DSA Trainer
New · Interview-ready track

System Design Foundations

System design has crept down into new-grad and mid-level loops, and most resources dump senior-level reference diagrams on you. This track does the opposite: one building block at a time, taught through the judgment an interviewer is really testing. When do you reach for a cache? Why this database and not that one? You will be able to make the call and defend it.

Every lesson is the same prediction-first format as the DSA path: read a concrete scenario, predict the move, then see why. No diagrams to memorize.

20
units
99
interactive lessons
159
checkpoints and drills
100
test questions

New here? Start at the beginning.

The track is ordered. Begin with How system design interviews work and work straight down, one building block at a time.

Start →

You’re not signed in. Your progress here won’t be saved.

Part 1 · The building blocks(20 units)

Caching, queues, sharding, and friends: when to reach for each, and what it costs.

1

How system design interviews work

Walk into a system design interview with a repeatable plan: clarify what you're building, size it, then design it in a fixed order instead of freezing.

7 steps
2

The request path

Trace what happens between a user clicking a link and getting a response, and name where each system-design component slots into that path.

7 steps
3

API design basics

Define a clean API for a system: pick the right verb for each action, keep it stateless, and make operations safe to retry.

6 steps
4

Back-of-envelope estimation

Produce rough scale numbers (requests per second, storage, read/write ratio) fast, and use them to justify design choices instead of guessing.

7 steps
5

SQL vs NoSQL

Choose between a relational and a non-relational database based on data shape and access patterns, and defend the choice instead of guessing.

7 steps
6

Transactions & ACID

Spot when a set of writes must succeed or fail together, explain what a transaction guarantees, and say when you genuinely need one.

7 steps
7

Database indexing

Explain why a query is fast or slow, what an index speeds up and what it costs, and decide when a column deserves one.

7 steps
8

Caching

Spot when a cache belongs in a design, say what it buys you and what it costs, and pick what to cache and where.

7 steps
9

Load balancing

Explain how a load balancer spreads traffic, routes around dead servers, and why you run more than one of everything, including the balancer itself.

7 steps
10

Availability & Failover

Find the single points of failure in a design, add redundancy where it counts, and explain how failover actually switches to the backup.

7 steps
11

Replication

Explain how copying a database across machines scales reads and survives failures, and name the price: replication lag.

7 steps
12

Sharding / partitioning

Explain how splitting data across machines scales writes and storage, how a shard key decides the split, and the hotspots and cross-shard costs it introduces.

7 steps
13

Consistent hashing

Explain why hash(key) mod N falls apart when servers are added or removed, how the hash ring fixes it, and what virtual nodes smooth out.

7 steps
14

Message queues & async processing

Recognize when to move work off the request path with a queue, and explain the three things a queue buys you: fast responses, spike absorption, and decoupling.

7 steps
15

Pub/sub vs queues

Tell a task apart from an event, pick a work queue or a pub/sub topic accordingly, and explain how the two combine in real systems.

7 steps
16

CAP & consistency

Tell strong from eventual consistency, explain the availability tradeoff during a network partition, and choose the right consistency per piece of data.

7 steps
17

Rate limiting

Explain why systems cap how often a client can call them, the token-bucket intuition for allowing bursts, and where a rate limiter belongs.

7 steps
18

Real-time & push

Explain why servers can't push over plain HTTP, compare polling, long polling, and WebSockets, and pick the cheapest mechanism that meets the freshness need.

7 steps
19

Search & the inverted index

Explain why text search defeats a normal database index, how an inverted index answers word queries instantly, and where a search service fits next to your database.

7 steps
20

Blob storage & CDNs

Decide where files and media live (object storage, not the database) and how to serve them fast worldwide with a CDN, keeping only metadata in your database.

7 steps
The first two units are completely free. Lessons, drills, and graded tests included. Everything after unlocks with premium, along with the full DSA course.