System Design — Junior

1Client–server fundamentals

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At junior level the round checks you understand how a web request actually flows. Trace a click from the browser to the database and back.

The request lifecycle

DNS → HTTP request → server → response. What each hop does, and where latency hides.

MDN — HTTP overview.

Stateless vs stateful; why the server usually shouldn't remember you between requests.

Twelve-Factor — Processes.

TCP vs UDP in one sentence; what a port is.

MDN — TCP.

HTTPS/TLS in one sentence; why passwords never go over plain HTTP.

MDN — TLS.

2HTTP & APIs

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REST is the lingua franca of web services — know its shape cold.

Design a simple API

HTTP methods (GET/POST/PUT/PATCH/DELETE) and status codes (2xx/3xx/4xx/5xx).

MDN — Status codes.

REST: resources, nouns in the URL, verbs as methods, JSON bodies.

restfulapi.net.

Idempotency in one line: a retried "create" shouldn't make two records.

Stripe — Idempotency.

Pagination: offset/limit vs cursor, and why huge offsets are slow.

Use The Index, Luke — No OFFSET.

3Where data lives

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You should be able to sketch a simple schema and explain SQL vs NoSQL in plain words.

Storage fundamentals

SQL vs NoSQL in one sentence each; when a relational DB is the safe default.

System Design Primer — SQL or NoSQL.

Tables, primary keys, foreign keys — model "users have many posts".

SQLBolt.

What an index is and why it speeds up reads (and slows writes).

Use The Index, Luke — Anatomy.

Why you never store passwords in plain text; hashing at a high level.

OWASP — Password storage.

4The framework (starter)

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You are not expected to design Twitter. You are expected to be organised: ask questions, sketch an API, draw boxes, and talk.

Run it in order

1. Ask clarifying questions before designing anything.

System Design Primer — How to approach.

2. Write 2–3 API endpoints with methods and a sample JSON response.

restfulapi.net.

3. Sketch a simple data model (a couple of tables).

SQLBolt.

4. Draw the boxes: client → server → database. Add a cache only if asked.

Excalidraw.

5Building blocks

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Know what these are so the words don't surprise you when a senior mentions them.

One line each

Load balancer — spreads traffic across several servers.

AWS — Load balancing.

Cache — a fast store for hot data (Redis); trades freshness for speed.

AWS — Caching.

CDN — serves static files from a server near the user.

Cloudflare — CDN.

Queue — hands slow work to a background worker so the request returns fast.

AWS — Message queue.

6Auth & security basics

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A few security concepts come up even in junior rounds.

Know these

Authentication vs authorization; sessions vs JWT tokens.

jwt.io — Introduction.

What a rate limit is and why every public API has one.

Cloudflare — Rate limiting.

CORS in one sentence; why the browser blocks a cross-origin call.

MDN — CORS.

SQL injection & the fix (parameterized queries) at a high level.

OWASP — SQL Injection.

7More fundamentals

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A grab-bag of concepts worth knowing at a talking level.

Rapid fire

Sync vs async work: why send an email via a background job.

AWS — Message queue.

Horizontal vs vertical scaling in plain words.

System Design Primer — Scalability.

What "latency" and "throughput" mean; p50 vs p99.

Latency numbers.

Logs, metrics, and why you monitor a service.

Google SRE — Monitoring.