Can you explain the concept of streams?

Question

I understand that a stream is a representation of a sequence of bytes. Each stream provides means for reading and writing bytes to its given backing store. But what is the

Answer 1

I think you need to consider that the backing store itself is often just another abstraction. A memory stream is pretty easy to understand, but a file is radically different depending on which file system you're using, never mind what hard drive you are using. Not all streams do in fact sit on top of a backing store: network streams pretty much just are streams.

The point of a stream is that we restrict our attention to what is important. By having a standard abstraction, we can perform common operations. Even if you don't want to, for instance, search a file or an HTTP response for URLs today, doesn't mean you won't wish to tomorrow.

Streams were originally conceived when memory was tiny compared to storage. Just reading a C file could be a significant load. Minimizing the memory footprint was extremely important. Hence, an abstraction in which very little needed to be loaded was very useful. Today, it is equally useful when performing network communication and, it turns out, rarely that restrictive when we deal with files. The ability to transparently add things like buffering in a general fashion makes it even more useful.

Answer 2

A stream is an abstracting of a sequence of bytes. The idea is that you don't need to know where the bytes come from, just that you can read them in a standardized manner.

For example, if you process data via a stream then it doesn't matter to your code if the data comes from a file, a network connection, a string, a blob in a database etc etc etc.

There's nothing wrong per-se with interacting with the backing store itself except for the fact that it ties you to the backing store implementation.

Answer 3

The point is that you shouldn't have to know what the backing store is - it's an abstraction over it. Indeed, there might not even be a backing store - you could be reading from a network, and the data is never "stored" at all.

If you can write code that works whether you're talking to a file system, memory, a network or anything else which supports the stream idea, your code is a lot more flexible.

In addition, streams are often chained together - you can have a stream which compresses whatever is put into it, writing the compressed form on to another stream, or one which encrypts the data, etc. At the other end there'd be the reverse chain, decrypting, decompressing or whatever.

Answer 4

When I heard about streaming for the first time, it was in the context of live streaming with a webcam. So, one host is broadcasting video content, and the other host is receiving the video content. So is this streaming? Well... yes... but a live stream is a concrete concept, and I think that the question refers to the abstract concept of Streaming. See https://en.wikipedia.org/wiki/Live_streaming

So let's move on.

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Video is not the only resource that can be streamed. Audio can be streamed too. So we are talking about Streaming media now. See https://en.wikipedia.org/wiki/Streaming_media . Audio can be delivered from source to target in numerous of ways. So let's compare some data delivery methods to each other.

Classic file downloading Classic file downloading doesn't happen real-time. Before taking the file to use, you'll have to wait until the download is complete.

Progressive download Progressive download chunks download data from the streamed media file to a temporary buffer. Data in that buffer is workable: audio-video data in the buffer is playable. Because of that users can watch / listen to the streamed media file while downloading. Fast-forwarding and rewinding is possible, offcourse withing the buffer. Anyway, progressive download is not live streaming.

Streaming Happens real-time, and chunks data. Streaming is implemented in live broadcasts. Clients listening to the broadcast can't fast-forwarding or rewind. In video streams, data is discarded after playback.

A Streaming Server keeps a 2-way connection with its client, while a Web Server closes connection after a server response.

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Audio and video are not the only thing that can be streamed. Let's have a look at the concept of streams in the PHP manual.

a stream is a resource object which exhibits streamable behavior. That is, it can be read from or written to in a linear fashion, and may be able to fseek() to an arbitrary location within the stream. Link: https://www.php.net/manual/en/intro.stream.php

In PHP, a resource is a reference to an external source like a file, database connection. So in other words, a stream is a source that can be read from or written to. So, If you worked with fopen(), then you already worked with streams.

An example of a Text-file that is subjected to Streaming:

// Let's say that cheese.txt is a file that contains this content: 
// I like cheese, a lot! My favorite cheese brand is Leerdammer.
$fp = fopen('cheese.txt', 'r');

$str8 = fread($fp, 8); // read first 8 characters from stream. 

fseek($fp, 21); // set position indicator from stream at the 21th position (0 = first position)
$str30 = fread($fp, 30); // read 30 characters from stream

echo $str8; // Output: I like c 
echo $str30; // Output: My favorite cheese brand is L

Zip files can be streamed too. On top of that, streaming is not limited to files. HTTP, FTP, SSH connections and Input/Output can be streamed as well.

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What does wikipedia say about the concept of Streaming?

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 being processed one at a time rather than in large batches.

See: https://en.wikipedia.org/wiki/Stream_%28computing%29 .

Wikipedia links to this: https://srfi.schemers.org/srfi-41/srfi-41.html and the writers have this to say about streams:

Streams, sometimes called lazy lists, are a sequential data structure containing elements computed only on demand. A stream is either null or is a pair with a stream in its cdr. Since elements of a stream are computed only when accessed, streams can be infinite.

So a Stream is actually a data structure.

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My conclusion: a stream is a source that can contains data that can be read from or written to in a sequential way. A stream does not read everything that the source contains at once, it reads/writes sequentially.

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Usefull links:

1. http://www.slideshare.net/auroraeosrose/writing-and-using-php-streams-and-sockets-zendcon-2011 Provides a very clear presentation
2. https://www.sk89q.com/2010/04/introduction-to-php-streams/
3. http://www.netlingo.com/word/stream-or-streaming.php
4. http://www.brainbell.com/tutorials/php/Using_PHP_Streams.htm
5. http://www.sitepoint.com/php-streaming-output-buffering-explained/
6. http://php.net/manual/en/wrappers.php
7. http://www.digidata-lb.com/streaming/Streaming_Proposal.pdf
8. http://www.webopedia.com/TERM/S/streaming.html
9. https://en.wikipedia.org/wiki/Stream_%28computing%29
10. https://srfi.schemers.org/srfi-41/srfi-41.html

Answer 5

I'll keep it short, I was just missing the word here:

Streams are queues usually stored in buffer containing any kind of data.

(Now, since we all know what queues are, there's no need to explain this any further.)

Answer 6

Another point (For reading file situation):

1. stream can allow you to do something else before finished reading all content of the file.
2. you can save memory, because do not need to load all file content at once.