grpc-swift/Sources/GRPCCore/Transport/ClientTransport.swift

/*
 * Copyright 2023, gRPC Authors All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
public protocol ClientTransport: Sendable {
  typealias Inbound = RPCAsyncSequence<RPCResponsePart>
  typealias Outbound = RPCWriter<RPCRequestPart>.Closable

  /// Returns a throttle which gRPC uses to determine whether retries can be executed.
  ///
  /// Client transports don't need to implement the throttle or interact with it beyond its
  /// creation. gRPC will record the results of requests to determine whether retries can be
  /// performed.
  var retryThrottle: RetryThrottle? { get }

  /// Establish and maintain a connection to the remote destination.
  ///
  /// Maintains a long-lived connection, or set of connections, to a remote destination.
  /// Connections may be added or removed over time as required by the implementation and the
  /// demand for streams by the client.
  ///
  /// Implementations of this function will typically create a long-lived task group which
  /// maintains connections. The function exits when all open streams have been closed and new connections
  /// are no longer required by the caller who signals this by calling ``close()``, or by cancelling the
  /// task this function runs in.
  ///
  /// - Parameter lazily: Whether the transport should establish connections lazily, that is,
  ///     when the first stream is opened or eagerly, when this function is called. If `false`
  ///     then the transport should attempt to establish a connection immediately. Note that
  ///     this is a _hint_: transports aren't required to respect this value and you should
  ///     refer to the documentation of the transport you're using to check whether it's supported.
  func connect(lazily: Bool) async throws

  /// Signal to the transport that no new streams may be created.
  ///
  /// Existing streams may run to completion naturally but calling ``withStream(descriptor:_:)``
  /// should result in an ``RPCError`` with code ``RPCError/Code/failedPrecondition`` being thrown.
  ///
  /// If you want to forcefully cancel all active streams then cancel the task
  /// running ``connect(lazily:)``.
  func close()

  /// Opens a stream using the transport, and uses it as input into a user-provided closure.
  ///
  /// - Important: The opened stream is closed after the closure is finished.
  ///
  /// Transport implementations should throw an ``RPCError`` with the following error codes:
  /// - ``RPCError/Code/failedPrecondition`` if the transport is closing or has been closed.
  /// - ``RPCError/Code/unavailable`` if it's temporarily not possible to create a stream and it
  ///   may be possible after some backoff period.
  ///
  /// - Parameters:
  ///   - descriptor: A description of the method to open a stream for.
  ///   - options: Options specific to the stream.
  ///   - closure: A closure that takes the opened stream as parameter.
  /// - Returns: Whatever value was returned from `closure`.
  func withStream<T>(
    descriptor: MethodDescriptor,
    options: CallOptions,
    _ closure: (_ stream: RPCStream<Inbound, Outbound>) async throws -> T
  ) async throws -> T

  /// Returns the configuration for a given method.
  ///
  /// - Parameter descriptor: The method to lookup configuration for.
  /// - Returns: Configuration for the method, if it exists.
  func configuration(forMethod descriptor: MethodDescriptor) -> MethodConfig?
}