1. Scalability concepts (vertical vs horizontal scaling)

• https://www.geeksforgeeks.org/system-design/system-design-horizontal-and-vertical-scaling/ (clear pros/cons table)
• https://www.digitalocean.com/resources/articles/horizontal-scaling-vs-vertical-scaling (practical cloud examples)

2. Load balancing & reverse proxy concepts

• https://dev.to/rajrathod/load-balancers-and-reverse-proxy-system-design-k51 (algorithms + diagrams)
• https://www.geeksforgeeks.org/system-design/reverse-proxy-vs-load-balancer/ (differences table)
• System Design School: https://systemdesignschool.io/blog/reverse-proxy-vs-load-balancer

3. Caching fundamentals

• https://www.designgurus.io/blog/caching-system-design-interview (strategies + invalidation)
• https://igotanoffer.com/blogs/tech/caching-system-design-interview
• https://www.hellointerview.com/learn/system-design/core-concepts/caching (interview-focused)
• https://systemdesignschool.io/fundamentals/caching (eviction policies + visuals)

4. Databases basics (ACID, replication, sharding, partitioning)

• https://www.geeksforgeeks.org/system-design/database-sharding-a-system-design-concept/ (ACID recap + strategies)
• https://planetscale.com/blog/sharding-vs-partitioning-whats-the-difference
• https://dev.to/jayaprasanna_roddam/database-partitioning-sharding-and-replication-17oc

5. Message queues / pub-sub basics

• https://www.geeksforgeeks.org/system-design/message-queues-system-design/
• https://igotanoffer.com/blogs/tech/queues-pubsub-system-design-interview