Concurrency interview questions

GCD basics

What is GCD (Grand Central Dispatch), and why do you hand work to a queue instead of creating threads yourself?

#gcd #dispatch-queue #thread-pool #serial

sync/async vs serial/concurrent

Which of the following statements about sync/async and serial/concurrent queues are correct? Select all that apply.

#sync #async #serial #concurrent

Main thread and UI

In a UIKit screen you set label.text = result inside a network response callback, and sometimes the screen updates late or not at all. What's causing that?

#main-thread #ui #main-actor #runloop

await vs async let

In Swift Concurrency, if fetchProfile() and fetchFeed() each take 1 second, which statement about the total time of these two snippets is correct? …

#async-await #async-let #parallel #sequential

iOS concurrency tools

Which of the following are appropriate ways to run work in the background (concurrency) on iOS? Select all that apply.

#thread #gcd #operationqueue #async-await

ConcurrencyShort answerIntroOracleinterview report

Choosing concurrency tools

A new iOS feature needs a network result shared across several screens, and the UI has to update too. What role would you give each of Swift Concurrency's asyn…

#swift-concurrency #async-await #actor #mainactor

Serial queue self-sync

You run this GCD code inside viewDidLoad of a UIKit view controller (main thread). What happens? …

#deadlock #sync #main-queue #serial

GCD vs OperationQueue

You're building an image pipeline: download, resize, then save to the disk cache, and when the user scrolls you need to cancel work in progress. Would you pick…

#operationqueue #operation #gcd #dependency

QoS classes

The user tapped a button and is waiting for the result, and you want to send JSON parsing to a GCD background queue. Which QoS fits best?

#qos #quality-of-service #priority #global-queue

Race conditions

This code gets called from several threads, and count comes out lower than expected. Can you explain why at the CPU level? …

#race-condition #data-race #lock #serial-queue

Barrier and reader-writer

In this cache built on a GCD concurrent queue, which statement about the role of the .barrier flag is correct? …

#barrier #concurrent-queue #reader-writer #gcd

Combining async work

When a screen appears you call three APIs in parallel, profile, feed, and notifications, and you have to render the UI once when all of them finish. How would …

#dispatchgroup #semaphore #async-let #taskgroup

Semaphores and mutexes

Which of the following statements about DispatchSemaphore and mutexes (NSLock, Mutex) are correct? Select all that apply.

#semaphore #mutex #lock #synchronization

How async/await works

In Swift Concurrency, what actually happens when execution hits an await, and how is that different from blocking a thread?

#async-await #suspension #continuation #cooperative-thread-pool

How Task runs

In Swift Concurrency, when you create a Task { }, when and where does that code run?

#task #unstructured #detached #inheritance

Combine operators and schedulers

You want to call the API once, 300ms after typing stops, instead of on every keystroke in a search field, and receive the result on the main thread. Which Comb…

#combine #debounce #throttle #scheduler

Run loop modes

While the user scrolls, an on-screen countdown Timer stops, and when scrolling ends it catches up all at once. From the run loop's point of view, what's the ca…

#runloop #timer #main-thread #runloop-mode

ConcurrencyShort answerBasicsWalmartinterview report

class vs actor

In Swift, what's the difference between a class and an actor?

#actor #mainactor #isolation #class

ConcurrencyShort answerBasicsCapital One

Combine zip vs combineLatest

Write a Combine pipeline that sends two different network requests (user profile, account list) at the same time and emits a result once, only when both have c…

#combine #zip #combinelatest #network

ConcurrencyMultiple choiceBasicsXiaohongshu

Concurrent queue self-sync

What does this GCD code print? And does every call to queue.async create a new thread? …

#gcd #concurrent #sync #thread-pool

ConcurrencyShort answerBasicsDiDi

Dependent work, priority inversion

You have independent pieces of work A, B, and C. C must run only after both A and B have finished, the order of A and B doesn't matter, and they don't nest ins…

#dispatchgroup #barrier #operationqueue #async-let

ConcurrencyShort answerBasicsFlipkart

Search debounce

Calling the API on every keystroke in a search field floods it with requests. Can you implement a debounce with GCD yourself, so the call only fires when there…

#debounce #dispatchworkitem #task #cancellation

ConcurrencyShort answerBasicsCRED

Leaving a loading screen

When the user enters the detail screen, it calls an API like this and shows the data. … The interviewer asks: "What happens if the user taps back before the re…

#cancellation #task #weak-self #timeout

ConcurrencyShort answerBasicsGrindrinterview report

Offloading the main thread

A UIKit screen with image conversion and API response handling stutters while scrolling. Which work would you move off the main actor?

#mainactor #background-work #uikit #threading

ConcurrencyShort answerBasicsZalandointerview report

Shared cache data race

Several image requests read and write the same in-memory cache dictionary at the same time. It crashes now and then, and the logs show the same URL downloaded …

#actor #data-race #cache #inflight

ConcurrencyShort answerBasicsSquare

Structured cancellation

When the search query changes, the previous search's network, decoding, and image work all have to stop. How would you structure the task tree using cancellati…

#cancellation #structured-concurrency #task-group #search

ConcurrencyShort answerBasicsIntuitinterview report

Latest response wins

On a tax summary screen, changing the filter quickly makes several API requests finish almost at once. To make sure only the result for the last filter chosen …

#stale-response #cancellation #state #concurrency

ConcurrencyShort answerBasicsHSBCinterview report

async let vs TaskGroup

A banking app's home screen has to fetch independent account, card, and notification data at the same time. Between async let and TaskGroup, which would you pi…

#async-let #taskgroup #fanout #error-handling

ConcurrencyShort answerBasicsRazorpay

DispatchGroup fan-out

In a callback-based payments app, you have to build a payment summary after three independent APIs have all finished. Where would you put DispatchGroup's enter…

#dispatchgroup #callbacks #fanout #errors

ConcurrencyShort answerAdvancedApple

Diagnosing deadlocks

QA reports that the app sometimes freezes completely with no crash log. If you suspect a deadlock, in what order would you track down the cause?

#deadlock #debugging #lldb #lock-ordering

Structured concurrency

Which of the following statements about structured concurrency (async let, TaskGroup) are correct? Select all that apply.

#structured-concurrency #async-let #taskgroup #cancellation

Sendable conformance

Under Swift 6 strict concurrency, which of the following are treated as Sendable (or adopt it automatically) with no extra work? Select all that apply.

#sendable #swift-6 #strict-concurrency #value-type

Swift 6 migration

You've been asked to move a large app written with completion handlers and GCD to Swift 6 strict concurrency. In what order would you do it?

#swift-6 #strict-concurrency #migration #completion-handler

MainActor isolation

In Swift Concurrency, what's the difference between putting @MainActor on a whole view model class and putting it on specific methods only?

#mainactor #global-actor #nonisolated #isolation

Thread Sanitizer

How does Thread Sanitizer actually catch data races?

#tsan #thread-sanitizer #data-race #debugging

ConcurrencyShort answerAdvancedSnapinterview report

Async network client design

Design the API of an async network client that several screens use at the same time. It must never block the main thread, and it needs deduplication of identic…

#design #network-client #async #cancellation

Canceling delayed work

You scheduled DispatchQueue.main.asyncAfter(deadline: .now() + 2) { showTooltip() }, and if the user leaves the screen within 2 seconds it has to be canceled. …

#dispatchworkitem #asyncafter #cancellation #task-sleep

ConcurrencyShort answerAdvancedPayPal

Completion handlers to async

You have an old networking layer built on completion handlers, called from hundreds of places across the app. How would you move it to async/await without brea…

#async-await #continuation #migration #cancellation

ConcurrencyShort answerAdvancedPhonePe

Ordered concurrent downloads

You're given an array of image ids [3, 1, 2]. Download each image concurrently, but display them on screen in id order (1, 2, 3). How would you do that with Sw…

#taskgroup #structured-concurrency #ordering #cancellation

ConcurrencyMultiple choiceAdvancedZomato

Nested queue output

You run this GCD code on the main thread. What's the output? …

#gcd #main-queue #sync #deadlock

ConcurrencyMultiple choiceAdvancedKuaishou

Serial vs concurrent sync

You call this function, which takes a GCD queue, from the main thread. About how many seconds until it returns? Answer for (1) a serial queue and (2) a concurr…

#gcd #serial #concurrent #sync

ConcurrencyMultiple choiceAdvancedNetEase

Lock-free increment range

Five threads each run count += 1 20 times on a shared variable count (initially 0), with no lock. When they've all finished, what range of values can count hav…

#race-condition #data-race #atomicity #lock

Multi-reader single-writer queue

You want a property that several threads can read at the same time while writes happen one at a time. Back in the Objective-C days there was a pattern of dispa…

#gcd #barrier #reader-writer #objective-c

ConcurrencyShort answerAdvancedMicrosoft

Dictionary cache thread safety

In a message list screen, each cell downloads its profile image asynchronously and caches it in a [String: UIImage] dictionary. With several threads reading an…

#thread-safety #dictionary #nscache #lock

ConcurrencyShort answerAdvancedDropbox

Merging same-URL downloads

In a photo grid, several cells request the same photo URL at the same time, but the network download must happen only once. How would you keep in-flight Tasks …

#actor #image-download #deduplication #task

Actor reentrancy

This image cache actor built with Swift Concurrency sometimes sends two network requests for the same key. The actor protects its state, so why does this happe…

#actor #reentrancy #isolation #await

Cooperative cancellation

In this Swift Concurrency code, which statement about what actually happens right after t.cancel() is correct? …

#cancellation #task #checkcancellation #task-sleep

ConcurrencyShort answerExpertAmazon

Designing a dispatch queue

Design a MyQueue of your own without GCD. Submitting a closure with async(_:) should run it on a background thread pool, it should support serial and concurren…

#design #queue #thread-pool #condition-variable

Real-time pipeline

You're building an app that applies real-time filters to the camera preview. It has to hold 60fps while staying memory safe (no buffer leaks, no contention). W…

#realtime #video #60fps #backpressure

Swift 6 singleton build

The take-home app you submitted has a singleton ImageCache created with static let shared (with var storage: [URL: UIImage] inside) that several screens read a…

#swift6 #strict-concurrency #sendable #singleton