Whether you are a few years into your career or a senior engineer who hasn't interviewed in a while, these patterns fill the gap between knowing individual building blocks and actually designing a complete system under interview pressure. Design patterns are the **recurring solutions** that show up across almost every system design question. Building blocks (databases, caches, queues) are your raw materials. Design patterns are *how you wire them together* to solve real problems — handling millions of writes, keeping data in sync, recovering from failures, and pushing updates to users in real time. You do not need to memorize every detail. What matters is **recognizing which pattern applies** when a requirement appears in an interview, and knowing the key trade-offs well enough to discuss them confidently. ## How the Patterns Are Organized We group the 22 patterns into six categories based on the type of problem they solve. During an interview, the requirements you gather will naturally point you toward one or more of these categories. ### Scaling & Performance These patterns address the most fundamental challenge: your system has more traffic than a single machine can handle. ### Data Handling & Storage These patterns deal with how data enters the system, how it is identified, and how it is stored efficiently. ### Consistency & Correctness These patterns ensure your system does the right thing, especially when multiple operations happen at the same time or things fail mid-way. ### Async Processing & Scheduling These patterns handle work that cannot or should not happen synchronously in the request path. ### Real-Time & Communication These patterns deliver updates to users or between services as events happen. ### Specialized Domains These patterns apply to specific problem domains that appear frequently in interviews. ## How to Use These Patterns in an Interview Patterns rarely appear in isolation. A single design question typically combines three to five patterns. Here is how patterns map to some of our practice questions: You don't need to read all 22 patterns before practicing. Start with the **Scaling & Performance** group — these appear in nearly every question. Then read patterns as you encounter them in practice questions. Each pattern page links to the questions where it applies, so you can jump between theory and practice naturally.