3.1.1.2. Mitigating Single Points of Failure

💡 First Principle: A resilient architecture is achieved by systematically identifying and eliminating any single component whose failure would cause the entire system or a critical function to fail.

Scenario: An architect is reviewing an existing application that runs on a single "EC2 instance" and uses an "Amazon RDS" database deployed in a single "Availability Zone (AZ)". The architect identifies both as single points of failure.

A robust architecture actively seeks out and mitigates Single Points of Failure ("SPOFs") at every layer.

• Compute Layer:
  • "SPOF": Single "EC2 instance", single "ECS task", single "Lambda" function deployment.
  • Mitigation: Deploy across "Multi-AZs" using "Auto Scaling Groups" ("EC2"/"ECS"), or use managed services like "Lambda"/"Fargate" that inherently distribute resources. Use "Placement Groups" for specific isolation needs.
• Data Layer:
  • "SPOF": Single-"AZ" database (e.g., "RDS"), unreplicated data.
  • Mitigation: "RDS Multi-AZ", "Aurora Global Database", "DynamoDB Global Tables", "S3 Cross-Region Replication", "EBS Snapshots". Ensure data is always replicated and backed up.
• Networking Layer:
  • "SPOF": Single "Internet Gateway", single "NAT Gateway" in an "AZ", reliance on a single "Direct Connect" link.
  • Mitigation: "IGWs" are inherently redundant. Deploy "NAT Gateways" in each "AZ" where private subnets need outbound internet access. For "Direct Connect", implement multiple links over diverse paths and multiple locations, or use "VPN" as backup.
• Application Layer:
  • "SPOF": Hardcoded endpoints, shared libraries on a single server, tightly coupled services.
  • Mitigation: Use load balancers ("ALB", "NLB") for traffic distribution, implement service discovery (e.g., "Cloud Map"), design for loose coupling with message queues ("SQS") or event buses ("EventBridge"), use immutable infrastructure.

Visual: Mitigating Single Points of Failure (SPOFs)

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⚠️ Common Pitfall: Overlooking dependencies on services in a single "AZ". For example, if all your "EC2 instances" rely on a single "NAT Gateway" in one "AZ" for internet access, the failure of that "AZ" will cut off internet connectivity for all instances, even those in other healthy "AZs".

Key Trade-Offs:

• Redundancy vs. Cost: Eliminating "SPOFs" often involves duplicating resources or infrastructure (e.g., "Multi-AZ" deployments), which increases the overall cost of the solution.

Reflection Question: How would you mitigate the single points of failure identified in this existing application (single "EC2 instance" and single-"AZ RDS" database) using AWS services like "Auto Scaling Groups" and "RDS Multi-AZ", to enhance the application's overall resilience without dramatically increasing cost?