Changelog

Track all the latest content additions and updates to Hello Interview. We continuously add new system design breakdowns, deep dives, and learning materials.

February 9 - 15, 2026

28 updates

New Videos

Fixed Length Sliding Window

Feb 11, 2026 by Shivam Chauhan | Content

Updated Content

Numbers to Know for System Design Interviews

Premium

Updated network bandwidth figures (50-100 Gbps), fixed SSD latency claim (sub-millisecond, not 10ms), corrected unit notation (KB/B vs kb/b), updated Aurora storage limit to 256 TiB, and refreshed all numbers for 2026.Feb 15, 2026 by Evan King | Deep Dive

Design a Ticket Booking Site Like Ticketmaster

Clarified seat map data model, added read-path complexity for distributed lock, explained Stripe payment flow with PCI compliance, added idempotency for webhooks, detailed virtual queue implementation, addressed OCC scalability limits, and fixed locking terminology.Feb 15, 2026 by Evan King | System Design

Design a File Storage Service Like Dropbox

Fixed SharedFiles composite key, added CompleteMultipartUpload step, corrected compression algorithm claims, clarified CDN signed URLs vs S3 presigned URLs, added content-defined chunking for delta sync, clarified download flow for chunked files, and improved hybrid sync explanation.Feb 15, 2026 by Evan King | System Design

Design a Coding Platform Like LeetCode

Fixed leaderboard SQL query to count distinct solved problems instead of all submissions, corrected tree serialization from 'inorder' to 'level-order (BFS)', updated deprecated Redis ZREVRANGE to ZRANGE REV, fixed DynamoDB partition key guidance, added cursor-based pagination, clarified container-worker communication flow, and fixed multiple typos.Feb 15, 2026 by Evan King | System Design

Design a Ride-Sharing Service Like Uber

Clarified Ride entity lifecycle, Redis geohash encoding details, service routing flow, and updated DynamoDB cost estimates.Feb 15, 2026 by Evan King | System Design

Design an Online Auction Platform Like eBay

Premium

Fixed SQL examples, corrected Redis/Kafka/SQS technical claims, and updated throughput calculations for consistencyFeb 15, 2026 by Evan King | System Design

Design a Distributed Job Scheduler Like Airflow

Premium

Corrected SQS claims about exponential backoff, throughput limits, and priority queue behavior; fixed DynamoDB partition capacity and hot partition claims; updated RabbitMQ HA and delayed message details; added write sharding guidance for Executions table.Feb 15, 2026 by Evan King | System Design

Design a Web Crawler

Corrected AWS bandwidth from 400 Gbps to 200 Gbps and updated machine count math. Added distributed crawl-delay locking with Redis. Clarified content vs URL deduplication, DNS bottleneck at scale, crawler trap depth definition, and SQS backoff as a pattern. Added priority crawling and HTTP HEAD deep dives.Feb 15, 2026 by Evan King | System Design

Design an Ad Click Aggregator

Clarified 100M clicks/day math, added OLAP vs Time Series DB comparison, explained why Flink over Kafka consumers, named Lambda architecture pattern, added Scaling Writes pattern callout, and fixed several factual inaccuracies.Feb 15, 2026 by Evan King | System Design

Kafka Deep Dive for System Design Interviews

Clarified offset ordering, replication semantics, delivery guarantees, and queue vs stream distinction. Updated CLI flags and KafkaJS code examples.Feb 15, 2026 by Evan King | Deep Dive

DynamoDB Deep Dive for System Design Interviews

Fixed GSI/LSI consistency claims, corrected partition throughput (3,000 RCU), rewrote CAP/consistency section to clarify per-request strong consistency, fixed replica configuration claims, corrected pricing math, updated DAX description, added PartiQL support, fixed hash partitioning terminology, and addressed 60+ community comments.Feb 15, 2026 by Evan King | Deep Dive

Design a Ticket Booking Site Like Ticketmaster

Fixed PostgreSQL lock timeout and Lucene claims, clarified Elasticsearch caching, added TTL race condition handling, and explained shared database design choice.Feb 14, 2026 by Evan King | System Design

Design a URL Shortener Like Bitly

Clarified Redis counter mechanics, multi-region deployment, URL expiration handling, and deduplication tradeoffs based on user feedback.Feb 14, 2026 by Evan King | System Design

Design a Web Crawler

Corrected SQS retry mechanism explanation (use ChangeMessageVisibility for exponential backoff, not built-in). Fixed DelaySeconds vs ChangeMessageVisibility confusion. Clarified RedisBloom module requirement for Bloom filters.Feb 14, 2026 by Evan King | System Design

Design an Ad Click Aggregator

Added explanations for hot shard re-aggregation, exactly-once semantics, and event time windowing.Feb 13, 2026 by Evan King | System Design

Design Facebook's Live Comments System

Added deep dives on mega-streams (CDN-based delivery, sampling) and client reconnection handling.Feb 13, 2026 by Evan King | System Design

Meta E4 Interview Guides & Questions (2026)