API Integration and Networking Questions

// Swift (Apollo iOS)
let sqliteCache = try SQLiteNormalizedCache(fileURL: cacheURL)
let store = ApolloStore(cache: sqliteCache)
let client = ApolloClient(networkTransport: transport, store: store)

client.perform(mutation: AddItemMutation(...), optimisticResult: ["addItem": ["id": "temp1", "name": "X"]]) { ... }

// Interfaces
interface Request { val url: String; val headers: MutableMap<String,String>; val body: ByteArray? }
interface HttpClient { suspend fun send(req: Request): HttpResponse }
data class RetryConfig(val maxAttempts: Int = 5, val baseDelayMs: Long = 200, val maxDelayMs: Long = 5000,
                       val jitter: Boolean = true)

// Simple persistence (file/DB abstraction)
interface KeyStore { fun save(keyId: String, key: String); fun load(keyId: String): String?; fun delete(keyId: String) }

// Retry framework
class RetryFramework(private val client: HttpClient, private val keyStore: KeyStore, private val config: RetryConfig) {
  suspend fun sendWithRetry(req: Request, idempotent: Boolean, keyId: String? = null): HttpResponse {
    var attempt = 0
    val idempKey = if (!idempotent) {
      // reuse existing persisted key if present, or generate+persist
      keyId?.let { keyStore.load(it) } ?: run {
        val generated = UUID.randomUUID().toString()
        keyId?.let { keyStore.save(it, generated) }
        generated
      }
    } else null

    while (true) {
      attempt++
      // attach idempotency header only for non-idempotent
      idempKey?.let { req.headers["Idempotency-Key"] = it }

      val resp = try { client.send(req) } catch (e: Throwable) { null }

      if (resp != null && resp.isSuccessful()) {
        // success -> cleanup persisted key when server confirms idempotent outcome
        if (idempKey != null && keyId != null) keyStore.delete(keyId)
        return resp
      }

      if (attempt >= config.maxAttempts) {
        // give up; keep key persisted so external retry after restart can continue (policy)
        throw RetryFailedException("failed after $attempt")
      }

      // backoff with optional jitter
      val delayMs = calculateBackoff(config.baseDelayMs, attempt, config.maxDelayMs, config.jitter)
      delay(delayMs)
    }
  }

  private fun calculateBackoff(base: Long, attempt: Int, max: Long, jitter: Boolean): Long {
    val exp = min(max.toDouble(), base * 2.0.pow(attempt - 1)).toLong()
    return if (jitter) (exp / 2 + Random.nextLong(exp / 2)) else exp
  }
}

import Foundation

struct User: Codable {
    let id: Int
    let name: String
    let email: String?          // can be null or missing
    let metadata: Metadata

    struct Metadata: Codable {
        let lastLogin: Date?

        private enum CodingKeys: String, CodingKey { case lastLogin = "last_login" }
    }

    private enum CodingKeys: String, CodingKey { case id, name, email, metadata }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(Int.self, forKey: .id)
        name = try container.decode(String.self, forKey: .name)
        // decodeIfPresent handles null or missing
        email = try container.decodeIfPresent(String.self, forKey: .email)
        metadata = try container.decode(Metadata.self, forKey: .metadata)
    }
}

enum FetchError: Error {
    case network(Error)
    case httpStatus(Int)
    case decoding(Error)
    case noData
}

func fetchUser(from url: URL, completion: @escaping (Result<User, FetchError>) -> Void) {
    var request = URLRequest(url: url)
    request.httpMethod = "GET"

    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        if let error = error {
            DispatchQueue.main.async { completion(.failure(.network(error))) }; return
        }
        if let http = response as? HTTPURLResponse, !(200...299).contains(http.statusCode) {
            DispatchQueue.main.async { completion(.failure(.httpStatus(http.statusCode))) }; return
        }
        guard let data = data else {
            DispatchQueue.main.async { completion(.failure(.noData)) }; return
        }

        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = .iso8601

        do {
            let user = try decoder.decode(User.self, from: data)
            DispatchQueue.main.async { completion(.success(user)) }
        } catch {
            DispatchQueue.main.async { completion(.failure(.decoding(error))) }
        }
    }
    task.resume()
}