System Design Tutorials
Architecture, scalability, and distributed systems concepts for system design interviews. Each tutorial includes HLD diagrams, sequence diagrams, and deep dives.
What is System Design?
Why system design matters and how to prepare.
8 min read →
Load Balancing
Algorithms, L4 vs L7, health checks, sticky sessions.
7 min read →
Caching Strategies
Cache-aside, write-through, write-behind, Redis vs Memcached.
8 min read →
CDN
Edge caching, pull vs push, TTL, invalidation.
7 min read →
SQL vs NoSQL
When to pick relational vs document, sharding reality.
7 min read →
Database Sharding
Horizontal scaling, shard keys, rebalancing strategies.
7 min read →
Consistent Hashing
Hash ring, virtual nodes, minimal data migration.
7 min read →
CAP Theorem
Consistency vs availability trade-offs in distributed systems.
7 min read →
Message Queues
Kafka vs RabbitMQ, pub/sub, delivery guarantees.
7 min read →
Rate Limiting
Token bucket, sliding window, Redis/Lua at scale.
8 min read →
ID Generation
Snowflake, UUID, Redis sequences at scale.
7 min read →
URL Shortener
Design bit.ly: hashing, base62, caching, analytics.
10 min read →
What this round tests
System design is the round senior and staff candidates fear most, because it is the one interview with no right answer. You are not given a function signature or a failing test. You are given a product and asked to explain, out loud, how you would build it to serve millions of users. The interviewer is not checking whether you can recite a diagram of Instagram from memory. They are checking whether you can break an enormous problem into pieces, attach numbers to your assumptions, and change your mind gracefully when a constraint shifts.
That is why almost every candidate who fails this round fails the same way: they skip the estimation step and start naming technologies. Choosing Kafka, Redis, and Cassandra before you know the read volume, the write volume, or which operations need to be fast is not architecture, it is decoration. Numbers first. The same set of components can be a good or a terrible design depending entirely on whether you sized the problem correctly.
The tutorials below are built around a small set of patterns that cover most interview prompts: how to shard, when to cache, what a queue is actually for, and how load balancing works under the hood. Learn those once and you can reason about an unfamiliar product out loud, which is the real skill being tested. InterviewSkool pairs each pattern with a full worked design and a mock interview, so the reasoning gets rehearsed before it counts.
Practice
Read a tutorial, then try it out loud with Alex, our AI interviewer.
Start a mock interview →Frequently asked questions
What is a system design interview?
A system design interview asks you to architect a large-scale product out loud, usually in 35 to 45 minutes. You are given a vague prompt like “design Instagram,” and you are expected to clarify requirements, estimate scale, choose components, and defend your trade-offs. There is no single correct answer: the interviewer scores whether your design is coherent, whether you quantified your assumptions, and whether you can adapt when constraints change.
How should I prepare for a system design interview?
Learn a small number of core patterns deeply rather than memorizing many designs. Sharding, caching, load balancing, replication, queues, and rate limiting cover most interview problems, and once you can apply those patterns to an unfamiliar prompt you can reason out loud convincingly. Practice by designing the same well-known system repeatedly, because the goal is to make the fundamentals automatic.
How long should I spend in the clarification phase?
Spend about five of your 45 minutes on requirements before proposing anything. Ask how many users the system needs to serve, which operations matter most, and what the latency and consistency requirements are. Skipping this is the single most common way candidates go wrong, because you cannot pick a sensible design until the scale is known, and guessing the scale means every decision downstream is ungrounded.
Should I lead with the high-level design or the details?
Lead with a rough high-level design, then drill down. Sketch the major components and how data flows between them, get agreement from the interviewer, and only then go deep on the two or three areas most relevant to the prompt. If you start at the database schema you will burn twenty minutes on a table the interviewer never cared about, and you may design it wrong because you skipped the scale estimate.
What should I do if I do not know a technology in the prompt?
Say so, then reason about what it would need to do. Interviewers are not testing whether you have used a specific database; they are testing whether you can reason about an unfamiliar tool. Describe what the system requires, compare the requirements against what you know about common options, and pick one out loud. That reasoning is far more valuable than a vague claim, and bluffing is easy to spot.