Okay, let's discuss 2.5.b Fault Tolerance.

• Definition: Fault tolerance is the property that enables a system to continue operating properly, potentially at a reduced level (graceful degradation), in the event of the failure of one or more of its components. It's about designing systems that can withstand and recover from failures without causing a complete outage.

• Goal: To increase availability and reliability by ensuring that component failures do not necessarily lead to system failure.

• Fault Tolerance vs. Redundancy:

  • Redundancy is a technique (having duplicate components).
  • Fault Tolerance is the system property achieved, often through redundancy, but also involving mechanisms for detecting failures and managing the switchover or recovery process. A system can have redundant components but still not be truly fault-tolerant if it can't automatically detect failures and switch to the backups effectively.

• Techniques and Principles for Achieving Fault Tolerance:

  1. Redundancy: (As previously discussed) Duplicating hardware, software instances, data (replication), network paths, and even entire data centers/availability zones is the foundation.

  2. Failure Detection: Implementing mechanisms to quickly detect when a component has failed.

     • Health Checks: Load balancers or monitoring systems periodically pinging services to check their status.
     • Heartbeats: Components periodically sending "I'm alive" signals to each other or a central monitor.
     • Monitoring Metrics: Tracking error rates, latency, resource usage to identify abnormal behavior.

  3. Failover Mechanisms: Automatic processes to switch system operation from a failed component to a redundant one.

     • Load Balancer Failover: Automatically removing failed instances from the server pool.
     • Database Failover: Promoting a replica database to become the new primary database.
     • DNS Failover: Updating DNS records to point to a healthy IP address or data center if the primary becomes unavailable.

  4. Isolation / Bulkheading: Preventing failures in one part of the system from cascading and affecting other parts.

     • Microservices: The architecture itself promotes isolation.
     • Resource Pooling: Limiting resources (connections, threads) used by calls to specific components so their failure doesn't exhaust resources for others.
     • Circuit Breaker Pattern: (Covered next) Prevents repeated calls to a failing service.

  5. Graceful Degradation: Designing the system to maintain essential functionality even when some non-critical components are unavailable or underperforming.

     • Example: An e-commerce site might disable personalized recommendations if the recommendation service fails but keep core search and checkout functions operational.

  6. Statelessness: Designing services to be stateless makes fault tolerance easier, as failed requests can simply be retried on any available healthy instance without loss of context.

  7. Error Handling and Retries: Implementing smart error handling and retry logic (often with exponential backoff) can handle transient network issues or temporary component unavailability.

• In an Interview: Fault tolerance is crucial for demonstrating you can design robust systems. When discussing your design:

  • Point out potential single points of failure and how you mitigate them (usually with redundancy).
  • Explain how failures would be detected (e.g., load balancer health checks).
  • Describe the failover process (e.g., LB stops sending traffic, replica DB promoted).
  • Mention specific patterns like Circuit Breakers or considering graceful degradation if relevant to the system's complexity.
  • Connect these techniques back to the goal of achieving high availability.