You’ve done LeetCode. Real interviews still break you.
LeetCode trained you for algorithms. Real backend rounds hand you an unfamiliar repo and ask you to find the bug and make the failing tests green. Practice exactly that — in your browser, no setup — across Java, Python, Node.js and C++, from concurrency races to database incidents and runtime failures.
Built for SDE1+ and SDE2 backend interviews.
$ ./verify.sh
PASS OrderServiceTest.createsOrder
PASS OrderServiceTest.appliesDiscount
FAIL OrderServiceTest.processesConcurrentCheckouts
expected inventory: 0
actual inventory: -7 ← race condition
2 passed, 1 failed
Find the bug. Fix it. Make it green.Why this is different
Practice reading existing repository structure, contracts, and failing states — the rounds LeetCode and HackerRank don’t cover — instead of only writing fresh code.
Who it is for
Mid-level and aspiring mid-level backend engineers who are interviewing now, plus anyone leveling up debugging, concurrency, and code-reading.
How it works
Open a free challenge in the browser, read the failing tests, fix the source, and run the full suite in a real sandbox — no setup, no signup to try — then compare against a reviewed solution.
The interview starts after your tests turn green.
Face an AI interviewer's live follow-ups after you solve — like the real machine-coding round. It changes a requirement, reviews your updated code, and probes the trade-offs behind your design.
Multiple challenge types, one real repo each
Every challenge is a production-style repository with failing tests — the format changes, the realism doesn’t.
Repository Implementation
Implement and fix real backend features — bookings, payments, wallets — inside an existing codebase.
Concurrency
Races, deadlocks, and locking bugs that only show up when requests collide.
Architecture Extension
Take a working service and extend it to support a new feature under interview constraints — the round startups actually give you.
API Contract Debugging
Hit an API, compare expected vs actual responses — wrong statuses, broken idempotency, leaky auth — and fix the handler until the contract holds.
Incident Debugging
A service crashed in production. Read the logs, find the root cause, and fix it until it comes back green.
Database Engineering
NewFix PostgreSQL transaction, indexing, migration, replication, partitioning, CDC, and consistency failures against real database invariants.
Runtime Diagnostics
NewUse heap evidence, thread dumps, and GC logs to diagnose retention, deadlocks, starvation, allocation churn, and collector pressure.
Prepare by interview track
Choose a guide based on the round, language, or environment you need to practise next.
Backend coding interview questions
Review the repository skills, failure modes, and service patterns that backend rounds test.
Machine coding round practice
Prepare to extend a working service under time pressure while keeping its contracts intact.
Java backend interview coding problems
Focus on concurrency, transactions, deadlock prevention, API correctness, and idempotency.
Linux shell scripting interview practice
Work through log parsing, permissions, data pipelines, and reliable Bash failure handling.
Data engineer interview questions
Debug pipelines, joins, and window queries — DAG idempotency, pandas fan-out, SQL partitions, and late-arriving data.
Machine learning engineer interview questions
Catch the failures that break models in production — data leakage, serving contracts, metric pooling, and training loops.
Forward deployed engineer interview questions
Integration work under real constraints — API pagination and retries, schema mapping, webhook idempotency, and reconciliation.
Database engineering interview problems
Repair transactions, indexes, migrations, CDC, replicas, partitioning, pagination, and outbox guarantees against PostgreSQL.
Runtime diagnostics interview problems
Investigate heap evidence, thread dumps, and GC logs to fix memory retention, deadlocks, starvation, and collector pressure.
Learn how real systems are built, one annotated diagram at a time.
Study reference architectures with detailed, full-size diagrams and clear written walkthroughs — from core building blocks to trade-offs interviewers actually probe. Included with Gronex Pro, with free guides to get you started.
Problem formats
Representative interview tasks
Zero setup