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Lessons (30)

• The L6 System-Design Thinking Loop

  … define correctness, size the load, design failure paths, and measure recovery. The big idea An L6 answer is …

• Design a Chat Application

  … presence and offline push paths](/img/system-design-l6/chat.svg) Persist a message before acknowledging it, then …

• Design a URL Shortener

  … durable mappings, cache-backed redirects and asynchronous analytics](/img/system-design-l6/url-shortener.svg) Generate a Base62 …

• Design a Ride-Sharing Service

  … and live updates flow separately](/img/system-design-l6/ride-sharing.svg) A driver must not be committed …

• Design a Polite Web Crawler

  … a URL frontier schedules polite fetches while deduplication and storage process discovered links](/img/system-design-l6/web …

• Design a File Storage Service

  … multipart uploads go to object storage; the API owns metadata, sharing and versioned changes](/img/system-design-l6 …

• Design an E-Commerce Checkout

  … transitions are owned and idempotent](/img/system-design-l6/ecommerce-checkout.svg) Catalog, search and recommendations can be …

• Design a Reliable Notification System

  … preferences feed priority queues, channel providers and idempotent delivery tracking](/img/system-design-l6/notifications.svg) Create an …

• Batch vs Stream and Data Modeling

  … controls read and write cost](/img/system-design-l6/batch-stream.svg) Choice Use when Cost --- --- --- Batch Reports …

• Design a Logging and Monitoring Platform

  … search, metrics and cold archival](/img/system-design-l6/telemetry-platform.svg) Agents batch and compress logs, metrics …

• Architecture Patterns at Scale

  … central services, independently deployed services, event functions and decentralized peers](/img/system-design-l6/architecture-patterns.svg) Style …

• Releases, Observability and Production Safety

  … rollback path limits blast radius](/img/system-design-l6/release-operations.svg) Tool Purpose --- --- Rolling rollout Replace instances …

• System Design Interview Framework

  … dive into risks and recovery](/img/system-design-l6/interview-framework.svg) 1. Clarify users, core flows, scope …

• Distributed Coordination: Consensus, Heartbeats and Gossip

  … leader agreement, heartbeats, gossip membership and checksums solve different failure questions](/img/system-design-l6/coordination.svg) Problem …

• Big Data: ETL, Warehouses and Streaming

  … raw events land durably, ETL/stream processing creates curated lake and warehouse datasets](/img/system-design-l6/big …

• API Contracts, Gateways and Security

  … services enforce resource-level policy](/img/system-design-l6/api-security.svg) Define resource/command contracts, error shapes …

• Proxies, APIs and Real-Time Protocols

  … reverse proxy at the edge, request APIs, queues and persistent real-time connections](/img/system-design-l6/protocols …

• Real-Time, Pub/Sub, Webhooks and CDC

  … updates and database change capture](/img/system-design-l6/realtime-events.svg) Pattern Best for Key rule --- --- --- SSE …

• Data Structures for Scale

  … spatial indexes locate nearby items; probabilistic sketches answer large-set questions cheaply](/img/system-design-l6/scale-data …

• Design a Social Media Feed

  … behind a cached timeline API](/img/system-design-l6/social-feed.svg) Posts are durable objects; the home …

• Capacity, Availability, Latency and CAP

  … a design chooses latency targets, availability targets and a partition-time consistency policy](/img/system-design-l6/capacity …

• Caching, Consistent Hashing and Sharding

  … and shards for database scale](/img/system-design-l6/data-distribution.svg) Tool Solves Risk --- --- --- Cache Repeated read …

• Networking, DNS and Traffic Routing

  … balancer choose a healthy service](/img/system-design-l6/network-routing.svg) The OSI model is a troubleshooting …

• Database Internals, Replication and Scaling

  … writes become durable log records, replicas serve compatible reads, and pools protect scarce connections](/img/system-design-l6 …

• Time, Consistency and Distributed Transactions

  … clocks order observations, fencing protects ownership, and sagas compensate cross-service work](/img/system-design-l6/time-transactions …

• Flash Sale: 10 Million Buyers, No Oversell

  … queueing controls traffic while the inventory owner atomically reserves the unit](/img/system-design-l6/flash-sale.svg …

• Timeouts, Retries & Backpressure

  … deadline budgets, limited concurrency, a queue and explicit overload shedding](/img/system-design-l6/backpressure.svg) One deadline …

• Debug Latency Like an Investigator

  … database waits and downstream services](/img/system-design-l6/latency-investigation.svg) Symptom Test first --- --- p50 and p99 …

• Exactly-Once Payment Processing

  … durable attempt, provider idempotency key, then recovery for an unknown result](/img/system-design-l6/payment-idempotency.svg …

• Premiere-Night Video Streaming

  … edge, regional and origin tiers](/img/system-design-l6/premiere-streaming.svg) Plane Handles Goal --- --- --- Control Auth, entitlement …