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Fault Tolerance and System Resilience Questions

Designing systems to anticipate, tolerate, contain, and recover from component and network failures while minimizing customer impact and preserving correctness. Topics include identifying common failure modes and single points of failure, redundancy and isolation patterns at hardware, service, and geographic levels, and failover strategies including active active and active passive. Cover retry policies with exponential backoff, timeouts, circuit breaker and bulkhead patterns, graceful degradation, rate limiting, and backpressure techniques to protect systems during overload. Discuss orchestration of node rejoin and state rebuild, replication strategies and consistency trade offs, leader election and consensus implications, and techniques to avoid and mitigate split brain. Explain monitoring, health checks, alerting, and metrics such as mean time to recovery and mean time between failures to guide operational improvements. Include testing for resilience through chaos engineering and fault injection, handling flaky components in test environments, analysis of past failures and refactoring for resiliency, and operational practices that reduce blast radius and speed recovery.

MediumSystem Design
80 practiced
You run a stateful session service that stores user sessions in-memory for low latency behind a router. Traffic is 30k req/s. Design a failover strategy to preserve sessions across instance failures with minimal latency impact. Consider sticky sessions, active-passive replication, shared storage options, consistency vs performance trade-offs, and automation for failover and recovery.
HardSystem Design
85 practiced
Design a resilience strategy for a payment processing stack that must maintain 99.99% availability under heavy traffic spikes and protect the core processing path from a slow fraud-detection service. Include per-merchant bulkheads, throttling tiers, prioritized queues, offline-safe fallbacks, capacity planning, and test approaches to validate the strategy.
HardSystem Design
66 practiced
Design an active-active, multi-region user profile service for 100M users with global low-latency reads (<50ms) and strong read-after-write consistency for updates within a user's primary region. Discuss data replication choices (synchronous vs asynchronous), conflict resolution, routing and locality, RPO/RTO targets, and graceful degradation strategies when a region is partitioned.
MediumTechnical
71 practiced
A downstream payment validation service has become slow, causing upstream request queues to grow and increasing end-to-end latency. Propose immediate mitigations and long-term fixes. Consider timeouts, retry strategies, bulkheads, backpressure, queueing, circuit breakers, and capacity planning. For each measure, explain why it helps and what new trade-offs it introduces.
MediumTechnical
80 practiced
Design client retry policies to avoid a thundering-herd when a service recovers. Include exponential backoff with jitter, randomized initial delays, progressive ramp-up windows, use of server-provided 'retry-after' signals, and coordination strategies (client-local vs server-coordinated). Explain how to avoid making recovery worse and how to enforce reasonable client behavior.

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