๐ง The Ultimate System Design Cheat Sheet (2025 Edition) ๐ Stop memorizing system design questions. Start mastering the concepts that power every scalable system. Here are the core pillars of modern backend design ๐ โ๏ธ A. Data Distribution & Scalability Sharding / Partitioning โ break massive data across DB nodes. Consistent Hashing โ distribute keys evenly, handle node changes smoothly. Replication โ ensure fault tolerance with data copies. LeaderโFollower Architecture โ manage consistency in writes. Load Balancing โ smart traffic routing across servers. โก B. Caching & Performance Caching Strategies โ LRU / LFU / TTL. CDN (Content Delivery Network) โ global edge delivery. App-Level Cache โ Redis / Memcached. Cache Invalidation โ write-through / write-back / write-around. Compression & Serialization โ reduce latency & bandwidth. ๐งพ C. Messaging & Asynchronous Systems Message Queues โ Kafka, RabbitMQ, SQS. Pub/Sub Pattern โ async notifications. Backpressure & Flow Control โ balance producers & consumers. Dead-Letter Queues โ capture failures safely. Delivery Guarantees โ at-least-once / exactly-once. ๐งฎ D. Storage & Indexing Object Storage (S3 Model) โ separate data & metadata. Chunking & Resumable Uploads โ handle large files. Time-Series DBs โ for metrics. Geospatial Indexing โ Geohash, QuadTree. Search Indexing โ Trie, Inverted Index. ๐ฐ E. Payments, Transactions & Ledgers Idempotency Keys โ avoid double payments. Double-Entry Ledger โ track debits & credits. State Machine โ initiated โ success/failure. Reconciliation Jobs โ sync internal & external records. ๐งฉ F. Real-Time Systems WebSockets / Long Polling โ live communication. Real-Time Analytics โ Flink, Spark Streaming. Leaderboards โ Redis Sorted Sets. Adaptive Bitrate (ABR) โ dynamic video streaming. ๐ก๏ธ G. Reliability & Fault Tolerance Idempotent Operations โ safe retries. Retry with Backoff โ avoid downstream overloads. Circuit Breakers โ stop cascading failures. Replication + Quorum โ ensure availability. Consensus (Raft, Paxos) โ elect leaders safely. ๐ง H. Algorithms for Scale Rate Limiting โ Token / Leaky Bucket. Priority Queues โ for scheduling. Bloom Filters โ fast membership checks. Tries โ autocomplete & search. Ranking Functions โ personalized feeds. ๐ I. Security & Access Control OAuth 2.0, JWT โ user identity & sessions. Encryption โ at-rest & in-transit. Tokenization โ protect card data. Abuse Prevention โ rate limits, auth walls. ๐ J. Observability & Monitoring Metrics โ Prometheus, Grafana. Logging & Tracing โ ELK, OpenTelemetry. Alerting Rules โ real-time failure detection. Health Checks & Circuit Breakers โ resilience. ๐งฐ K. Patterns & Trade-offs CAP Theorem โ Consistency vs Availability. CQRS + Event Sourcing โ separate read & write models. Eventual Consistency โ async updates at scale. Backpressure & Throttling โ control system load. Horizontal Scaling โ scale out, not up. #SystemDesign #BackendDevelopment #SoftwareEngineering #DistributedSystems