add batch reader into reader design doc

doc/design/reader/README.md

@@ -4,9 +4,10 @@ At training and testing time, PaddlePaddle programs need to read data. To ease t
 
 - A *reader* is a function that reads data (from file, network, random number generator, etc) and yields data items.
 - A *reader creator* is a function that returns a reader function.
-- A *reader* decorator is a function, which accepts one or more readers, and returns a reader.
+- A *reader decorator* is a function, which accepts one or more readers, and returns a reader.
+- A *batch reader* is a function that reads data (from *reader*, file, network, random number generator, etc) and yields a batch of data items.
 
-and provide frequently used reader creators and reader decorators.
+and provide function which converts reader to batch reader, frequently used reader creators and reader decorators.
 
 ## Data Reader Interface
 
@@ -37,9 +38,54 @@ def reader_creator_random_imageand_label(widht, height, label):
 	return reader
 ```
 
+## Batch Reader Interface
+
+*batch reader* can be any function with no parameter that creates a iterable (anything can be used in `for x in iterable`). The output of the iterable should be a batch (list) of data items. Each item inside the list must be a tuple.
+
+Here are valid outputs:
+```python
+# a mini batch of three data items. Each data item consist three columns of data, each of which is 1.
+[(1, 1, 1),
+(2, 2, 2),
+(3, 3, 3)]
+
+# a mini batch of three data items, each data item is a list (single column).
+[([1,1,1],),
+([2,2,2],),
+([3,3,3],),
+```
+
+Please note that each item inside the list must be a tuple, below is an invalid output:
+```python
+ # wrong, [1,1,1] needs to be inside a tuple: ([1,1,1],).
+ # Otherwise it's ambiguous whether [1,1,1] means a single column of data [1, 1, 1],
+ # or three column of datas, each of which is 1.
+[[1,1,1],
+[2,2,2],
+[3,3,3]]
+```
+
+It's easy to convert from reader to batch reader:
+```python
+mnist_train = paddle.dataset.mnist.train()
+mnist_train_batch_reader = paddle.batch(mnist_train, 128)
+```
+
+Also easy to create custom batch reader:
+```python
+def custom_batch_reader():
+	while True:
+		batch = []
+		for i in xrange(128):
+			batch.append((numpy.random.uniform(-1, 1, 28*28),)) # note that it's a tuple being appended.
+		yield batch
+
+mnist_random_image_batch_reader = custom_batch_reader
+```
+
 ## Usage
 
-data reader, mapping from item(s) read to data layer, batch size and number of total pass will be passed into `paddle.train`:
+batch reader, mapping from item(s) read to data layer, batch size and number of total pass will be passed into `paddle.train`:
 
 ```python
 # two data layer is created:
@@ -47,8 +93,8 @@ image_layer = paddle.layer.data("image", ...)
 label_layer = paddle.layer.data("label", ...)
 
 # ...
-
-paddle.train(paddle.dataset.mnist, {"image":0, "label":1}, 128, 10, ...)
+batch_reader = paddle.batch(paddle.dataset.mnist.train(), 128)
+paddle.train(batch_reader, {"image":0, "label":1}, 128, 10, ...)
 ```
 
 ## Data Reader Decorator
@@ -64,7 +110,7 @@ Since reading data may take time and training can not proceed without data. It i
 Use `paddle.reader.buffered` to prefetch data:
 
 ```python
-buffered_reader = paddle.reader.buffered(paddle.dataset.mnist, 100)
+buffered_reader = paddle.reader.buffered(paddle.dataset.mnist.train(), 100)
 ```
 
 `buffered_reader` will try to buffer (prefetch) `100` data entries.
@@ -91,10 +137,10 @@ def reader_creator_bool(t):
 true_reader = reader_creator_bool(True)
 false_reader = reader_creator_bool(False)
 
-reader = paddle.reader.compose(paddle.dataset.mnist, data_reader_creator_random_image(20, 20), true_reader, false_reader)
-# Skipped 1 because paddle.dataset.mnist produces two items per data entry.
+reader = paddle.reader.compose(paddle.dataset.mnist.train(), data_reader_creator_random_image(20, 20), true_reader, false_reader)
+# Skipped 1 because paddle.dataset.mnist.train() produces two items per data entry.
 # And we don't care second item at this time.
-paddle.train(reader, {"true_image":0, "fake_image": 2, "true_label": 3, "false_label": 4}, ...)
+paddle.train(paddle.batch(reader, 128), {"true_image":0, "fake_image": 2, "true_label": 3, "false_label": 4}, ...)
 ```
 
 ### Shuffle
@@ -103,16 +149,20 @@ Given shuffle buffer size `n`, `paddle.reader.shuffle` will return a data reader
 
 Example:
 ```python
-reader = paddle.reader.shuffle(paddle.dataset.mnist, 512)
+reader = paddle.reader.shuffle(paddle.dataset.mnist.train(), 512)
 ```
 
 ## Q & A
 
-### Why return only a single entry, but not a mini batch?
+### Why reader return only a single entry, but not a mini batch?
+
+Always returning a single entry make reusing existing data readers much easier (e.g., if existing reader return not a single entry but 3 entries, training code will be more complex because it need to handle cases like batch size 2).
+
+We provide function `paddle.batch` to turn (single entry) reader into batch reader.
 
-If a mini batch is returned, data reader need to take care of batch size. But batch size is a concept for training, it makes more sense for user to specify batch size as a parameter for `train`.
+### Why do we need batch reader, isn't train take reader and batch_size as arguments sufficient?
 
-Practically, always return a single entry make reusing existing data readers much easier (e.g., if existing reader return not a single entry but 3 entries, training code will be more complex because it need to handle cases like batch size 2).
+In most of the case, train taking reader and batch_size as arguments would be sufficent. However sometimes user want to customize order of data entries inside a mini batch. Or even change batch size dynamically.
 
 ### Why use a dictionary but not a list to provide mapping?
 
@@ -137,7 +187,7 @@ def image_reader_creator(image_path, label_path, n):
 
 # images_reader_creator creates a reader
 reader = image_reader_creator("/path/to/image_file", "/path/to/label_file", 1024)
-paddle.train(reader, {"image":0, "label":1}, ...)
+paddle.train(paddle.batch(reader, 128), {"image":0, "label":1}, ...)
 ```
 
 ### How is `paddle.train` implemented
@@ -145,17 +195,8 @@ paddle.train(reader, {"image":0, "label":1}, ...)
 An example implementation of paddle.train could be:
 
 ```python
-def make_minibatch(reader, minibatch_size):
-	def ret():
-		r = reader()
-		buf = [r.next() for x in xrange(minibatch_size)]
-		while len(buf) > 0:
-			yield buf
-			buf = [r.next() for x in xrange(minibatch_size)]
-	return ret
-
-def train(reader, mapping, batch_size, total_pass):
+def train(batch_reader, mapping, batch_size, total_pass):
 	for pass_idx in range(total_pass):
-		for mini_batch in make_minibatch(reader): # this loop will never end in online learning.
+		for mini_batch in batch_reader(): # this loop will never end in online learning.
 			do_forward_backward(mini_batch, mapping)
 ```