背景

这里说的Cancel是指Client可以主动取消request的执行，包括在request还没有通过网络发送时取消发送，或者将Cancel命令通过网络传递到server端以提前终止服务的执行（在server端的服务实现支持Cancel功能的情况下），其目的都是避免不必要的资源消耗，无论是网络资源还是计算资源。

protobuf的RPC框架中定义了Cancel功能的接口和语义，具体在RpcController类中进行了定义：

  // Client-side methods ---------------------------------------------

  // Advises the RPC system that the caller desires that the RPC call be
  // canceled.  The RPC system may cancel it immediately, may wait awhile and
  // then cancel it, or may not even cancel the call at all.  If the call is
  // canceled, the "done" callback will still be called and the RpcController
  // will indicate that the call failed at that time.
  virtual void StartCancel() = 0;

  // Server-side methods ---------------------------------------------

  // If true, indicates that the client canceled the RPC, so the server may
  // as well give up on replying to it.  The server should still call the
  // final "done" callback.
  virtual bool IsCanceled() const = 0;

  // Asks that the given callback be called when the RPC is canceled.  The
  // callback will always be called exactly once.  If the RPC completes without
  // being canceled, the callback will be called after completion.  If the RPC
  // has already been canceled when NotifyOnCancel() is called, the callback
  // will be called immediately.
  //
  // NotifyOnCancel() must be called no more than once per request.
  virtual void NotifyOnCancel(Closure* callback) = 0;

考虑server端用户是如何使用cancel功能的：在用户实现的服务处理函数开始时，通过RpcController::NotifyOnCancel()注册一个Closure回调函数，回调函数的处理逻辑是提前终止服务处理，避免不必要的资源消耗。

实现

考虑的一种实现方案是：在client端调用RpcController::StartCancel的时候，发送一个special的cancel_request到server端，server收到cancel_request后，找到这个request对应的RpcController，然后调用服务实现者通过RpcController::NotifyOnCancel()注册的回调函数。

baidu / sofa-pbrpc

增加Cancel支持 #85

背景

实现