Cruscible: The Platform Where System Design Meets Real Code

85+ engineering challenges with real Redis, PostgreSQL, and Kafka. Not a browser sandbox — your code runs in actual containers.

By SysAdmin · Published 2026-05-27

Cruscible: The Platform Where System Design Meets Real Code

Most coding platforms ask you to solve puzzles. Cruscible asks you to build systems.

When you implement a rate limiter on Cruscible, your code doesn't run against mock objects in a browser. It connects to a real Redis instance, stores sliding window counters, and gets tested under simulated load. When you build a cab booking system, your data lives in a real PostgreSQL database — complete with transactions, indexes, and constraints.

This is the difference between knowing system design and doing system design.

Two Tracks, One Goal

Cruscible has two problem tracks, both designed to bridge the gap between interview prep and real engineering:

🏗️ LLD (Low-Level Design) — 40+ Problems

LLD problems give you a contract (a Java interface) and real infrastructure (Redis, PostgreSQL, KV Store, or in-memory). You implement the contract, and the grader runs a comprehensive test suite against your implementation.

Categories include:

Every problem is scored on three dimensions:

• Functional — do the tests pass?
• Performance — does it handle load?
• Code Quality — is the code clean, well-structured, and idiomatic?

The composite of these three scores determines your ranking on the global leaderboard.

🔬 Applied DSA — 45+ Problems

Applied DSA takes classic algorithm patterns and puts them in real-world contexts:

• JSON & Data: JSONPath Reader, JSON Diff, Deep Clone
• String Parsing: CSV Parser, Email Validator, URL Query Parser
• SQL & Queries: Criteria to SQL Converter, Query Builder
• Algorithms: Sliding Window Max, Balanced Brackets, Rate Limiter

These problems use the same three-dimensional scoring. It's not enough to get the right answer — your solution needs to be fast and well-written.

How Cruscible Works

Step 1: Choose a Challenge

Browse 85+ problems across LLD and Applied DSA. Each problem page shows:

• The contract (interface you'll implement)
• The infrastructure available (Redis, PostgreSQL, etc.)
• Difficulty rating (Easy → Expert)
• A detailed problem description with examples and constraints

Step 2: Write Your Solution

A full-featured Monaco editor (the same editor that powers VS Code) with Java support. For LLD problems, you get SDK clients for the infrastructure:

// Your code connects to REAL infrastructure
public class RateLimiterImpl implements RateLimiterContract {
    private final RedisClient redis;
    
    public RateLimiterImpl(RedisClient redis) {
        this.redis = redis;  // Real Redis connection
    }
    
    @Override
    public boolean allowRequest(String clientId) {
        // Your sliding window logic here
        // redis.zadd(), redis.zrangeByScore() — real Redis commands
    }
}

Step 3: Submit and Get Graded

When you submit:

1. Your code is compiled in an isolated container
2. Real infrastructure is provisioned (Redis instance, PostgreSQL database, etc.)
3. The test suite runs against your implementation
4. You get a three-dimensional score: Functional + Performance + Quality
5. Results appear in seconds via WebSocket — no polling, no refreshing

Step 4: Compete and Climb

Every submission contributes to your global ranking. The leaderboard tracks:

• Total problems solved
• Average composite score
• Difficulty-weighted points
• Streak and consistency metrics

Earn achievements and titles as you progress — from Bronze through Mythic, across 31 achievement types.

What Makes Cruscible Different

vs. LeetCode / HackerRank

vs. System Design Interviews

vs. Take-Home Projects

For Hiring Teams

Cruscible isn't just for practice — it's also an assessment platform. Hiring teams can:

• Build assessments from 43+ battle-tested problems
• Send invite links — no candidate sign-up required
• Compare candidates with side-by-side 3D scoring
• Review code playback — replay the entire coding session keystroke by keystroke
• Detect AI usage — identify AI-generated code patterns and external pastes
• Integrate with ATS — Zoho Recruit integration and custom webhooks

Candidates build real systems against real infrastructure. No more whiteboard theater.

Built by One Developer

Cruscible is a passion project built by a solo developer who believes coding platforms should test what actually matters. Running on a single 16GB VPS, it's a full-stack platform:

• Backend: Spring Boot 3.5, Java 17
• Frontend: Angular 19 with SSR
• Infrastructure: Redis, PostgreSQL, Kafka, MinIO
• Execution: Isolated containers with real JVM per submission

It's currently in beta. Things will break. But that's how steel is forged.

Start Building

The best way to understand Cruscible is to try it:

1. Explore LLD Problems — Browse 40+ problems across 8 categories
2. Try Applied DSA — 45+ real-world algorithm challenges
3. Read Walkthrough Blogs — Detailed guides for every LLD problem
4. Join a Contest — Compete in live coding contests with real-time chat

Stop memorizing patterns. Start building systems.

👉 Get started for free: cruscible.com