[Zero Two]

System design interviews trip a lot of people up because there's no definitive "correct" answer and the scope feels enormous. Here's a curated list of resources that are actually worth your time, free or nearly free.

Books

• Designing Data-Intensive Applications (Martin Kleppmann) - not free but worth every penny. The most useful technical book I've read. Covers databases, replication, consensus, stream processing properly.
  
• System Design Interview Vol 1 & 2 (Alex Xu) - more interview-focused, covers the walk-through format well. Available as PDFs online if you look.
  

Free online resources

• ByteByteGo newsletter - Alex Xu's weekly system design breakdowns. Excellent diagrams, free archive.
  
• High Scalability blog - real architecture breakdowns from actual companies (Slack, Twitter, Netflix). Aged but the fundamentals are evergreen.
  
• The System Design Primer (GitHub, donnemartin) - comprehensive open-source guide covering all the major components. Good reference doc.
  
• roadmap.sh/system-design - structured learning path with free content.
  

YouTube

• ByteByteGo channel - animated explainers of system design concepts
  
• Gaurav Sen - good for understanding distributed systems concepts
  
• Hussein Nasser - deep dives on specific technologies (databases, proxies, protocols)
  

Practice
Ask yourself: how would I design [Twitter / YouTube / URL shortener / ride sharing / Google Drive]? Work through it out loud or on paper. Then look up how they actually do it.

The components worth understanding first: load balancers, CDNs, SQL vs NoSQL trade-offs, caching strategies (read-through/write-through/write-behind), message queues, horizontal vs vertical scaling, CAP theorem.