Alamofire Patterns — Expert Decisions

Alamofire Patterns — Expert Decisions

Expert decision frameworks for Alamofire choices. Claude knows Alamofire syntax — this skill provides judgment calls for when Alamofire adds value and how to design interceptor chains.

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Decision Trees

Alamofire vs URLSession

What networking features do you need?
├─ Basic REST calls with JSON
│  └─ Modern URLSession is sufficient
│     async/​await + Codable works well
│
├─ Complex authentication (token refresh, retry)
│  └─ Alamofire's RequestInterceptor shines
│     Built-in retry coordination
│
├─ Request/​Response inspection and modification
│  └─ Does app need centralized logging/​metrics?
│     ├─ YES → Alamofire EventMonitor
│     └─ NO → URLSession delegate suffices
│
├─ Certificate pinning
│  └─ Alamofire ServerTrustManager simplifies this
│     But URLSession can do it with delegates
│
└─ Multipart uploads with progress
   └─ Alamofire upload API is cleaner
      URLSession works but more boilerplate

The trap: Adding Alamofire for simple apps. If you just need basic GET/​POST with JSON, URLSession's async/​await API is clean enough and avoids a dependency.

Interceptor Chain Design

What cross-cutting concerns exist?
├─ Just auth token injection
│  └─ Single RequestAdapter
│
├─ Auth + retry on 401
│  └─ Authenticator pattern (Alamofire's built-in)
│     Handles refresh token race conditions
│
├─ Multiple concerns (auth, logging, caching headers)
│  └─ Compositor pattern
│     Interceptor(adapters: [...], retriers: [...])
│
└─ Request modification varies by endpoint
   └─ Per-router interceptors
      Different Session instances or conditional logic

Retry Strategy Selection

What kind of failure?
├─ Auth failure (401)
│  └─ Refresh token and retry once
│     Use Authenticator, not generic retry
│
├─ Transient network error
│  └─ Is request idempotent?
│     ├─ YES → Retry with exponential backoff (3 attempts)
│     └─ NO → Don't retry (may cause duplicates)
│
├─ Server error (5xx)
│  └─ Retry for 503 (Service Unavailable) only
│     Other 5xx usually won't recover
│
└─ Client error (4xx except 401)
   └─ Never retry
      Request is malformed, retry won't help

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NEVER Do

Interceptor Design

NEVER refresh tokens in generic retry logic:

// ❌ Race condition — multiple requests refresh simultaneously
final class BadInterceptor: RequestRetrier {
    func retry(_ request: Request, for session: Session, dueTo error: Error, completion: @escaping (RetryResult) -> Void) {
        if response.statusCode == 401 {
            Task {
                try await refreshToken()  // 5 requests = 5 refresh calls!
                completion(.retry)
            }
        }
    }
}

// ✅ Use Alamofire's Authenticator — coordinates refresh across requests
final class TokenAuthenticator: Authenticator {
    func refresh(_ credential: OAuthCredential, for session: Session, completion: @escaping (Result<OAuthCredential, Error>) -> Void) {
        // Single refresh, all waiting requests resume
    }
}

NEVER create new Session instances per request:

// ❌ Loses connection pooling, memory inefficient
func fetchUser() async throws -> User {
    let session = Session()  // New session per call!
    return try await session.request(endpoint).serializingDecodable(User.self).value
}

// ✅ Reuse session — connection pooling, shared interceptors
final class NetworkManager {
    private let session: Session  // Single instance

    func fetchUser() async throws -> User {
        try await session.request(endpoint).serializingDecodable(User.self).value
    }
}

NEVER use Interceptor for endpoint-specific logic:

// ❌ Interceptor has complex conditionals
func adapt(_ urlRequest: URLRequest, ...) {
    if urlRequest.url?.path.contains("/​admin") {
        // Add admin header
    } else if urlRequest.url?.path.contains("/​public") {
        // Skip auth
    }
}

// ✅ Use Router pattern — endpoint defines its own needs
enum APIRouter: URLRequestConvertible {
    case adminEndpoint
    case publicEndpoint

    var requiresAuth: Bool {
        switch self {
        case .adminEndpoint: return true
        case .publicEndpoint: return false
        }
    }
}

Session Configuration

NEVER disable SSL validation in production:

// ❌ Security vulnerability
let manager = ServerTrustManager(evaluators: [
    "api.production.com": DisabledTrustEvaluator()  // MITM vulnerable!
])

// ✅ Use DisabledTrustEvaluator only for development
#if DEBUG
let evaluator = DisabledTrustEvaluator()
#else
let evaluator = DefaultTrustEvaluator()
#endif

NEVER ignore response validation:

// ❌ Silently accepts 4xx/​5xx as success
session.request(endpoint)
    .responseDecodable(of: User.self) { response in
        // May decode error response as User!
    }

// ✅ Always validate before decoding
session.request(endpoint)
    .validate(statusCode: 200..<300)
    .responseDecodable(of: User.self) { response in
        // Only called for 2xx responses
    }

Retry Logic

NEVER retry non-idempotent requests:

// ❌ May create duplicate orders
func placeOrder() {
    session.request(APIRouter.createOrder)
        .validate()
        .response { response in
            if response.error != nil {
                self.placeOrder()  // Retry — may duplicate!
            }
        }
}

// ✅ Use idempotency keys for non-idempotent operations
func placeOrder(idempotencyKey: String) {
    session.request(APIRouter.createOrder(idempotencyKey: idempotencyKey))
    // Server uses key to prevent duplicates
}

NEVER retry immediately without backoff:

// ❌ Hammers server during outage
let retryPolicy = RetryPolicy(retryLimit: 5)  // Immediate retries

// ✅ Exponential backoff
let retryPolicy = RetryPolicy(
    retryLimit: 3,
    exponentialBackoffBase: 2,
    exponentialBackoffScale: 0.5
)

---

Essential Patterns

Authenticator with Refresh Coordination

final class OAuthAuthenticator: Authenticator {
    private let tokenStore: TokenStore
    private let refreshService: RefreshService

    func apply(_ credential: OAuthCredential, to urlRequest: inout URLRequest) {
        urlRequest.headers.add(.authorization(bearerToken: credential.accessToken))
    }

    func refresh(_ credential: OAuthCredential, for session: Session, completion: @escaping (Result<OAuthCredential, Error>) -> Void) {
        // Alamofire ensures only ONE refresh happens
        // Other 401 requests wait for this to complete
        refreshService.refresh(refreshToken: credential.refreshToken) { result in
            switch result {
            case .success(let tokens):
                let newCredential = OAuthCredential(
                    accessToken: tokens.accessToken,
                    refreshToken: tokens.refreshToken,
                    expiration: tokens.expiration
                )
                self.tokenStore.save(newCredential)
                completion(.success(newCredential))
            case .failure(let error):
                completion(.failure(error))
            }
        }
    }

    func didRequest(_ urlRequest: URLRequest, with response: HTTPURLResponse, failDueToAuthenticationError error: Error) -> Bool {
        response.statusCode == 401
    }

    func isRequest(_ urlRequest: URLRequest, authenticatedWith credential: OAuthCredential) -> Bool {
        urlRequest.headers["Authorization"] == "Bearer \(credential.accessToken)"
    }
}

Compositor Interceptor

// Combine multiple adapters and retriers
let interceptor = Interceptor(
    adapters: [
        AuthAdapter(tokenStore: tokenStore),
        LoggingAdapter(),
        DeviceInfoAdapter()
    ],
    retriers: [
        AuthRetrier(authenticator: authenticator),
        NetworkRetrier(retryLimit: 3)
    ]
)

let session = Session(interceptor: interceptor)

Certificate Pinning

let evaluators: [String: ServerTrustEvaluating] = [
    "api.yourapp.com": PinnedCertificatesTrustEvaluator(
        certificates: Bundle.main.af.certificates,
        acceptSelfSignedCertificates: false,
        performDefaultValidation: true,
        validateHost: true
    )
]

let session = Session(
    serverTrustManager: ServerTrustManager(evaluators: evaluators)
)

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Quick Reference

When Alamofire Adds Value

Interceptor Checklist

• Single Session instance shared across app
• Authenticator for token refresh (not generic retry)
• Exponential backoff for transient failures
• Only retry idempotent requests
• Validate responses before decoding
• Certificate pinning for production

Red Flags