Design Doc: Session

doc/design/session.md

+# Design Doc: Session
+
+## Abstract
+
+This design doc proposes to have an object called *Session* which
+encapsulates the environment in which the computation graph is
+executed.
+
+## Background
+
+A computation graph is executed in an environment which contains the
+[scope](./scope.md) and other states. PaddlePaddle used to only have
+an implicit global session on which `paddle.eval()` is executed.
+
+This has the limitation that the user can not create two independent
+environments. For example, in reinforcement learning, the user may
+want to have a stale model for inference and a fresh model for
+training, and only replace the stale model with the fresh model
+periodically. Also, we have no concept that can encapsulate a remote
+environment that could execute a computation graph.
+
+## Session
+
+Session is an object that owns all runtime states such as scope,
+reader OP's file handles, connection to a remote PaddlePaddle cluster,
+etc.
+
+Session has two methods: `eval` and `close`. `eval` executes the
+target OP in a given graph, and `close` closes the session and
+releases all related resources:
+
+```Python
+a = paddle.constant(1.0)
+b = paddle.constant(2.0)
+c = a + b
+sess = paddle.session()
+sess.eval(c)
+sess.close()
+```
+
+### Remote Session
+
+Paddle Cloud will support user creating a remote session pointing to
+the Paddle Cloud cluster. The user can send the computation graph to
+be executed on the Paddle Cloud. In this way, the user can control a
+cluster from her local computer:
+
+```Python
+reader = paddle.reader.recordio("/pfs/home/peter/mnist-train-*") # data stored on Paddle Cloud
+image = reader.column(0)
+label = reader.column(1)
+fc1 = paddle.op.fc(image, size=256, act="sigmoid")
+fc2 = paddle.op.fc(fc1, size=10, act="softmax")
+cost = paddle.op.cross_entropy(fc2)
+opt = paddle.optimizer.sgd(cost)
+
+remote_config = ... # remote configuration such as endpoint, number of nodes and authentication.
+sess = paddle.remoteSession(remote_config)
+for i in range(1000):
+    sess.eval(opt)
+sess.close()
+```