If you are looking for free preparatory materials, consider these highly-regarded community resources:

To effectively use the material provided in Alex Wu's guides, follow this study roadmap: 1. Master the Fundamentals

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Alex Wu’s book is highly regarded because it strips away the ambiguity of designing massive systems. The author, having worked at companies like Oracle and Facebook, breaks down complex architectures into actionable frameworks. 1. The 4-Step Framework

PE 4