Everything is a tradeoff

• Designing a scalable system is an optimization task: there are tons of constraints (time, budget, knowledge, complexity, technologies currently available, etc.), and you need to build the best thing that fits those constraints.
• Every technology, every pattern is great for some things, and not so great for others. Understanding these pros and cons, the advantages and disadvantages, is key.
• Remember: there is no one optimal system design.
• In your preparation, don’t try to find silver bullets. Instead, focus on what each scalability pattern is good for, what its shortcomings are, and why people prefer it over other patterns.

At the interview

• First of all, follow the System Design Process. You already know how to apply it, so we'll be brief. Don't skip steps, don't make assumptions, start broad and go deep when asked.
• Second, keep in mind that system design questions serve as an idea exchange platform. Be prepared for discussions about tradeoffs, about pros and cons. Be prepared to give alternatives, to ask questions, to identify and solve bottlenecks, to go broad or deep depending on your interviewer's preferences.
• Be open to new constraints to pop up during the discussion and to adjust your architecture on the fly.