Swarmkit notes (--SWARMCTL) controlclient to swarm cluster by command

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swarmctlEssentially, by controlClient swarm cluster sending an order. controlClientdefined in api/control.pb.go :

// Client API for Control servicetype ControlClient interface {    GetNode(ctx context.Context, in *GetNodeRequest, opts ...grpc.CallOption) (*GetNodeResponse, error)    ListNodes(ctx context.Context, in *ListNodesRequest, opts ...grpc.CallOption) (*ListNodesResponse, error)    ......}type controlClient struct {    cc *grpc.ClientConn}func NewControlClient(cc *grpc.ClientConn) ControlClient {    return &controlClient{cc}}func (c *controlClient) GetNode(ctx context.Context, in *GetNodeRequest, opts ...grpc.CallOption) (*GetNodeResponse, error) {    out := new(GetNodeResponse)    err := grpc.Invoke(ctx, "/docker.swarmkit.v1.Control/GetNode", in, out, c.cc, opts...)    if err != nil {        return nil, err    }    return out, nil}......