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Swift Actors for Thread-Safe Persistence
Patterns for building thread-safe data persistence layers using Swift actors. Combines in-memory caching with file-backed storage, leveraging the actor model to eliminate data races at compile time.
When to Activate
- Building a data persistence layer in Swift 5.5+
- Need thread-safe access to shared mutable state
- Want to eliminate manual synchronization (locks, DispatchQueues)
- Building offline-first apps with local storage
Core Pattern
Actor-Based Repository
The actor model guarantees serialized access — no data races, enforced by the compiler.
swift
public actor LocalRepository<T: Codable & Identifiable> where T.ID == String {
private var cache: [String: T] = [:]
private let fileURL: URL
public init(directory: URL = .documentsDirectory, filename: String = "data.json") {
self.fileURL = directory.appendingPathComponent(filename)
// Synchronous load during init (actor isolation not yet active)
self.cache = Self.loadSynchronously(from: fileURL)
}
// MARK: - Public API
public func save(_ item: T) throws {
cache[item.id] = item
try persistToFile()
}
public func delete(_ id: String) throws {
cache[id] = nil
try persistToFile()
}
public func find(by id: String) -> T? {
cache[id]
}
public func loadAll() -> [T] {
Array(cache.values)
}
// MARK: - Private
private func persistToFile() throws {
let data = try JSONEncoder().encode(Array(cache.values))
try data.write(to: fileURL, options: .atomic)
}
private static func loadSynchronously(from url: URL) -> [String: T] {
guard let data = try? Data(contentsOf: url),
let items = try? JSONDecoder().decode([T].self, from: data) else {
return [:]
}
return Dictionary(uniqueKeysWithValues: items.map { ($0.id, $0) })
}
}Usage
All calls are automatically async due to actor isolation:
swift
let repository = LocalRepository<Question>()
// Read — fast O(1) lookup from in-memory cache
let question = await repository.find(by: "q-001")
let allQuestions = await repository.loadAll()
// Write — updates cache and persists to file atomically
try await repository.save(newQuestion)
try await repository.delete("q-001")Combining with @Observable ViewModel
swift
@Observable
final class QuestionListViewModel {
private(set) var questions: [Question] = []
private let repository: LocalRepository<Question>
init(repository: LocalRepository<Question> = LocalRepository()) {
self.repository = repository
}
func load() async {
questions = await repository.loadAll()
}
func add(_ question: Question) async throws {
try await repository.save(question)
questions = await repository.loadAll()
}
}Key Design Decisions
| Decision | Rationale |
|---|---|
| Actor (not class + lock) | Compiler-enforced thread safety, no manual synchronization |
| In-memory cache + file persistence | Fast reads from cache, durable writes to disk |
| Synchronous init loading | Avoids async initialization complexity |
| Dictionary keyed by ID | O(1) lookups by identifier |
Generic over Codable & Identifiable | Reusable across any model type |
Atomic file writes (.atomic) | Prevents partial writes on crash |
Best Practices
- Use `Sendable` types for all data crossing actor boundaries
- Keep the actor's public API minimal — only expose domain operations, not persistence details
- Use `.atomic` writes to prevent data corruption if the app crashes mid-write
- Load synchronously in `init` — async initializers add complexity with minimal benefit for local files
- Combine with `@Observable` ViewModels for reactive UI updates
Anti-Patterns to Avoid
- Using
DispatchQueueorNSLockinstead of actors for new Swift concurrency code - Exposing the internal cache dictionary to external callers
- Making the file URL configurable without validation
- Forgetting that all actor method calls are
await— callers must handle async context - Using
nonisolatedto bypass actor isolation (defeats the purpose)
When to Use
- Local data storage in iOS/macOS apps (user data, settings, cached content)
- Offline-first architectures that sync to a server later
- Any shared mutable state that multiple parts of the app access concurrently
- Replacing legacy
DispatchQueue-based thread safety with modern Swift concurrency
