
• Example: Show cached data if DB is slow

🔁 Retries with Backoff

• Retry transient failures with exponential backoff
• Add jitter to avoid thundering herd

retryCount = 0
while retryCount < maxRetries:
    wait = 2 ** retryCount + random_jitter()
    sleep(wait)
    retryCount += 1

🔒 Circuit Breaker

• Avoid repeated calls to failing services
• Example tools: Netflix Hystrix, Resilience4j

🪝 Health Checks

• Liveness and readiness probes
• Useful for auto-recovery and autoscaling

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🔄 3. Data Resiliency Techniques

🧬 Replication

• Real-time copies of data across nodes or regions
• Synchronous (strong consistency) vs. Asynchronous (eventual)

⛓️ Event Sourcing

• Record all state changes as events
• Enables replaying to restore state

💾 Backups

• Automate frequent backups
• Verify recovery process regularly

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📉 4. Failure Handling Patterns

🌐 Bulkheads

• Isolate failures to prevent cascading
• Example: Separate thread pools or service instances per feature

📦 Queue-Based Decoupling

• Use queues to absorb load spikes and smooth traffic
• Helps when downstream services are slow or fail

⏳ Timeouts

• Fail fast instead of hanging indefinitely
• Set appropriate timeouts per service

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📊 5. Monitoring & Alerting

• Track availability, errors, latency, and saturation (Google's Four Golden Signals)
• Use observability tools: Prometheus, Grafana, Datadog, Sentry
• Alert on symptoms, not causes

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🧪 6. Chaos Engineering

• Introduce failures in controlled ways to test resilience
• Tools: Gremlin, Chaos Monkey, LitmusChaos

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🧩 7. Common Tools & Libraries

Area	Tools / Frameworks
Circuit Breakers	Hystrix, Resilience4j
Retries & Backoff	Polly (.NET), Tenacity (Python)
Load Balancing	HAProxy, Envoy, NGINX, Istio
Chaos Engineering	Gremlin, Chaos Monkey, LitmusChaos
Observability	Prometheus, Grafana, ELK, Sentry

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📋 8. Reliability/Resilience Design Checklist

• [ ] Is every component redundant or recoverable?
• [ ] Are retries implemented with backoff and limits?
• [ ] Are circuit breakers and timeouts in place?
• [ ] Do we gracefully degrade during failures?
• [ ] Is observability set up for all critical paths?
• [ ] Are backups regularly tested for restoration?
• [ ] Have we performed chaos testing?

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