Trainer Communication Library design doc, the Go interface part

doc/design/cluster_train/trainer.md

+# Design Doc: Trainer Communication Library
+
+For an overview of trainer's role, please refer to [distributed training design doc](README.md). In this design doc, we will discuss the trainer's communication library, which will manage communication with parameter servers and the [master server](master_server.md). The library will be implemented in [Go](https://golang.org/) and made available as a static or dynamic library with a C header file.
+
+## Go Interface
+
+The Go interface is the basic abstraction of communications with the master server and parameter servers. We will add another layer on top (add retry logic, polish interface with C idiom) before exposing the library with a [C interface](#c-interface).
+
+```go
+// MasterClient is the client to the master server.
+type MasterClient struct {}
+
+// GetTask gets a new task by telling the master server the finished task.
+// Use nil as the finished task when getting the task for the first time.
+func (*MasterClient) GetTask(finished master.Task) (master.Task, error)
+
+// ElementType is the type of elements of a Parameter.
+type ElementType int
+
+// Different element types.
+const (
+	Int32 ElementType = iota
+	UInt32
+	Int64
+	UInt64
+	Float32
+	Float64
+)
+
+// Parameter is a piece data to sync with the parameter server.
+type Parameter struct {
+	Name        string
+	ElementType ElementType
+	Buffer      []byte
+}
+
+// Gradient is the gradient of the parameter.
+type Gradient Parameter
+
+// PServerClient is the client to parameter servers.
+type PServerClient struct {}
+
+// UpdateRule specifies the rule for updating parameters with gradients.
+type UpdateRule struct {
+	UpdateMethod pserver.UpdateMethod
+	LearningRate float32
+}
+
+// ParamInitChans returns a send channel for parameter initialization.
+//
+// ParamInitChans will be called from multiple trainers, only one trainer should
+// initialize the parameters on parameter servers, other trainers will instead
+// get the initialized parameters from parameter servers using GetParam.
+//
+// If send channel is not nil, the trainer is selected to do the initialization,
+// the trainer needs to signal for finishing initializing the parameters by
+// closing the send channel.
+func (*PServerClient) ParamInitChan() (send chan<- Parameter, err error)
+
+// SendGrad sends gradients to parameter servers.
+func (*PServerClient) SendGrad(method UpdateMethod, grads []Gradient) error
+
+// GetParam gets parameters from parameter servers.
+func (*PServerClient) GetParam(names []string) ([]Parameter, error)
+
+// Save indicates parameters to save the parameter to the given path.
+//
+// Path needs to be the path to a distributed file system which is visible
+// to all parameter servers.
+func (*PServerClient) Save(path string) error
+```
+Please see [master server design doc](master_server.md) for the definition of `master.Task`.
+
+## C Interface
+
+TODO