Swift Developer interview questions

A media export can run for 20 minutes and spawns 40 child tasks. What cancellation contract would you design for app, server, and CLI callers?

A log ingestion pipeline receives 50,000 events per second but storage sustains 30,000. How would you express backpressure with Swift concurrency?

A legacy C callback API completes once on a private queue and has a 5-second timeout. How would you expose it as async without leaks or hangs?

An Objective-C delegate emits 2,000 sensor updates per second and can call back after stop. How would you bridge it into Swift concurrency?

A sync engine creates tasks that may outlive a screen by 30 seconds and uses a delegate for progress. How would you define ARC ownership?

A Codable event is stored by Linux services and read by app versions up to 18 months old. How would you evolve the shared contract?

Four Swift clients and a Vapor service share a 120-endpoint contract that changes weekly. Would you generate models or hand-maintain them?

You are designing a Vapor API for 15,000 requests per second, 99.95% availability, and three independently owned domains. What service shape would you choose?

A Vapor endpoint performs 20 ms of CPU parsing and two database calls under a p99 target of 120 ms. How would you protect NIO event loops?

A multi-tenant Vapor service handles 6,000 concurrent requests, and no mutable request context may cross tenants. How would you isolate request state?

A Vapor service has 120 database connections and traffic spikes to 4,000 requests per second. How would you size Fluent concurrency and pools?

Creating an order touches inventory, payment intent, and an audit row, with a 300 ms SLA. Where would you draw Fluent transaction boundaries?

A public Vapor API serves 3 million users and internal operators with stronger privileges. How would you structure authentication and authorization?

An API must enforce 100 requests per minute per user across 20 Vapor instances, while allowing a 10-request burst. What limiter would you design?

A mobile client may retry order creation for 24 hours over unreliable networks, and the service handles 2,000 orders per second. How would you guarantee idempotency?

A Vapor endpoint streams 8 GB exports to 500 concurrent clients with a 256 MB memory limit per instance. How would you design it?

A collaboration service maintains 80,000 WebSocket connections and broadcasts documents to rooms of up to 2,000 users. What Swift architecture would you use?

A Vapor deployment gets 30 seconds to terminate while requests last up to 10 seconds and background jobs up to 2 minutes. How would you implement graceful shutdown?

A Linux Swift service has a 99.9% SLO and 150 ms p99 target across 60 endpoints. What observability contract would you build into shared packages?

A Swift CLI transforms 1 TB overnight on 200 hosts, and operators need to resume failed runs. What observability would you expose?

A package must support macOS and Ubuntu for 5 years, but currently imports Darwin and assumes case-insensitive paths. How would you make portability enforceable?

A SwiftUI trading screen receives 500 price updates per second, while the same domain package powers a CLI. Where would you place state boundaries?

A UIKit editor is embedded in a SwiftUI app used by 4 million users, and both frameworks currently own the document state. How would you remove the dual ownership?

A cross-platform Swift platform has 600 tests but still ships contract failures monthly. What test pyramid would you establish?

A Swift SDK and Vapor backend release independently, with 10 supported client versions and weekly schema changes. How would you build contract testing?

An actor-based ledger must preserve ordering under 5,000 concurrent test operations. How would you test it without sleeps?

A package supports Swift 5.10 and Swift 6 on macOS and Ubuntu, plus two iOS deployment targets, and CI must finish in 18 minutes. What matrix would you run?

Eight teams lose 35 engineer-hours daily to Swift builds. How would you prioritize build-performance work over one quarter?

A Swift package handles OAuth tokens for 6 apps and 3 Linux services, and a compromised dependency must not expose them. How would you design the security boundary?

A public Swift SDK parses untrusted 25 MB archives on client devices and Linux workers. What safeguards belong in the architecture?

During Swift 6 strict-concurrency migration, warnings jump from 40 to 2,800 across 70 targets and the release is due in six weeks. How do you stage the migration?

After enabling strict-concurrency checks and parallel request execution, a Linux service corrupts 0.03% of account balances under 6,000 requests per second, while unit tests pass. How do you diagnose the race?

An actor-backed inventory check oversells 17 items during a sale because it awaits pricing between read and write. What evidence confirms reentrancy, and what do you change?

After annotating a telemetry dashboard view model @MainActor, frame drops rise from 1% to 17% and main-thread CPU reaches 91% while decoding 2,400 sensor samples per second. How do you fix it?

A Vapor deploy increases live tasks from 4,000 to 240,000 and memory from 700 MB to 9 GB in eight minutes. What do you inspect first?

Clients cancel 35% of searches, but database CPU remains 92% for 40 seconds afterward. How do you trace and repair cancellation propagation?

A wrapper around a photo callback hangs 1 in 20,000 requests, always near its 10-second timeout. How do you prove a continuation is never resumed?

Production crashes with SWIFT TASK CONTINUATION MISUSE at a rate of 2 per 1 million requests after adding timeout support. How do you find the double resume?

A user-initiated export waits 12 seconds behind background indexing even though its task priority is high. How do you diagnose priority inversion?

An AsyncSequence consumer handles 8,000 messages per second while producers send 25,000, and RSS grows 400 MB per minute. What evidence and fix do you want?

An iOS sync session remains allocated after logout, retaining 600 MB and 18 child tasks. How do you isolate the ARC leak?

A macOS controller and Objective-C delegate survive window closure in 4% of sessions. What cycle do you look for and how do you prove the fix?

Processing a 120 MB value-type document suddenly peaks at 1.8 GB after one mutation stage. How do you confirm a CoW regression?

Replacing a concrete parser with a generic pipeline lowers runtime CPU 8% but grows the Linux binary from 80 MB to 310 MB and cold start by 900 ms. What do you do?

Moving 30 handlers into an any Handler registry raises p99 from 75 to 104 ms and allocations by 38%. How do you decide whether existentials caused it?

Package resolution starts taking 11 minutes and produces different versions on developer machines after adding two dependencies. How do you stabilize it?

Two shared packages require incompatible major versions of a logging library, blocking 14 teams. How do you resolve the SPM collision?

Incremental builds rise from 70 seconds to 9 minutes after a shared protocol gains one associated type. What evidence guides the fix?

After a route macro release, CI cache hits fall from 82% to 21%, and 7 of 50 clean builds produce different expansions for the same 180 routes. Do you roll back or repair?

A build plugin running in 18 repositories unexpectedly connects to the internet, and CI secrets are exposed in its child-process environment for 27 minutes. What is your incident response?

An XCFramework update crashes only on devices running the oldest supported OS, affecting 6% of users. How do you handle the binary artifact incident?

A backend changes retryAfterSeconds from a JSON number to a String, and 12% of active clients fail Codable decoding. What rollback do you execute?

A new enum case from the server causes 80,000 daily CLI jobs to exit before processing. How do you recover without hiding semantic errors?

A Vapor endpoint p99 jumps from 90 ms to 4.2 seconds, and event-loop lag is 1.8 seconds after adding PDF parsing. What do you do in the first 15 minutes?

After a deploy, service p99 doubles from 140 to 290 ms while CPU, memory, and error rate remain flat. How do you narrow the cause?

One NIO event loop shows 95% utilization while seven remain below 25%, and only long-lived connections are slow. What do you investigate?

A list endpoint grows from 12 to 1,201 SQL queries and p99 reaches 2.6 seconds for 100 records. How do you confirm and fix Fluent N+1?

Fluent pool acquisition p95 rises to 3 seconds, 18% of requests time out, but PostgreSQL uses only 35% CPU. What do you inspect?

Two Fluent transactions deadlock 40 times per hour after a batch feature launches. How do you identify and remove the cycle?

A Fluent migration adding an index locks a 400-million-row table and write latency exceeds 30 seconds. What is your recovery and safer plan?

After an auth-cache optimization, 23 requests return data from the wrong tenant in one hour. How do you contain and diagnose it?

A payment endpoint creates 31 duplicate charges during a 20-minute network incident despite idempotency keys. Where do you look?

A global rate limiter should allow 5,000 requests per second but admits 8,700 during Redis failover. How do you redesign degraded behavior?

A WebSocket room with 2,000 members causes one event to consume 1.4 CPU-seconds and fanout latency reaches 9 seconds. How do you respond?

Five percent of WebSocket clients consume 20 times slower, and per-connection queues grow until instances restart. What policy do you implement?

A Linux Swift service is OOM-killed at 12 GB every six hours while heap snapshots show no single dominant leak. How do you investigate?

During deployment, 2% of in-flight uploads truncate because instances exit 14 seconds after SIGTERM. How do you repair graceful shutdown?

A Swift CLI sorting a 300 GB file reaches 64 GB RSS and is killed halfway through. What redesign and evidence do you require?

A CLI receives SIGINT during a 40-minute import, exits after 90 seconds, and leaves 12 child processes. How do you fix signal handling?

A formatter CLI is interrupted while replacing 18,000 files and leaves 73 zero-byte files. How do you eliminate partial-file corruption?

A SwiftUI dashboard redraws 4,500 rows on every one-second tick and energy use doubles. How do you isolate the invalidation source?

Embedding a UIKit editor in SwiftUI duplicates save commands in 0.4% of sessions after navigation. How do you diagnose ownership?

A 900,000-line Objective-C product must move networking to Swift without changing behavior for 8 million users. What incremental sequence do you choose?

You must enable strict concurrency in a 45-target package while three teams ship weekly and cannot pause feature work. How do you prevent a flag-day migration?

A public library must replace a callback API with async/await while supporting clients for 12 months. How do you migrate without a breaking release?

In review, an engineer proposes @unchecked Sendable on a mutable cache used by 30 tasks and says tests pass. How do you mentor through the decision?

A teammate puts Data(contentsOf:) inside a Vapor route; staging p99 rises from 100 ms to 1.6 seconds with 200 concurrent requests. How do you handle the review and learning?

During a pool-exhaustion incident, a junior engineer wants to raise PostgreSQL connections from 120 to 500 immediately. How do you guide the response?

A code review adds a debug snapshot that retains every 50 MB CoW buffer for undo, and peak memory becomes 2.4 GB. How do you coach a better design?

A team wants to skip Linux integration tests to cut CI from 22 to 14 minutes, after three portability regressions last quarter. How do you lead the review?