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Lessons (40)

• 500 Users Upload Documents at Once: Design the Background Job Pipeline

  … Each takes about 60 seconds of worker time, which is 30,000 worker-seconds of work. With 50 …

• Message Queues & RabbitMQ

  … each message → ONE consumer"] Pr1[Producer] -- Q1[(jobs)] -- C1[Worker A] Q1 -- C2[Worker B] end subgraph PS …

• Kubernetes Cluster Architecture, Simply

  … the provider operates and backs up the control plane. Worker nodes: where your app runs Each worker node …

• AWS Messaging: Queues, Topics and Events

  … A message lets checkout save the order quickly and hand the email work to another worker. mermaid flowchart …

• Async JavaScript: Promises, async/await and Node's Event Loop

  … data }); worker.once("message", resolve); worker.once("error", reject); }); } Async iteration and streams js import { createReadStream } from "node …

• Go Concurrency: Goroutines, Channels and Context

  Concurrency vs parallelism, goroutines and the scheduler, buffered and unbuffered channels, select, WaitGroup, Mutex and atomics, worker pools …

• Design a Polite Web Crawler

  … Its lease expires and another worker repeats the task; durable identity makes reprocessing safe. A 429 response backs …

• Time, Consistency and Distributed Transactions

  … Their wall clocks differ, and a paused worker resumes after its lease expires. What must be true Define …

• Concurrency, Resilience & Distributed Workflows

  Keep backend systems correct under contention, failure and traffic spikes with idempotency, retries, circuit breakers, workers, webhooks, backpressure …

• Design a Reliable Notification System

  … Then workers send through email, push or SMS providers. Provider acceptance is not proof a person read it …

• Distributed Coordination: Consensus, Heartbeats and Gossip

  … It pauses for 30 seconds; its lease expires and Worker B receives a new lease. If A wakes …

• C4 Architecture Diagrams: Context, Container, Component and Code

  … a web app, API, database, queue, worker, or mobile app. Think of a shopping mall: - The context diagram …

• Flash Sale: 10 Million Buyers, No Oversell

  … The expiry worker must coordinate with the payment state and reconcile an ambiguous charge. Blindly returning this unit …

• Resilient Service Calls: Retries, Breakers and Load Shedding

  … Bound concurrency so a slow email provider cannot occupy every request worker. Then decide whether the work can …

• Payment Retries: Prevent Duplicate Charges

  … Store UNKNOWN , query the provider, and reconcile in a worker. Step-by-step flow 1. The browser creates …

• Count, for_each, Functions, and Stable Identities

  … Add worker = { port = 9000 } ; plan should add one file without changing the other identities. Why list indexes can …

• The Hot SKU Alert Problem

  … Use bounded worker concurrency, per-provider rate limits, channel-specific queues, circuit breakers and backpressure. Separate the work …

• Design a Social Media Feed

  … Precompute inbox entries for ordinary authors using bounded workers. 2. For high-fan-out authors, merge recent posts …

• Python Web APIs: FastAPI and Django

  … several workers don't share memory Key takeaways - FastAPI : type hints + Pydantic = validation and OpenAPI docs for free …

• Timeouts, Retries & Backpressure

  … During an incident it takes three seconds; retries occupy every worker and healthy endpoints become slow. What must …

• Design an E-Commerce Checkout

  … A workflow worker resumes from durable state after the API restarts. How to prove it worked Crash at …

• Event-Driven Patterns, Step Functions and Reliable Lambdas

  … Interrupt your worker after the provider stores success but before the worker stores its result. Retry the event …

• Containers at Scale: ECS Deep Dive and EKS

  … Scheduled Scale for known peaks Queue-based Scale workers on SQS backlog per task For EC2 launch type …

• GitOps, Progressive Delivery and Observability

  … sh/v1alpha1 kind: ScaledObject metadata: name: worker spec: scaleTargetRef: name: worker minReplicaCount: 0 maxReplicaCount: 50 triggers: - type: aws …

• 10,000 Users Do Not Need 10,000 Database Connections

  … 10 × 20 = 200 before admin sessions or workers; doubling instances would request 400. 2. Estimate active query demand …

• IAM: Who Can Do What

  … an invoice-export worker The worker reads database rows and writes objects under exports/ in one bucket. It …

• JavaScript Essentials: Event Loop, Closures & this

  … Split heavy work into chunks, or move it to a Web Worker . async/await is promises underneath js …

• Serverless Architecture

  … AWS Lambda, Google Cloud Functions, Azure Functions, Cloudflare Workers, Vercel Functions BaaS (Backend as a Service) Managed building …

• Frontend Performance & Core Web Vitals

  … ship less JS, break up long tasks, use Web Workers and transitions. - CLS: reserve space with width/height …

• Practical Project: Deploy a Real CRM App on EKS

  … code change → new versioned image → ECR push → EKS rollout. Never edit code directly on a worker node or …

• CloudWatch: Know What Your App Is Doing

  … Log a stable request/correlation ID across the API and worker so you can follow the same business …

• Backend Design Scenarios

  … Use provider-specific rate limits, bounded worker concurrency, connection reuse, backoff with jitter and a dead-letter queue …

• Design a File Storage Service

  … A worker then scans the object, verifies size/type, creates derivatives, and marks it READY . Downloads check the …

• Storage, Search and Location Queries

  … upload directly Storage - scanner/worker: validate, scan, resize API/database: mark file usable only after validation The API …

• Design a Ride-Sharing Service

  … Use a version or fencing token so an old timeout worker cannot later release a newer assignment by …

• L6 Production Debugging Casebook

  … After every old application version and long-running worker is gone, stop writing old column . 6. In a …

• Design a Chat Application

  … database - MessageAccepted event - online recipient gateway - offline push worker The client sends client message id , for example phoneA …

• Message Delivery: Queues, Kafka, Ordering and DLQs

  … always Kafka A work queue usually means "one worker should handle this job." Kafka is a durable, partitioned …

• AWS Interview Questions: Clear Answers

  … If the worker fails first, it can become visible again, so side effects must tolerate retries. Example: generate …

• Comments & Formatting

  … Call only from the main worker. Legal / licence // Copyright 2026 … TODO with owner // TODO(ana): remove after v2 …