addrss https://github.com/PaddlePaddle/Paddle/issues/1319 as well

doc/design/api.md

@@ -166,14 +166,19 @@ There are some open questions here:
 ### Training
 
 The recommended way to training a model is to call `paddle.train`,
-which simply calls `paddle.optimizer.Default`, a global variable of
-type `paddle.optimizer.SGD`.  Equivalently, we can do
+which simply calls `paddle.trainer.Default`, a global variable of
+type `paddle.trainer.SGD`.  Equivalently, we can do
 
 ```python
-opt = paddle.optimizer.SGD(...)
+opt = paddle.trainer.SGD(..., paddle.updater.Adam(...))
 opt.train(model, reader=read, ...)
 ```
 
+Please be aware that a trainer requires an updater as its data
+member.  This is to make it easier to customize trainers, as
+discussed [here](https://github.com/PaddlePaddle/Paddle/issues/1319).
+
+
 #### Distributed Training
 
 If users want to do distributed training on a cluster, s/he should
@@ -185,8 +190,9 @@ access a Kubernetes cluster, s/he should be able to call
 
 ```python
 paddle.dist_train(model,
+                  trainer=paddle.trainer.SGD(..., 
+                                             paddle.updater.Adam(...)),
                   reader=read,
-                  optimizer=paddle.optimizer.SGDOptimizer(...),
                   k8s_user="yi",
                   k8s_token="kube_cluster_tls.pem",
                   k8s_job="hello",
@@ -196,7 +202,7 @@ paddle.dist_train(model,
 The pseudo code if `paddle.dist_train` is as follows:
 
 ```python
-def dist_train():
+def dist_train(topology, parameters, trainer, reader, ...):
     if os.getenv("KUBERNETES_SERVICE_HOST") == None:
         image_name = k8s_user + '/' + k8s_job
         docker_build(image_name)
@@ -209,7 +215,7 @@ def dist_train():
         elif rank < 15:
             parameter_server()
         else:
-            optimizer.train(model, reader=read)
+            trainer.train(model, reader=read)
 ```
 
 Please be aware that if a process is running on the Kubernetes