Machine Coding Round: The Complete Guide (2026)
The machine coding round is a 60–120 minute interview where you build a small but complete, working system. Think of a parking lot, a rate limiter or a booking service. You write real, runnable code, and you're judged on whether it works, whether it's cleanly structured, and whether it can be extended. It sits between a DSA round, which tests algorithms, and a system design round, which tests architecture on a whiteboard: you design the classes and ship the code.
This guide covers what the round looks like, how interviewers score it, the systems that come up most, and a practice plan you can start today.
Updated 17 October 2026
What is a machine coding round?
A machine coding round (also called an LLD round or a low-level design coding round) asks you to implement a system from a short problem statement inside a fixed time. Unlike a whiteboard design round, outlining classes isn't enough. The code has to compile, run and handle the required cases. One widely shared CodeChef write-up puts it simply: it's like an LLD round, except you must actually write the code.
Indian product companies use it heavily. Reported formats:
| Company | Reported format | Source |
|---|---|---|
| Flipkart | ~90 minutes to write fully functional, production-quality code, followed by a live review | TechPrep company page |
| Razorpay | 60–90 minute live session building a functional, modular system from scratch | TechPrep company page |
Formats vary by team and change over time. Your recruiter's brief always wins over any guide, this one included.
How is the machine coding round scored?
Interviewers usually score four things:
- Does it work? The required features run, edge cases are handled, and the code doesn't crash on input it should accept.
- Is it well modelled? Sensible entities, clear responsibilities, and no god classes. Design patterns when they help, not as decoration.
- Can it be extended? The follow-up ("now add a second payment method", "now support multiple floors") shouldn't need a rewrite.
- Is it readable? Naming, small functions, and tests or a runnable demo. Reviewers read your code live.
What usually sinks candidates isn't a missing pattern. It's running out of time with nothing that runs. Get a thin working version first, then deepen it.
What systems do they ask you to build?
The same families come up again and again:
| Family | Typical prompts | Core concepts |
|---|---|---|
| Caching | LRU cache, TTL cache | Eviction policy, O(1) operations, hash map + linked list |
| Traffic control | Rate limiter, circuit breaker, retry with backoff | Sliding windows, state machines, failure handling |
| Booking & allocation | Parking lot, cab booking, railway reservation | Entity modelling, concurrency on shared inventory, state transitions |
| Routing | Load balancer, URL shortener | Algorithms behind an interface, persistence, collision handling |
| Workflows | Task queue, deployment pipeline | Priorities, retries, dead-letter handling, rollback |
| Games / simulations | Snake and ladder, chess validator, Splitwise | Rules engines, clean domain models |
How to prepare: a 4-week practice plan
Week 1: foundation (easy). Build a retry engine with exponential backoff and an employee management system. The goal is a clean, working implementation inside 60 minutes.
Week 2: core patterns (medium). Build an LRU cache, a sliding-window rate limiter, a circuit breaker, a priority task queue and a parking lot. These cover most of the follow-up questions you'll hear.
Week 3: production systems (hard). Build a load balancer, a URL shortener, a cab booking system and a railway reservation system. Practise the follow-up: after you pass, add one requirement and see what you have to change.
Week 4: the final boss (expert) and mock rounds. Build a deployment pipeline engine with rollback. Then run timed mocks: 90 minutes, a fresh problem, no notes.
For each problem:
- Spend 10 minutes on entities and interfaces before writing code.
- Get the simplest working version in by the halfway mark.
- Use the rest of the time for edge cases, concurrency and the extension.
- Read your own code as the reviewer would. Would you approve this pull request?
Practise on Cruscible
Most machine coding practice is self-graded: you write the code, then compare it with someone else's solution. Cruscible grades it for you instead:
- A contract, not a blank page. Each problem gives you an interface to implement, like a real codebase.
- Real infrastructure. Your code runs in an isolated container wired to real PostgreSQL 16, Redis 7 or a key-value store, depending on the problem. No mocks.
- A three-part score. Every submission gets a weighted score for functional correctness, performance (latency targets from 1 ms to 20 ms p99, depending on the problem) and code quality, not just pass or fail.
- Your language. Java, Python, Go, Node.js or TypeScript.
- Free to start. No credit card required.
Machine coding vs. system design vs. DSA
| DSA round | Machine coding round | System design round | |
|---|---|---|---|
| Output | A function | A working, structured mini-system | A diagram and a discussion |
| Time | 30–60 min | 60–120 min | 45–60 min |
| Judged on | Correctness, complexity | Working code, modelling, extensibility, readability | Trade-offs, scale, components |
| Practise with | Algorithm problems | Building systems end to end | Design discussions and case studies |
Frequently asked questions
Which language should I use in a machine coding round?
Use the one you're fastest and cleanest in, unless the company specifies one. Java is the most common choice for LLD-style rounds, because interfaces and classes map directly onto the design.
Do I need design patterns?
You need clear responsibilities. Patterns like Strategy, Observer and State show up naturally in rate limiters, notification systems and booking state machines. Using one where it doesn't fit costs more than leaving it out.
Should I write tests?
If time allows, a small runnable demo or a few tests help a lot. They prove the code works and show how you think about edge cases. A working main method beats an untested "complete" design.
How is this different from an LLD whiteboard round?
A whiteboard LLD round stops at classes and relationships. In a machine coding round the code must run.
Related: How to practise system design: a roadmap · AI-enabled coding interviews