Swift Actors for Thread-Safe Persistence Skill

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.

Maestro Integration

Lifecycle: implement Activates when: maestro:new-track detects relevant tech in tech-stack.md, or maestro:implement encounters matching task types.

Phase Guidance

In maestro:implement: Apply actor-based persistence when implementing data storage. Use actors to eliminate data races by design with file-backed caching.

Related Skills

maestro:swift-concurrency-6-2
maestro:swift-protocol-di-testing
maestro:swiftui-patterns

Core Pattern

Actor-Based Repository

The actor model guarantees serialized access — no data races, enforced by the compiler.

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:

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

@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 DispatchQueue or NSLock instead 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 nonisolated to 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

Relationship to Maestro Workflow

/maestro:new-track -- Detects this skill during Step 9.5 (skill matching)
/maestro:implement -- Loads this skill's guidance during task execution
/maestro:review -- Uses checklists as review criteria

Agent Skills: Swift Actors for Thread-Safe Persistence

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