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Lessons (40)

• DBMS Foundations, Keys & Normalization

  Detailed interview answers for DBMS advantages, normalization, keys, abstraction and conceptual design. Covers PDF questions 1-5, 12 …

• Database Relations & Foreign Keys

  Model one-to-one, one-to-many and many-to-many data with keys, constraints and deletion rules …

• Topics, Partitions & Offsets

  … JSON.stringify({ type: "OrderPlaced" }) }, { key: "order-42", value: JSON.stringify({ type: "OrderPaid" }) }, // same partition, after Placed { key: "order …

• Normalization & Denormalization

  … no non-key depends on another non-key Rule: be in 2NF, and a non-key attribute must …

• Exactly-Once Payment Processing

  … Reject reuse of the key with a different amount. Send the same key to the provider: your database …

• Hash Maps & Sets

  … A hash function turns a key (like "apple" ) into a number (a bucket index), so the computer jumps …

• REST API Design

  … POST /payments, Idempotency-Key: abc123 S- DB: INSERT key abc123 (UNIQUE) → PROCESSING S- S: charge card 💳 S- DB …

• Caching & Redis

  … Hot key One key (a celebrity's profile) gets so much traffic it overloads a single Redis node …

• Replication, Partitioning & Sharding

  Scale reads with replicas, split data safely with partitions and shards, and plan for consistency, hot keys and …

• Caching, Consistent Hashing and Sharding

  … A hash ring moves only some keys when nodes change. Choose a stable shard key such as tenant …

• SQL vs NoSQL Databases

  … tables, rows, relationships sql CREATE TABLE customers ( id BIGSERIAL PRIMARY KEY, email TEXT NOT NULL UNIQUE, name TEXT …

• Structural Patterns: How to Connect Objects

  … loadMesh(name) }); // heavy return treeTypes.get(key); } // Light, unique data per object (extrinsic state) const forest = Array.from …

• React Fundamentals

  … Using the array index as the key causes bugs when items are reordered or removed. State: data that …

• Design a Chat Application

  … A client idempotency key makes reconnect/retry safe. Order messages by conversation key, not globally. Presence is temporary …

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

  … Include event ID, schema version, ordering key and trace context; consumers deduplicate and send exhausted failures to a …

• ConfigMaps, Secrets & Storage

  … shop-api-config, key: LOG LEVEL } a single key volumeMounts: - name: settings mountPath: /etc/shop → /etc/shop/app …

• Kafka Producers

  … CompressionTypes.GZIP, messages: [ { key: order.id, // keeps all events for this order in order value: JSON.stringify({ orderId …

• Flash Sale: 10 Million Buyers, No Oversell

  … A Redis Lua script can gate a hot key, but strict no-oversell still needs a durable owner …

• Scaling, Load Balancing & Rate Limiting

  … the key to scaling out If a server keeps user sessions in its own memory , the next request …

• Database Indexing & Transactions

  … Check pg stat user indexes and remove them. Index these: primary keys (automatic), foreign keys, columns in frequent …

• React Hooks in Practice

  … It's the main way to share stateful logic between components. jsx function useLocalStorage(key, initialValue) { const [value …

• Delivery Guarantees: At-Most, At-Least & Exactly Once

  … async ({ topic, partition, message }) = { const tx = await producer.transaction(); try { await tx.send({ topic: "ledger", messages: [{ key: message …

• Message Queues & RabbitMQ

  … using bindings . mermaid flowchart LR P[Producer] -- "routing key:<br/ order.created" X{{Exchange}} X -- binding order. Q1 …

• D — Dependency Inversion Principle

  … The key to swappable, testable code. The big idea Your lamp doesn't have a wire soldered directly …

• Design a URL Shortener

  … Store code, destination, owner, expiry and status with a unique key. Redirects read edge/cache then durable storage …

• Design an E-Commerce Checkout

  … reserve inventory atomically, create/get a payment attempt with an idempotency key, then commit an order and publish …

• Design a Reliable Notification System

  … notifications.svg) Create an immutable request with idempotency key, audience, channel, template version, priority and expiry. Enforce consent …

• How a Web Request Works

  … TLS handshake (certificate, keys) B- LB: GET /products/42 LB- S: forward to a healthy server S- DB …

• What Is Apache Kafka?

  … append an event const producer = kafka.producer(); await producer.connect(); await producer.send({ topic: "orders", messages: [{ key: "customer …

• Batch vs Stream and Data Modeling

  … must stay in sync Streams need an ordering key, idempotency, retention and DLQ. Batch jobs need isolation so …

• Deployments, ReplicaSets & Rollouts

  … how objects find each other Labels are key/value tags on objects. Selectors pick objects by label. This …

• Data Structures for Scale

  … distinct items? Approximate count Count-Min Sketch Which keys are frequent? Overestimation MinHash How similar are two sets …

• Design a Social Media Feed

  … storage, eventual consistency and safeguards for celebrity hot keys. ![Social feed: write a post once, then fan out …

• Database Internals, Replication and Scaling

  … relational, document, key-value, wide-column, graph, time-series, search and vector stores solve different queries.

• API Gateway, BFF & Service Discovery

  … ask at the desk, which checks their room key and sends the request to the right place. An …

• Serverless Architecture

  … getObject(record.s3.bucket.name, record.s3.object.key); const thumbnail = await sharp(image).resize(200).webp().toBuffer …

• Onion Architecture & Clean vs Hexagonal vs Onion

  … at the bottom that everything depends on, or at the edge where it depends on the domain. Key …

• Creational Patterns: How to Make Objects

  … config.method = method; return this; } header(key, value) { this. config.headers[key] = value; return this; } timeout(ms) { this …

• Sorting Algorithms

  … That matters when you sort by several keys. The simple O(n²) sorts Bubble sort: big items "bubble …

• Timeouts, Retries & Backpressure

  … temporary failure. Payment 450 ms Reconcile with idempotency key. Safety margin 150 ms Return before caller gives up …