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https://github.com/joyent/node-vstream

instrumented streams
https://github.com/joyent/node-vstream

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instrumented streams

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# node-vstream: instrumented streams



When working with Node streams, particularly object-mode streams, it's often

helpful to be able to inspect a pipeline.  This module instruments objects to

provide:



**For all objects**:



* a name: used in debug output and especially useful for data pipelines

* custom counters: create your own counters for events of interest like

  errors, requests handled, messages sent, or the like

* custom warnings: Error objects that are counted, forwarded to subscribers, but

  otherwise ignored



**For all streams**:



* forward and back pointers: vstream watches `pipe` events and records upstreams

  and downstreams.  You can walk to the head of a pipeline and iterate

  downstream streams.

* debug methods to dump stream state, including high watermarks and data

  buffered on the upstream and downstream sides



**For object-mode transform streams**:



* automatic counters for inputs processed and outputs emitted

* provenance: history information for objects passed through the pipeline (so

  you can report errors with precise information, like "object X from line N")



## Usage



### Instrumenting a simple pipeline



You can instrument one or more regular Node streams by calling

`vstream.wrapStream` on them.  wrapStream returns the same stream, but attaches

a few new functions (and private properties).  Here's a simple pipeline that

reads data from "/usr/bin/more", sends it through a pass-through stream, and

then writes it to "/dev/null":



```javascript

var instream, passthru, outstream;

instream = vstream.wrapStream(fs.createReadStream('/usr/bin/more'), 'source');

passthru = vstream.wrapStream(new stream.PassThrough(), 'passthru');

outstream = vstream.wrapStream(fs.createWriteStream('/dev/null'), 'devnull');



instream.pipe(passthru);

passthru.pipe(outstream);

```



As an example, we'll attach a listener to each `'data'` event that dumps the

debug information from each stream in the pipeline.  The debug information

includes the stream's name, what kind of stream it is (readable, writable, or

duplex), the amount of data buffered, and the high watermark.  We'll also dump

this when the pipeline finishes (when the last stream emits `'finish'`).



```javascript

instream.on('data', report);

outstream.on('finish', report);



function report()

{

	var head = outstream.vsHead();

	assert.ok(head == instream);

	head.vsWalk(function (stream) { stream.vsDumpDebug(process.stdout); });

	console.error('-----');

}

```



On my system, this prints:



```

source               (readable, rbuf: 0/65536)

passthru             (duplex, wbuf: 0/16384, rbuf: 0/16384)

devnull              (writable, wbuf: 65536/16384)

-----

source               (readable, rbuf: 0/65536)

passthru             (duplex, wbuf: 0/16384, rbuf: 65536/16384)

devnull              (writable, wbuf: 65536/16384)

-----

source               (readable, rbuf: 0/65536)

passthru             (duplex, wbuf: 0/16384, rbuf: 6640/16384)

devnull              (writable, wbuf: 65536/16384)

-----

source               (readable, rbuf: 0/65536)

passthru             (duplex, wbuf: 0/16384, rbuf: 0/16384)

devnull              (writable, wbuf: 0/16384)

-----

```



"source", "passthrough", and "devnull" are the names we gave these streams,

which are readable, duplex, and writable, respectively.  Notice that the default

high watermark for a readable file stream ("source") is 64K, or 65536 bytes.

The default high watermarks for the PassThrough and the writable file streams

are 16K, or 16384 bytes.



Seeing this while writing this example, I picked /usr/bin/more because it's

just over 128K.  On my system, this causes Node to read two full 64K chunks,

plus one smaller chunk.  This makes for an interesting example:



* When we get the first `data` event (the first printout above), the data has

  already been written to the PassThrough, which wrote it downstream to the

  /dev/null stream.  The /dev/null stream has it all buffered, since it hasn't

  had a chance to do I/O yet.  The /dev/null stream buffered all 64K, not just

  16K, presumably because it came in as one chunk.  (Remember that the high

  watermark is a guideline, but it's possible to buffer more bytes than that --

  as we see here.)

* When we get the second `data` event (the second printout above), the data is

  still buffered on the /dev/null stream and has backed up to the PassThrough.

  For whatever reason, the /dev/null stream isn't keeping up with the source

  read stream, and we can see the buffering here.  This is flow control

  (backpressure) in action.

* At this point, if the /dev/null stream were totally blocked, we'd expect the

  PassThrough stream to buffer at least 16K on the *write* side as well, then

  the source would back up on its readable side, and then we'd stop reading from

  the original file.

* This doesn't happen, because by the time we get the third `data` event, the

  /dev/null stream must have written some data out, because the PassThrough has

  fewer bytes buffered.

* When the pipeline has finished (the last printout), there are no bytes

  buffered anywhere.



The example's pretty trivial, but you can learn a lot about Node streams by

understanding what's going on at each stage.  Of course, the real point of

`vstream` isn't to demonstrate this, but to help debug situations where things

*aren't* going as expected.  The debug information can help you understand where

data has buffered up way more than you wanted (usually a memory leak) or where

the pipeline's plugged up.



### Instrumenting a transform stream



When you wrap Transform streams, vstream modifies `_transform`, `_flush`, and

`push` to keep track of which outputs were generated by which inputs.  You can

use this to generate useful error messages when you encounter bad input.  Here's

a little program that reads /etc/passwd and emits a warning when it finds the

"nobody" user:



```javascript

var instream, linestream, mystream;

var user = 'nobody';



/*

 * Read the contents of /etc/passwd and chunk it into lines.

 */

instream = vstream.wrapStream(fs.createReadStream('/etc/passwd'), 'source');

linestream = vstream.wrapTransform(new lstream());



/*

 * Pipe the lines into a stream that looks for the "nobody" user and emits a

 * warning, which will include the line number.

 */

mystream = new stream.Transform({ 'objectMode': true });

mystream._transform = function myTransform(line, _, callback) {

	if (line.substr(0, user.length + 1) == user + ':') {

		this.push(user);

		this.vsWarn(new Error('found "' + user + '"'), 'nfoundusers');

	}



	callback();

};

mystream = vstream.wrapTransform(mystream, 'UserSearcher');



instream.pipe(linestream);

linestream.pipe(mystream);



mystream.on('warn', function (context, kind, error) {

	console.log('kind:    %s', kind);

	console.log('error:   %s', error.message);

	console.log('context: %s', context.label());



	mystream.vsHead().vsWalk(function (s) {

		s.vsDumpDebug(process.stdout);

	});

});

```



When I run this on OS X Mavericks, I get:



```

kind:    nfoundusers

error:   found "nobody"

context: UserSearcher input 11 from LineStream input 1: value 'nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/false'

source               (readable, rbuf: 0/65536)

LineStream           (duplex, wbuf: 1/16384, rbuf: 0/16384)

    ninputs:         1

    noutputs:        11

UserSearcher         (duplex, wbuf: 1/16384, rbuf: 1/16384)

    nfoundusers:     1

    ninputs:         11

    noutputs:        1

```



The context for each warning keeps track of the history through all

vstream-wrapped Transform streams.  vstream also bumps a counter for each

warning, which is why "nfoundusers" is 1.



## PipelineStream



vstream also provides a PipelineStream class, which takes an array of streams

that should be piped into each other (A piped to B piped to C) and wraps them

into a single stream P.  Writes to P are directed to A, and reads from P read

from C.  This is mainly useful when you want to expose a single stream that's

logically made up of a couple of existing streams.



## Design notes



A key design principle for this module is that it should be possible to

usefully instrument streams that were not written to support this module.  Some

features (like bumping custom counters or emitting warnings) may require adding

code, but basic features should work without special support, and instrumenting

a stream should not break existing functionality.  That's why this module is

implemented (somewhat regrettably) by attaching properties to existing objects

rather than requiring users to inherit from a custom class.



Public properties attached to instrumented objects use camel case and start with

a "vs" prefix to avoid colliding with other properties a user might be using.

Similarly, private properties use snake case with a "vs\_" prefix.



## Caveats



### Memory usage



vstream tracks pipes to add upstream and downstream references, but it does not

track unpipes to remove these references.  That's because streams are unpiped

when the end-of-stream is reached, but this is often when it's most useful to

debug the pipeline, so it's useful to keep these references around.  As a

result, if you keep a reference to any stream in the pipeline, you'll likely

have references to the whole pipeline.  If you implement a stream that makes

heavy use of pipe and unpipe, you may end up referencing more memory than you'd

expect.  In all cases, once all references to all streams in the pipeline have

been removed, all of these objects can be garbage collected.



### API stability



The stream debugging information relies on accessing the internal Node state of

the stream, which is not guaranteed to remain stable across Node releases.  It's

possible that Node upgrades could break this behavior.  If you notice incorrect

output or unexpected crashes, please file an issue.



## Contributions



Contributions welcome, and should be 'make prepush' clean.  The prepush checks

use [javascriptlint](https://github.com/davepacheco/javascriptlint) and

[jsstyle](https://github.com/davepacheco/jsstyle).