As a Principal Backend Engineer with over 12 years of experience, I can tell you quite certainly that if you're still getting rejections in system design interviews after good efforts, I think your fundamentals are not strong... Dedicate 2-3 months to mastering these design fundamentals, then practice designing a few systems(and do plenty of mock interviews). Scaling & Architecture ↬ CDN ↬ Caching ↬ Sharding ↬ Queueing ↬ Replication ↬ Partitioning ↬ API Gateway ↬ Rate Limiting ↬ CAP Theorem ↬ Microservices ↬ Load Balancing ↬ Fault Tolerance ↬ Database Scaling ↬ Service Discovery ↬ Consistency Models ↬ Eventual Consistency ↬ Distributed Transactions ↬ Monolith vs Microservices ↬ Leader Election Databases & Storage ↬ Leader-Follower Replication ↬ WAL (Write Ahead Log) ↬ Asynchronous Processing ↬ Transaction Isolation ↬ Read/Write Patterns ↬ Consistent Hashing ↬ Redis/Memcached ↬ Backup & Restore ↬ Hot/Cold Storage ↬ Data Partitioning ↬ Object Storage ↬ SQL vs NoSQL ↬ Data Retention ↬ Data Modeling ↬ OLAP vs OLTP ↬ ACID & BASE ↬ Bloom Filters ↬ File Systems ↬ S3 Basics ↬ B+ Trees ↬ Indexing Communication & APIs ↬ JWT ↬ CORS ↬ OAuth ↬ Throttling ↬ Serialization ↬ API Security ↬ Long Polling ↬ WebSockets ↬ API Gateway ↬ Idempotency ↬ Service Mesh ↬ Retry Patterns ↬ REST vs gRPC ↬ API Versioning ↬ Circuit Breaker ↬ API Rate Limits ↬ Fan-out/Fan-in ↬ Protocol Buffers ↬ Message Queues ↬ Dead Letter Queue Reliability & Observability ↬ Metrics ↬ Alerting ↬ Failover ↬ Logging ↬ Rollbacks ↬ Monitoring ↬ Heartbeats ↬ Retry Logic ↬ Autoscaling ↬ SLO/SLI/SLA ↬ Load Testing ↬ Error Budgets ↬ Health Checks ↬ Circuit Breaker ↬ Incident Response ↬ Chaos Engineering ↬ Distributed Tracing ↬ Canary Deployments ↬ Graceful Degradation ↬ Blue-Green Deployment