Image transform pipeline with a wall-clock budgetCodingApply a fixed sequence of image transforms (grayscale, flip, scale, blur, rotate) to every image in a directory, then speed the batch up with threads or processes to hit a…
Last asked Oct 2026
#concurrency
- +2Concurrent same-host web crawlerCodingImplement a web crawler that finds every page reachable from a start URL on the same host, and fetches pages concurrently. You are given a blocking helper: Write: python def…Last asked Oct 2026
Rotating title picker for a home-page billboardCodingBuild a component that picks which title to show next in a billboard (or plays next in a playlist). Every title has a relevance score, and scores can change at any time. Each call…Last asked Sep 2026
Rule-driven file review agentAI system designDesign an agent workflow that applies supplied rules to a file or contract: classify and recommend fixes, or revise a contract against dozens of rules while merging overlapping…Last asked Sep 2026
+2Design an image sharing service that stores each file onceSystem designDesign a service where users upload, view, download and delete images, and identical images are stored only once, keyed by a content hash. Explain how metadata and blobs stay…Last asked Sep 2026
Find the bugs in a dictionary memoization cache under concurrencyCodingA slow function is wrapped by a module-level dictionary that stores its results: look up the key, otherwise compute and store. You are shown the code and asked what breaks in…Last asked Sep 2026
+7LRU cacheCodingBuild a key-value cache with a fixed capacity that evicts the least recently used entry when it is full. get(key) returns the value or a miss, put(key, value) inserts or updates…Last asked Sep 2026
Thread-safe reference-counted smart pointer in C++CodingImplement a reference-counted smart pointer class template in C++ from scratch, in the spirit of std::sharedptr. Several owners can share one object; it is destroyed exactly once…Last asked Sep 2026- First successful result from parallel calls under a global timeoutCodingA backend API is slow and sometimes fails. To answer quickly, call several of these functions in parallel and return the first result that succeeds. Write a function that takes…Last asked Sep 2026
Durable event log writer shared by thousands of threadsCodingMany threads on one server append small opaque records to a single durable log. Design and partly build the writer: how callers hand off records, how writes are batched and…Last asked Sep 2026- +1Count and map a cluster tree with message passingCodingA cluster of n machines is wired as a rooted tree (any number of children per node). Each machine knows only three things: its own id (a string such as "4"), its parent's id ("-1"…Last asked Sep 2026
Per-key ordered task executor on a thread poolCodingBuild a component that takes a maximum concurrency and an existing thread-pool executor and exposes submit(key, task). Tasks with the same key must run one at a time in the order…Last asked Sep 2026
Find and fix the races in a bank account classCodingYou are shown a small bank account class used from several threads, with a deposit method that loses updates. Find every concurrency bug, fix it, and explain why your fix is…Last asked Sep 2026- Sort an array with k worker threadsCodingImplement parallelsort(nums: listint, k: int) - listint, which returns every element of nums in ascending order and uses k worker threads for the heavy lifting. Rules - Split nums…Last asked Sep 2026
Keep GPUs fed: fix a training job that waits on its dataML designA training job's GPUs sit idle a third of the time waiting for the next batch. How do you prove the input pipeline is the bottleneck, and then fix it? Cover profiling, parallel…Last asked Sep 2026
Locks versus MVCC in a versioned storeCodingYou have a store that keeps versions of its data. Compare protecting concurrent readers and writers with locks against multi-version concurrency control: how each works, what…Last asked Aug 2026- Hit counter for the last five minutesCodingRecord hits (or events) as they arrive with a timestamp, and answer how many happened in the last five minutes (more generally, the last W seconds) at any moment. Discuss which…Last asked Aug 2026
- Bounded blocking queueCodingImplement a first-in, first-out queue with a fixed capacity that producer and consumer threads share. put blocks while the queue is full and take blocks while it is empty, and…Last asked Aug 2026
- Reusable cyclic barrierCodingImplement a barrier for a fixed number of threads. Each thread calls wait() at a sync point and blocks until all of them have arrived; then all are released together and the…Last asked Aug 2026
- Implement a mutex without built-in locksCodingBuild a lock with lock() and unlock() without using any lock, mutex, semaphore, monitor or condition variable the language or a library provides. Build it from atomic operations…Last asked Aug 2026
+2Typed task lock for mutually exclusive job typesCodingSome jobs on a shared accelerator pool must not run alongside jobs of a different kind. Each job runs on its own thread and has one of two types: "gen" or "score". Any number of…Last asked Aug 2026
+3Fixed-capacity circular bufferCodingImplement a generic FIFO buffer with a fixed capacity, allocated once up front, with constant-time push and pop. Discuss how to signal full and empty and how the buffer would be…Last asked Aug 2026
Two threads updating a shared counter with different stop conditionsCodingStart two threads that both update a shared counter. One stops when the counter reaches 10 and returns a snapshot of it; the other stops after one second and returns its snapshot…Last asked Aug 2026- Dependency-aware concurrent tool schedulerCodingBuild a scheduler that runs a set of tool calls concurrently while respecting their dependencies: a call starts only after every call it depends on has finished, and independent…Last asked Aug 2026
- Design an incident ticket correlation platformSystem designDesign a platform that takes in a high volume of incident tickets and alerts at the same time and groups the ones caused by the same underlying problem, so responders see one…Last asked Aug 2026
- Video playback loop at 25 frames per secondCodingYou get a VideoPlayer with a Read() that returns the next frame and a call that renders a frame. Write the playback loop: frames must be shown at a steady 25 frames per second…Last asked Aug 2026
Rate limiter with multiple sliding-window rulesCodingBuild a rate limiter that enforces several rules at once. Each rule is a pair (limit, window): at most limit accepted requests may fall in any window of window time units. Use the…Last asked Aug 2026- Design trackable product links that reward every Nth visitorSystem designCompanies create and edit product pages with images and descriptions, generate unique links to share on social media, and see each link's reach and engagement. The platform also…Last asked Aug 2026
- Tiered rate limiter with limits that change at runtimeCodingBuild a rate-limiting service in Python. Each client belongs to a tier (for example free, pro, enterprise), and each tier has its own limit. A request arrives with a client id…Last asked Aug 2026
- Thread safety and deadlocks, explained and demonstratedCodingA fast concepts round. What makes code thread-safe? Show the common ways a deadlock happens (for example two threads taking two locks in opposite order) and the ways to prevent…Last asked Aug 2026
- Design a collaboratively edited playlistSystem designSeveral people add, remove and reorder songs in the same playlist at once. Scale is not the point: model the ordered list so reorders are cheap and concurrent edits resolve…Last asked Jul 2026
Run blocking API calls in parallel with a thread poolCodingA program makes many blocking API calls one after another. Make them run concurrently with a thread pool, collect the results in the right order, and handle calls that fail or…Last asked Jul 2026
+4Design a ticket booking systemSystem designSell seats for concerts and games where demand spikes the moment sales open: hold seats briefly, never sell one twice, and stay up under the rush.Last asked Jul 2026
Concurrent restaurant waitlist and seatingOO designDesign and code a restaurant's waitlist and seating: parties join the list, tables free up, and parties get seated. Keep it correct when many requests happen at the same time.Last asked Jul 2026- Thread-safe tally counter with bounded memoryCodingBuild a tally service that many threads call at once to count occurrences of keys, and that answers queries for counts. Memory is bounded, so it cannot keep every key forever…Last asked Jul 2026
Build your own CompletableFutureCodingImplement a small version of CompletableFuture: a result that some thread completes later, that other threads can wait on, and that can be chained with callbacks. Then use it to…Last asked Jun 2026- Priority task executor with duplicate task idsCodingImplement an executor that accepts tasks, each with an id and a priority, and runs them highest priority first. The same task id can be submitted more than once. Decide and…Last asked Jun 2026
- Thread-safe scheduler for periodic jobsCodingWrite a scheduler that runs functions periodically, at a frequency such as 50 Hz, with schedule and cancel. One thread runs the jobs while other threads may add or cancel them at…Last asked May 2026
Run promise tasks with bounded concurrencyCodingGiven a list of async tasks (functions returning promises) and a limit k, run them with at most k in flight at once, start the next as soon as one finishes, and resolve with all…Last asked May 2026- Database connection poolCodingAgainst a mocked database query, implement a connection pool with acquire and release over a fixed set of reusable connections. Reported on 1point3acres' Harvey bank;…Last asked Apr 2026
- Design calendar event conflict handlingSystem designIn a calendar service, design how conflicts between events are found and handled: overlapping events for the same person or room, two bookings made at the same moment, and…Last asked Apr 2026
Deduplicate concurrent downloads of the same URLCodingWrite a downloader that many threads call with URLs. Downloads of different URLs run in parallel. When several threads ask for the same URL at the same time, only one of them…Last asked Apr 2026- Build a hash map, make it thread-safe, and review a producer-consumer queueCodingImplement a hash map from scratch: hashing, buckets, collisions, resizing and the edge cases (null keys, rehashing while iterating). Then make it safe for many threads and discuss…Last asked Apr 2026
- Design a disk-backed key-value store under heavy contentionSystem designDesign a key-value store that keeps its data on disk and serves many concurrent readers and writers, some of them hammering the same keys. Cover the on-disk layout, the write path…Last asked Mar 2026
- Systems fundamentals quick-fire: memory, processes, threads and transactionsCodingA short screen of basics, answered out loud: 1. What an array is, how it is laid out in memory, and how it differs from a linked list. 2. What a smart pointer is and the kinds you…Last asked Feb 2026
- Concurrent latency percentile trackerCodingBuild a component that records request latencies from many threads and answers percentile queries such as p50, p95 and p99. Discuss exact versus approximate approaches (sorted…Last asked Feb 2026
- Debug a staged C++ project with AI tools allowedCodingA C++ program is split into stages, each hiding a logic error, a memory bug or a race. Run it, observe the output, and fix each stage, using an AI assistant if you like while…Last asked Feb 2026
- Parallel chunked download with integrity checkCodingChange a sequential chunked file download to fetch chunks in parallel, then verify the reassembled file's integrity. Reported on 1point3acres' Baseten bank; reconstructed in our…Last asked Feb 2026
- Lazy in-order BST iteratorCodingWrite an iterator over a binary search tree that returns values in sorted order one at a time, with next() and hasnext(). Do it lazily with a stack, so memory is proportional to…Last asked Feb 2026
- Idempotent request handling with idempotency keysCodingImplement a request handler that uses client-supplied idempotency keys so that retrying the same request returns the original result instead of doing the work twice. Handle two…Last asked Feb 2026
- Thread-safe task queue on a linked listCodingA task queue was written for one thread: a singly linked list with head and tail, where put appends at the tail and get removes from the head. Explain what goes wrong when many…Last asked Feb 2026
- Choosing lock granularity for concurrent storageCodingFor a storage component used by many threads, compare one global lock, per-partition locks and per-object locks, plus reader-writer locks. Discuss throughput, deadlock risk and…Last asked Jan 2026
- Runnable versus Callable and a thread-safe singletonCodingJava concurrency questions: explain the difference between Runnable and Callable (return value, checked exceptions, how each is run through an executor and Future), then write a…Last asked Jan 2026
- Robot charging controller with exclusive stationsCodingA fleet of robots shares a small number of charging stations. A robot whose battery drops below 50% should go to a free station, and a robot charging above 95% may leave and go…Last asked Jan 2026
- Design a dependency-aware job schedulerSystem designDesign a job scheduler in which each job is a list of tasks with dependencies between them; a task may start only when every task it depends on has finished. Cover how you…Last asked Jan 2026
- Maximum value in a file too big for memoryCodingA very large text file holds one number per line. Return the largest value. Then speed it up: split the file into chunks, find each chunk's maximum in parallel, and combine. Write…Last asked Dec 2025
- Thread-safe job registry with expiryCodingBuild an in-memory registry of jobs that many threads read and write at once. Each job is registered with a time to live; lookups must not return a job whose time has passed, and…Last asked Nov 2025
- Thread-safe key-value storeCodingImplement an in-memory key-value store that many threads can use at once. Start with correctness, then reduce contention with finer-grained locks (striped locks or a lock per…Last asked Nov 2025
- Mutexes, memory alignment, polymorphism and idempotencyCodingA fully technical round of short questions: what a mutex guarantees and how to use one correctly, memory efficiency and why the compiler aligns and pads structures, how runtime…Last asked Oct 2025
- Thread-safe single-machine chat server with channelsCodingBuild an in-process chat system where many clients join channels and post messages that the other members receive. Make it correct under concurrent senders and receivers, and…Last asked Oct 2025
Batching service around an embedding modelCodingPut a service around an embedding model: many callers send texts at once, the service groups them into batches for the model, returns each caller its vectors, and stops without…Last asked Oct 2025- Design a distributed counter for many usersSystem designMany users increment shared counters concurrently. Start with a correct single-process counter with locking, then design it for many servers: atomic increments in a database…Last asked Sep 2025
- C++ performance round with provided threading codeCodingA 60-minute coding round for performance-minded roles, held in C++. The focus is on core C++ performance, with a smaller part on concurrency; the threading components are…Last asked Sep 2025
- Building H2OCodingHydrogen and oxygen threads arrive in any order. Each must wait until it can form a water molecule: exactly two hydrogen threads and one oxygen thread pass the barrier together…Last asked Aug 2025
- Concurrent caching client for byte-range file readsSystem designDesign a client that reads byte ranges of remote files and caches what it has fetched, so a later read of a range it already holds is served locally. Many threads use one client…Last asked Aug 2025
- Delayed job schedulerOO designDesign and code a scheduler that accepts a job and a delay and runs the job after that many seconds. Cover the data structure that finds the next due job, the worker thread or…Last asked Aug 2025
- Thread-safe document and tag classesOO designDesign classes that record documents and the tags on them: add and remove tags, list a document's tags, and find documents by tag. Follow-up: many threads use it at once; make it…Last asked Aug 2025
- Run three methods in a fixed order across threadsCodingThree methods, first(), second() and third(), are called by three different threads in any order. Make sure they always run in that order, using locks, events or semaphores.Last asked Aug 2025
- Design a string type: memory layout, moves and thread safetyCodingWhat is a string in a programming language? Describe what fields a string struct holds (for example in Rust: pointer, length, capacity) and how you would implement one yourself…Last asked Jul 2025
- Keep a shared payment account correct under concurrent paymentsSystem designTwo people share one online payment account and pay with it at the same moment. How do you make sure the balance never goes wrong? Answer from the API side (idempotency, request…Last asked Jul 2025
- Python lists, dicts and concurrencyCodingAnswer Python fundamentals questions: how lists and dicts work underneath and the cost of their operations, then multithreading versus multiprocessing, the GIL, and when to use…Last asked Jul 2025
- Group prompts into templates under concurrent ingestionCodingAn API receives prompts from many threads. Most come from a few templates filled with different values: Summarize ticket 48213 for alice@acme.com and Summarize ticket 1007 for…