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Coding interview practice in real repositories

Investigate a bug, review an AI proposal, and verify a change in an existing application. Explore each scenario before you start an interview.

Backend engineeringMachine learning engineeringPractice guides

21 problems

  • Close the Cross-Tenant Document Leak

    Pro

    Centralize authorization across reads, updates, lists, and administrative access.

    Difficulty
    Medium
    Estimated time
    45 min
    Topic
    Security

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Collapse the Order Report Query Storm

    Pro

    Replace a per-row lookup with one batched load without changing the report output.

    Difficulty
    Medium
    Estimated time
    45 min
    Topic
    Databases

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Detect an Edited Entry in the Audit Log

    Pro

    Re-derive every entry checksum so edited content cannot hide behind an intact chain.

    Difficulty
    Hard
    Estimated time
    50 min
    Topic
    Security

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Fix the Multi-Tenant Rate Limiter

    Pro

    Preserve fractional refill credits while isolating client quotas.

    Difficulty
    Medium
    Estimated time
    40 min
    Topic
    Performance

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Hybrid Retrieval

    Free

    Repair reciprocal rank fusion in a retrieval workbench while preserving catalog tools and search explanations.

    Difficulty
    Medium
    Estimated time
    40 min
    Topic
    AI

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Keep Latency Percentiles Inside the Window

    Pro

    Evict samples outside the window and rank percentiles correctly so a stale burst stops skewing the dashboard.

    Difficulty
    Medium
    Estimated time
    45 min
    Topic
    Performance

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Let the Circuit Breaker Recover

    Pro

    Track consecutive failures and a half-open trial so a recovered dependency is used again.

    Difficulty
    Medium
    Estimated time
    45 min
    Topic
    Reliability

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Make Activity Pagination Stable

    Pro

    Repair tied-timestamp cursors, tenant isolation, and end-of-feed detection.

    Difficulty
    Medium
    Estimated time
    40 min
    Topic
    APIs

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Make Payment Webhooks Idempotent

    Pro

    Handle retries, invalid events, and conflicting identities without corrupting balances.

    Difficulty
    Hard
    Estimated time
    50 min
    Topic
    Backend

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Make the Assertion Helper Compare Structurally

    Pro

    Replace serialized comparison with a structural walk that reports where values diverge.

    Difficulty
    Medium
    Estimated time
    40 min
    Topic
    Testing

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Parse Delimited Records with Quoted Fields

    Free

    Build a quote-aware field scanner so imported data survives commas inside values.

    Difficulty
    Easy
    Estimated time
    35 min
    Topic
    Feature Implementation

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Point-in-Time Training Features

    Free

    Repair historical feature reconstruction so late data, corrections and deletions cannot leak into training rows.

    Difficulty
    Hard
    Estimated time
    75 min
    Topic
    Machine Learning

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Rebuild State from Out-of-Order Events

    Pro

    Handle duplicate deliveries, stale versions, and tombstones in a projection service.

    Difficulty
    Hard
    Estimated time
    55 min
    Topic
    Distributed Systems

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Repair Retry Safety in the HTTP Client

    Free

    Ship resilient retries without duplicating writes or resetting request deadlines.

    Difficulty
    Medium
    Estimated time
    45 min
    Topic
    Reliability

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Repair the Classifier Evaluation Report

    Free

    Repair threshold decisions and confusion-matrix metrics in a model evaluation desk.

    Difficulty
    Easy
    Estimated time
    35 min
    Topic
    Machine Learning

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Repair the Deployment Dependency Planner

    Pro

    Build reproducible deployment waves with cycle and missing-service diagnostics.

    Difficulty
    Hard
    Estimated time
    55 min
    Topic
    Refactoring

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Repair the Expiring LRU Cache

    Pro

    Untangle expiration, recency, and cached null values in a shared cache.

    Difficulty
    Medium
    Estimated time
    45 min
    Topic
    Debugging

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Stop Concurrent Edits Overwriting Each Other

    Pro

    Enforce the version check a collaborative writer skips, so a stale edit loses instead of winning.

    Difficulty
    Hard
    Estimated time
    50 min
    Topic
    Concurrency

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Stop Double Claims in the Job Scheduler

    Free

    Introduce expiring leases and ownership checks for concurrent workers.

    Difficulty
    Hard
    Estimated time
    55 min
    Topic
    Concurrency

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Stop Secrets Escaping Structured Logs

    Pro

    Redact nested credentials without damaging ordinary event data.

    Difficulty
    Medium
    Estimated time
    40 min
    Topic
    Security

    TypeScript, Python, Java, C, C++, Go (Golang), Rust

  • Stop the Rebalance Storm When a Node Joins

    Pro

    Replace modulo sharding with a hash ring so a joining node moves few keys, not all of them.

    Difficulty
    Hard
    Estimated time
    50 min
    Topic
    Distributed systems

    TypeScript, Python, Java, C, C++, Go (Golang), Rust