Stream

“stream”即“流”，stream是一种非常强大的**抽象结构**，在computer science中被广泛应用：计算机科学是关于**数据**的科学，我们的计算机系统处理着各种各样的**数据**，我们可以形象地将**数据**的传递看做是"水流"一般: **数据**就像**水流**的流转一般在**计算机系统**中进行流转。

NOTE: 上面这段话中的**数据**、**计算机系统**都是抽象概念，它们可以指代很多内容

Stream可以用做描述**数据流**的**抽象结构**。

wikipedia Stream (computing)

In computer science, a stream is a sequence of data elements made available over time. A stream can be thought of as items on a conveyor belt (传送带) being processed one at a time rather than in large batches.

TRANSLATION: 在计算机科学中，stream是随时间推移可用的一系列数据元素。 可以将stream视为传送带上的物品，一次一个地处理，而不是大批量处理。

Streams are processed differently from batch data – normal functions cannot operate on streams as a whole(不能够整个的操作stream), as they have potentially unlimited data, and formally, streams are codata (potentially unlimited), not data (which is finite).

NOTE: 上面这段话所论述的是stream 和 batch。

Functions that operate on a stream, producing another stream, are known as filters, and can be connected in pipelines, analogously to function composition. Filters may operate on one item of a stream at a time, or may base an item of output on multiple items of input, such as a moving average.

NOTE:

1、上面这段话其实给出了stream的programming model，显然，它非常类似于functional programming的。

2、上述function composition，是如下形式:

z = f(g(x))

即上一个函数的输出作为下一个函数的输入，这是典型的pipeline

Examples

The term "stream" is used in a number of similar ways:

1) "Stream editing", as with sed, awk, and perl. Stream editing processes a file or files, in-place, without having to load the file(s) into a user interface. One example of such use is to do a search and replace on all the files in a directory, from the command line.

2) On Unix and related systems based on the C language, a stream is a source or sink of data, usually individual bytes or characters. Streams are an abstraction used when reading or writing files, or communicating over network sockets. The standard streams are three streams made available to all programs.

3) I/O devices can be interpreted as streams, as they produce or consume potentially unlimited data over time.

NOTE: 2、3所指为stream-based IO。

4) In object-oriented programming, input streams are generally implemented as iterators.

NOTE: 如何对iterator进行操作？

5) In the Scheme language and some others, a stream is a lazily evaluated or delayed sequence of data elements. A stream can be used similarly to a list, but later elements are only calculated when needed. Streams can therefore represent infinite sequences and series.

6) In the Smalltalk standard library and in other programming languages as well, a stream is an external iterator. As in Scheme, streams can represent finite or infinite sequences.

7) Stream processing — in parallel processing, especially in graphic processing, the term stream is applied to hardware as well as software. There it defines the quasi-continuous flow of data that is processed in a dataflow programming language as soon as the program state meets the starting condition of the stream.

NOTE: stream processing是parallel computing中的重要model，参见工程parallel-computing的Model\Stream-model章节。

Applications

Streams can be used as the underlying data type for channels in interprocess communication.

Other uses

NOTE: 非常重要的描述

Stream的单位

对stream的单位、组成的分析是非常重要的，一般将单位放到具体stream的命名中，下面是我目前所遇到的具体stream:

具体stream	说明
Bitstream	单位是bit
byte stream	单位是byte
character stream	单位是character
data stream	单位是data，显然data是一个抽象的概念，它是big data、parallel computing等时代背景的产物。 另外与它相关的是: dataflow programming paradigm，其实dataflow就是data stream
message/event stream	单位是message/event，参见下面的Stream-based message/event processing system章节

需要注意的是，上面这些stream并不是具体的分类，更多的是在具体的实际应用中的抽象。

Byte stream VS character stream

geeksforgeeks Character Stream Vs Byte Stream in Java

stackoverflow Byte Stream and Character stream

iitk Character Streams versus Byte Streams

通过上述文章可以总结：byte stream和character stream的主要差别在于每次处理的unit不同：

byte stream的unit是byte，character stream的unit是character；一个character由一个或者多个byte组成。

Byte stream VS bit stream

stackoverflow Difference between byte stream and bit stream

wikipedia Bitstream

A bitstream (or bit stream), also known as binary sequence, is a sequence of bits.

A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity (octets), and so the term octet stream is sometimes used interchangeably. An octet may be encoded as a sequence of 8 bits in multiple different ways (see endianness) so there is no unique and direct translation between bytestreams and bitstreams.

Bitstreams and bytestreams are used extensively in telecommunications and computing. For example, synchronous bitstreams are carried by SONET, and Transmission Control Protocol transports an asynchronous bytestream.

Stream的方向

在使用stream来进行抽象的时候，我们可以考虑stream的方向：

• 源: 数据从何处流入
• 目的: 数据流向何处

特性

stream有着非常好的特性，这些优良的特性决定了它在computer science中的广泛应用。

结构的简单

stream的结构是非常简单的: 相邻关系，这就决定了它是线性的，这种结构是一个非常普遍的结构，在computer science中广泛存在。

有序性

stream中的元素默认是有序的。

Computation的简单

stream结构的简单决定了对它进行computation是非常简单的。stream结构是discrete的，我们可以one-by-one地来对它计算，一种最最常见的方式就是iteration，关于此，参见工程discrete的Relation-structure-computation\Computation\Iteration章节，关于此，在维基百科Stream (computing)中也进行了详细地介绍。

Application

Stream-base IO

参见:

1) 工程Linux-OS的Programming\IO\IO-流派\Stream章节。

2) 工程programming-language的C-family-language\C++\Library\Standard-library\IO-library章节。

Stream-oriented protocol

TCP就是典型的例子，参见工程Linux-OS的Network\Theory\TCP章节。

Parallel computing

参见工程parallel-computing的Model\Stream-model章节。

Stream-based message/event processing system

参见工程parallel-computing的Application\Message-processing-system\Stream-based-message-processing-system章节。

Dataflow programming paradigm

关于Dataflow programming paradigm，参见工程programming-language的Theory\Programming-paradigm\Dataflow-programming章节。

TensorFlow

TensorFlow就是典型的采用dataflow programming的，在TensorFlow的中，显然它的stream的单位是tensor。