Persistence interview questions

UserDefaults scope and limits

A teammate's PR stores the access token, the profile image Data, and an array of 5,000 recently viewed products all in UserDefaults. What would you raise in re…

#userdefaults #plist #storage

Atomic file writes

A notes app saves Data to Documents/notes.json. Select all the correct statements about file writes. …

#filemanager #data-write #atomic #documents

Picking a local store

A new iOS project has to store a favorites list, the login session, downloaded PDFs, and display settings locally. Which store would you pick for each?

#storage-choice #userdefaults #keychain #filemanager

Keychain characteristics

Compared with UserDefaults, select all the correct characteristics of the Keychain.

#keychain #security #userdefaults

Choosing a directory

You want to store a few thousand thumbnail images locally that you can download again at any time. Which directory fits best?

#filemanager #caches #documents

PersistenceShort answerIntroHeadspace

Persistence tools you've used

This is a written question on a take-home: "Which tools have you used to persist data so far?" Answer with what data you used each tool for and why, not with a…

#userdefaults #keychain #swiftdata #file-storage

Choosing Core Data

Which requirements push you to Core Data (or SwiftData) instead of UserDefaults or a plain file?

#core-data #swiftdata #sqlite #storage-choice

Core Data stack setup

You have a bug where a screen backed by Core Data adds an item but the list never updates. Where is that bug most likely coming from?

#core-data #nspersistentcontainer #nsmanagedobjectcontext #nsfetchrequest

Context threading rules

This is a news app on Core Data. You fetched an Article on a background context (container.newBackgroundContext()), handed the object straight to a view on the…

#core-data #threading #nsmanagedobjectid #background-context

Core Data vs SQLite

A teammate says "Core Data is heavy and hard to understand, let's use SQLite directly (or GRDB)." What's the real difference between the two approaches?

#core-data #sqlite #grdb #realm

Archiving and plists

Legacy code saves model objects to a file with NSKeyedArchiver, and on recent OS versions unarchiving started warning and crashing. What's the difference betwe…

#nscoding #nssecurecoding #nskeyedarchiver #plist

Relational vs object DB

Select all the sound reasons to pick a SQLite-backed relational store (Core Data, SwiftData, GRDB) over a document or object database such as Realm for an iOS …

#sqlite #nosql #realm #rdbms

NSFetchedResultsController

On a Core Data list screen, what does NSFetchedResultsController do, most precisely?

#core-data #nsfetchedresultscontroller #uitableview

SwiftData building blocks

Say you're building a to-do app with SwiftData. What do @Model, ModelContainer, ModelContext, and @Query each do?

#swiftdata #model #modelcontainer #modelcontext

SwiftData facts

Select all the correct statements about SwiftData.

#swiftdata #model #core-data

Sandbox directories and backup

Users complain that the app takes up 2GB of their iCloud backup, and review flagged you on the offline data storage guideline. Walk me through the app sandbox …

#filemanager #documents #caches #tmp

PersistenceShort answerBasicsBloomberg

Server and local models

A phone screen asked how you design data models on the backend and how you handle them on mobile. If you use the server's response JSON structure as your local…

#dto #domain-model #swiftdata #schema-evolution

PersistenceShort answerBasicsPhonePe

Keychain access groups

Two payment apps from the same team, the main app and a merchant app, have to share one login token. UserDefaults and files are out, so on what conditions and …

#keychain #access-group #entitlements #app-group

NSCache vs custom LRU

NSCache exists, so why do image libraries write their own LRU cache?

#nscache #lru #linked-list #image-cache

PersistenceMultiple choiceBasicsShopee

LRU behavior and structure

You picked LRU for an image cache. Select all the correct statements about LRU.

#lru #cache #hit-rate #linked-list

PersistenceShort answerBasicsAlibaba Group

ObjC runtime model mapping

In the Objective-C days, YYModel and MJExtension turned JSON into models and even handled NSCoding archiving from nothing but the class definition. Which runti…

#objective-c #runtime #kvc #codable

PersistenceShort answerBasics왓챠interview report

Realm vs Apple stack

A legacy app leans on Realm heavily. As the new hire you're asked whether to stay on Realm or move to SwiftData/Core Data. How do Realm's threading and object …

#realm #swiftdata #core-data #threading

PersistenceMultiple choiceBasicsCapital Oneinterview report

Matching stores to requirements

Select all the pairs where a finance app's storage requirement is matched with the right store.

#storage #userdefaults #keychain #caches

Core Data migration

You shipped an update to a Core Data app and crash reports are pouring in. The log says "The model used to open the store is incompatible with the one used to …

#core-data #migration #lightweight-migration #mapping-model

Lightweight migration scope

Select all the changes Core Data handles with lightweight migration, with no mapping model.

#core-data #migration #lightweight-migration

Faulting and save

You fetched 10,000 objects from Core Data and memory barely moved, and then it grows a little as you scroll the list. Explain what's going on inside.

#core-data #faulting #nsmanagedobject #save

SwiftData schema migration

You shipped a SwiftData app and now want to add a priority field to Todo and clean up existing title values by trimming whitespace. Which change happens automa…

#swiftdata #migration #versionedschema #schemamigrationplan

Data Protection classes

You have logs where the app, woken by a background push while the screen is locked, fails to open its local database. What is file protection (Data Protection)…

#data-protection #file-protection #encryption #background

Background saves and merging

You have two complaints: data saved on a background context doesn't refresh the main screen, and the other way around, values being edited on main get overwrit…

#core-data #background-context #merge #automaticallymergeschangesfromparent

Merge policy behavior

This is a notes app on Core Data. A background context changed note.body to the server's value and finished saving first. The main context, which doesn't know …

#core-data #mergepolicy #conflict

PersistenceMultiple choiceAdvancedPhonePe

Contexts for parallel downloads

You're downloading image metadata in parallel on four threads and saving it to Core Data. Select all the correct statements about how many NSManagedObjectConte…

#core-data #managed-object-context #background-context #merge

PersistenceShort answerAdvancedZomato

Key-value store with expiry

You're given 15 minutes to design a key-value store whose values carry an expiry time. set(key, value, ttl) and get(key) must never hand back an expired value,…

#userdefaults #ttl #sqlite #index

PersistenceShort answerAdvanced당근interview report

Offline chat storage design

Chat in a secondhand marketplace app has to work for reading and writing on the subway. Design the local database schema and the sync flow so that messages sen…

#offline-first #chat #outbox #sync

PersistenceMultiple choiceAdvancedPinduoduo

LFU cache characteristics

Someone proposes moving the image cache from LRU to LFU. Select all the correct statements about LFU.

#lfu #lru #cache #eviction

CloudKit sync constraints

You want to add iCloud sync across devices to a SwiftData/Core Data app that has been local only. What do you have to change in the schema and the design befor…

#cloudkit #nspersistentcloudkitcontainer #swiftdata #sync

Bulk insert performance

An initial sync saves 100,000 records into Core Data, memory climbs to 1GB and the app gets killed, and the UI freezes the whole time. What do you think the ca…

#core-data #nsbatchinsertrequest #performance #memory

Store sharing with extensions

You want a widget extension to read the main app's SwiftData/Core Data store. Select all the correct statements.

#app-group #widget #sqlite #persistent-history

Server sync design

You need to sync a notes app's local database with the server. Downloading everything each time is slow and burns data. How would you design a delta sync proto…

#sync #delta-sync #tombstone #offline