CODEWITHSUNDEEP
javascriptnode.js
Kartik Anand
Kartik Anand

Reputation: 4609

How can you pipe a readable stream to another readable stream?

I am doing exercises from the "stream-adventure", and I'm very uncertain about the stream-combiner module.

Specifically it says that:

The stream-combiner module creates a pipeline from a list of streams, returning a single stream that exposes the first stream as the writable side and the last stream as the readable side like the duplexer module, but with an arbitrary number of streams in between. Unlike the duplexer module, each stream is piped to the next. For example:

var combine = require('stream-combiner');
var stream = combine(a, b, c, d);

will internally do a.pipe(b).pipe(c).pipe(d) but the stream returned by combine() has its writable side hooked into a and its readable side hooked into d.

Now as it says, "its writable side hooked into a and its readable side hooked into d", one can use the above stream like follows:

someReadableStream.pipe(stream).pipe(someWritableStream)

But won't the above just become:

someReadableStream.pipe(a).pipe(b).pipe(c).pipe(d).pipe(someWritableStream)

My question is how can a readable stream pipe to another readable stream. And at the end, how can the result of piping to a writable stream again pipe to a writable stream.

Upvotes: 2

Views: 1258

Answers (2)

dominic
dominic

Reputation: 571

you are correct that it is the same as readable.pipe(a).pipe(b).pipe(c).pipe(d).pipe(writable), the advantage of combiner is that you cannot return a.pipe(b).pipe(c).pipe(d) from a function, so you must always manually connect them. with combiner you can create a stream out of other streams, and then tidy that away into a function or module.

for this to work, though, a,b,c,d all must be transform streams, that are both readable and writable.

Upvotes: 0

Peter Lyons
Peter Lyons

Reputation: 146134

Here's a textual example that hopefully clarifies the rules about the streams in order for stream-combiner to do something useful:

var rw = combine(r_or_rw, rw1, rw2, rw3, ...rwN, w_or_rw)

So the first stream needs to be at least readable, the interior streams all need to be read/write, and the final stream needs to at least be writeable. The returned new stream is read/write. When you write to the returned stream, that sends new data down through the pipeline, and when you read that gives you data from the end of the pipeline.

Upvotes: 1

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