A collection of 75 real-world backend engineering interview problems — machine coding & low-level design (LLD), repository-based tasks, forward-deployed-engineer (FDE) / integration rounds, AI-researcher & data-pipeline bugs, and production-incident debugging. Each is written the way the round actually plays out: you inherit a service that mostly works, the requirements say what correct looks like, and the edge cases are where the grading happens. Statements only — bring your own language and implementation.
Every problem here can also be practiced against a real repository with a failing test suite, free, at gronex.org — solve it in the browser workspace or download the starter repo and run the bundled verify script in Java, Python, or C++.
| Problem | Difficulty | Core topic |
|---|---|---|
| Food Delivery Order Status Tracker | Easy | state machines |
| CMS Article Publishing and Versioning Workflow | Medium | versioning, optimistic locking |
| CRM Lead Assignment and Escalation System | Medium | assignment, load balancing |
| Course Enrollment and Progress Tracker | Medium | progress tracking, one-time side effects |
| Customer Support Ticket SLA System | Medium | SLA tracking, injected clock |
| E-Commerce Coupon Application Engine | Medium | pricing rules, idempotent redemption |
| Hotel Room Booking System | Medium | date-range logic |
| Inventory Stock Reservation System | Medium | inventory accounting, rollback |
| Kanban Board with WIP Limits and Card Flow | Medium | ordering, WIP limits |
| Loyalty Points and Tier Management System | Medium | FIFO accounting and idempotency |
| Notification Preferences and Delivery System | Medium | policy evaluation, idempotent delivery |
| Online Exam Attempt and Grading Engine | Medium | attempt state machine, idempotency |
| Poll and Voting System with Quorum | Medium | deduplication, quorum tally |
| Ride Booking and Driver Matching System | Medium | matching, dispatch lifecycle |
| Seat Reservation System | Medium | state machines, cross-entity validation |
| Tournament Bracket and Match Progression System | Medium | bracket tree, match progression |
| URL Shortener with Custom Aliases and Quotas | Medium | uniqueness, quota accounting |
| Warranty RMA Return Authorization System | Medium | state machine, transactional consistency |
| Expense Report Approval and Reimbursement System | Hard | approval routing, budget state |
| Feature Flag and Rollout Targeting System | Hard | deterministic rule evaluation |
| Insurance Claim Adjudication System | Hard | coverage math, approval authority |
| IoT Device Provisioning and Firmware Rollout System | Hard | device lifecycle, staged rollout |
| Multi-Tenant API Key and Scope Management System | Hard | tenant isolation, scope inheritance |
| Project Task Dependency Manager | Hard | dependency graphs, cycle detection |
| Pull Request Review and Merge Gate System | Hard | review state machine, merge gates |
| Purchase Order Receiving and Three-Way Match System | Hard | cumulative receipts, three-way match |
| Referral Program and Reward Attribution System | Hard | attribution, idempotency |
| Subscription Billing and Proration Engine | Hard | proration math, lifecycle state machines |
| Wallet Transaction and Refund System | Hard | ledger discipline, idempotency |
| Problem | Difficulty | Core topic |
|---|---|---|
| API Rate Limiter | Medium | time-based accounting, concurrency |
| Auction Bid Placement and Closing System | Medium | concurrency, critical sections |
| Concurrent Bank Transfer | Medium | deadlock prevention, atomicity |
| SaaS License Seat Allocation and Reclamation System | Medium | seat capacity, concurrency |
| API Rate Limiter and Quota Enforcement System | Hard | token-bucket concurrency |
| Ad Campaign Budget Pacing and Spend Ledger | Hard | concurrency-safe budget accounting |
| Bank Account Transfer and Deadlock Prevention System | Hard | concurrency, deadlock prevention |
| Distributed Job Scheduler with Singleton Execution | Hard | distributed coordination, leases |
| File Upload Deduplication and Multipart Finalization | Hard | content-addressed storage, reference counting |
| Flash Sale Inventory Purchase System | Hard | concurrency, atomic check-and-decrement |
| Hotel Room Reservation with Expiring Holds System | Hard | concurrency, expiring holds |
| Marketplace Escrow: Payment Hold and Release | Hard | money under concurrency, ledger reconciliation |
| Message Queue Consumer: Idempotency, Retries, DLQ | Hard | at-least-once delivery, exactly-once effects |
| Ride Booking Driver Assignment Race Resolution System | Hard | assignment races, ordered locking |
| Shared Calendar Slot Booking System | Hard | multi-resource locking, deadlock prevention |
| Problem | Difficulty | Core topic |
|---|---|---|
| In-Memory Cache: TTL Expiry + LRU Eviction | Medium | caching, LRU eviction |
| Job Queue: Delayed Jobs & Priority Ordering | Medium | scheduling, priority queue |
| Rate Limiter: Sliding-Window Algorithm & Per-Tier Limits | Medium | rate limiting, sliding window |
| Resumable Multipart Upload | Hard | multipart upload, dedup |
| Problem | Difficulty | Core topic |
|---|---|---|
| API Contract Debugging: Authorization Leak (Broken Access Control) | Medium | access control, authorization |
| API Contract Debugging: Cursor Pagination | Medium | cursor pagination, contract testing |
| API Contract Debugging: Idempotent POST /orders | Medium | idempotency, HTTP contracts |
| API Contract Debugging: Input Validation & Error Envelope | Medium | API contract, validation |
| Model Serving Prediction API Contract | Hard | model serving, batch inference |
| Webhook Event Idempotency and Ordering | Hard | idempotency, event ordering |
| Problem | Difficulty | Core topic |
|---|---|---|
| Incident Debugging: Crash Loop from a Bad Config | Easy | config validation, crash loop |
| Incident Debugging: Checkout Hangs When Recommendations Stall | Medium | timeouts, graceful degradation |
| Incident Debugging: Connection Pool Leak (Service Outage Under Load) | Medium | resource leak, error handling |
| Incident Debugging: OOM Crash-Loop on Startup | Medium | memory budget, lazy loading |
| Incident Debugging: Poison Message Kills the Queue Worker | Medium | poison messages, dead-lettering |
| Incident Debugging: Retry Storm Without Backoff | Medium | retry backoff, resilience |
| API Client Pagination and Retry | Hard | paginated sync, retry idempotency |
| Airflow Revenue DAG: Early Publish and Retry Inflation | Hard | DAG dependencies, idempotent load |
| Incident Debugging: Late CDC Corrupts a Customer Snapshot | Hard | CDC merge, late-arriving data |
| Incident Debugging: Ledger Transfers Freeze in Production | Hard | lock ordering, deadlock |
| Problem | Difficulty | Core topic |
|---|---|---|
| Connector Record Reconciliation Sync | Hard | reconciliation, sync idempotency |
| Customer Data Schema Mapping Ingestion | Hard | schema mapping, deduplication |
| Debug Batched Classification Metrics | Hard | classification metrics, batch pooling |
| Debug Data Leakage in a Retention Feature Pipeline | Hard | data leakage, feature engineering |
| Debug a Logistic Training Loop | Hard | gradients, training loop |
| Pandas Order Reconciliation: Join Fan-out and Deduplication | Hard | joins, change-data-capture |
| SQL Window Isolation in Fulfillment Event History | Hard | window functions, partitioning |
| Problem | Difficulty | Core topic |
|---|---|---|
| Access Log Triage & Request Analysis | Easy | shell log analysis |
| File Permission & Secret Hardening | Easy | file permissions, hardening |
| CSV Group-By Aggregation in the Shell | Medium | shell text processing |
| Hardening a Fragile Deploy Script | Medium | shell reliability, error handling |
These are original problems in the style of common interview rounds — not questions leaked from any company's process. Solutions, starter repos, and a graded test suite for each live at gronex.org.