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‫Streams are yet another fundamental concept

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‫in Node.JS.

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‫So let's now learn all about them.

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‫So first of all, what are streams?

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‫Well, with streams we can process meaning read and write

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‫data piece by piece without completing the whole read

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‫or write operation.

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‫Therefore we don't have to keep all the data in memory

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‫to do these operations.

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‫For example, when we read a file using streams,

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‫we read part of the data, do something with it,

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‫then free our memory, and repeat this until the entire file

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‫has been processed.

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‫Or think of YouTube or Netflix,

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‫which are both called streaming companies

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‫because they stream video using the same principle.

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‫So instead of waiting until the entire video file loads,

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‫the processing is done piece by piece or in chunks

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‫so that you can start watching even before the entire file

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‫has been downloaded.

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‫So the principal here is not just about Node.JS.

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‫But universal to computer science in general.

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‫So as you can see, this makes streams the perfect candidate

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‫for handing large volumes of data like for example,

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‫video or also data that we're receiving piece by piece

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‫from an external source.

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‫Also, streaming make the data processing more efficient

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‫in terms of memory because there is no need

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‫to keep all the data in memory and also in terms of time

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‫because we can start processing the data as it arrives,

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‫rather than waiting until everything arrives.

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‫Okay, so now that we know what streams are,

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‫let's talk a little bit about how they are implemented

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‫in Node.JS.

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‫So in Node, there are four fundamental types of streams:

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‫readable streams, writable streams, duplex streams,

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‫and transform streams.

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‫But the readable and writeable ones

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‫are the most important ones.

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‫And so we're gonna focus more on these two.

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‫So readable streams are the ones from which we can read.

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‫We can consume data, makes sense right?

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‫Now streams are everywhere in the core Node modules.

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‫So a bit like events, like we talked about before.

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‫For example, the data that comes in when an http server

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‫gets a request is actually a readable stream.

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‫So all the data that is sent with the request comes in

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‫piece by piece and not in one large piece.

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‫Also another example from the file system

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‫is that we can read a file piece by piece

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‫by using a read screen from the FS module,

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‫which can actually be quite useful for large text files.

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‫All right, now another important thing to note

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‫is that streams are actually instances

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‫of the EventEmitter class.

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‫Meaning that all streams can emit and listen

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‫to named events.

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‫Just like we learned in the last lecture.

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‫In the case of readable streams, they can emit,

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‫and we can listen to many different events.

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‫But the most important two are the data and the end events.

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‫The data event is emitted when there is a new piece

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‫of data to consume,

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‫and the end event is emitted as soon as

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‫there is no more data to consume.

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‫And of course, we can then react

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‫to these events accordingly.

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‫And actually we're gonna do exactly that in the next video.

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‫So to practice how to work with streams.

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‫Finally, besides events, we also have important functions

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‫that we can use on streams.

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‫And in the case of readable streams,

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‫the most important ones are the pipe and the read functions.

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‫And again, you will see these in action in the next video,

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‫especially the super important pipe function,

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‫which basically allows us to plug streams together,

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‫passing data from one stream to another

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‫without having to worry much about events at all.

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‫Okay, next up, writeable streams are the ones

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‫to which we can write data.

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‫So basically, the opposite of readable streams.

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‫A great example is the http response that we can send back

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‫to the client and which is actually a writeable stream.

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‫So a stream that we can write data into.

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‫So when we want to send data, we have to write it somewhere,

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‫right?

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‫And that somewhere is a writeable stream,

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‫and that makes perfect sense, right?

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‫For example, if we wanted to send a big video file

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‫to a client, we would stream that result

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‫just like Netflix or YouTube do.

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‫Now about events, the most important ones are

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‫the drain and the finish events.

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‫And the most important functions are

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‫the write and end functions,

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‫some of which you will see in action in the next lecture.

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‫Now quickly about duplex streams.

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‫They're simply streams that are both readable and writeable

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‫at the same time.

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‫These are a bit less common.

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‫But anyway, a good example would be a web socket

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‫from the net module.

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‫And a web socket is basically just a communication channel

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‫between client and server that works in both directions

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‫and stays open once the connection has been established.

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‫Finally, transform streams are duplex streams,

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‫so streams that are both readable and writeable,

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‫which at the same time can modify or transform

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‫the data as it is read or written.

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‫A good example of this one is the zlib core module

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‫to compress data which actually uses a transform stream.

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‫All right, and that's the four types of streams

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‫and a broad overview of how we can use them.

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‫Now there is something important to mention here

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‫before we move on,

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‫which is a fact that these events and functions

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‫that I showed you are for consuming streams

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‫that are already implemented like the ones

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‫that I showed you here as our examples.

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‫So for example, Node implemented these http requests

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‫and responses as streams,

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‫and we can then consume, we can use them

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‫using the events and functions that are available

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‫for each type of stream.

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‫We could of course also implement our own streams

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‫and then consume them using these same events and functions.

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‫That however would be a video for another time

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‫because in order to build most apps,

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‫it's most important to know how to actually consume streams,

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‫not really how to implement them.

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‫All right, so let's now move on

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‫and actually use streams in practice.