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writable stream _write and _writev #28408

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roman-io opened this issue Jun 24, 2019 · 6 comments · May be fixed by #28690
Closed

writable stream _write and _writev #28408

roman-io opened this issue Jun 24, 2019 · 6 comments · May be fixed by #28690
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@roman-io
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@roman-io roman-io commented Jun 24, 2019

Version: v12.4.0 or v10.15.3
Platform: Linux 4.15.0-51-generic #55~16.04.1-Ubuntu SMP Thu May 16 09:24:37 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Subsystem: Stream Duplex, Object mode, piped into itself.

The documentation says that when I implement _writev method, it will receive chunks of data available in the buffer.

What I actually see is that both _write and _writev methods are used simultaneously.
If _write is fast enough to process the buffer until it fills up to more than one object, then only _write is used. Otherwise, for the first object in the buffer _write will be used, and the rest will be sent to _writev.

Example:

DEBUG: Calling _write with 1
DEBUG: Calling _writev with 15
DEBUG: Calling _write with 1
DEBUG: Pushing null to readable
DEBUG: Calling _writev with 15
DEBUG: Calling _write with 1
DEBUG: Calling _writev with 2
DEBUG: Writable final
DEBUG: _writev 16, _write 16
DEBUG: Read 243, Written 243

Is this an expected behavior? I was assuming that when _writev method is implemented, it will be used exclusively.

Thanks
@Fishrock123 Fishrock123 added the stream label Jun 28, 2019
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@lpinca lpinca commented Jul 10, 2019

This is by design. The first chunk is written with _write(). When the write is complete, buffered chunks, if available, will be written all at once with _writev().
@addaleax addaleax added the doc label Jul 10, 2019
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@addaleax addaleax commented Jul 10, 2019

I’ve added the doc label because this is indeed missing from the documentation and I think a PR to address that would make sense.
tyof45 added a commit to tyof45/node that referenced this issue Jul 15, 2019
@tyof45
docs: fix description of _writev()
53d83f0 
The documentation gave the impression that when the _writev method is
used, it is used instead of _write. However, when _writev is used,
_write loads the first chunk of data, then _writev loads all the
remaining bufferred chunks in the write queue. The docs have been
changed to reflect this behavior.

Fixes: nodejs#28408
@tyof45 tyof45 mentioned this issue Jul 15, 2019
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@asbillings07

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@asbillings07 asbillings07 commented Oct 4, 2019

Does this still need to be worked on? If so I'd like to take it.
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@ronag ronag commented Oct 8, 2019

This might be relevant #29639
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@addaleax addaleax commented Oct 8, 2019

@asbillings07 You might want to look at #28690 – it’s an open PR for this, where @tyof45 started working on it but that has kind of stalled out. I’d suggest waiting a short time to see if there’s any movement in that PR, and if not, you can feel free to open a new one.
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@asbillings07 asbillings07 commented Oct 8, 2019 •
edited

@addaleax Okay sounds good.
@Trott Trott removed the Hacktoberfest label Nov 3, 2019
HarshithaKP added a commit to HarshithaKP/node that referenced this issue Jan 14, 2020
@HarshithaKP
doc: explain _writev() API
Loading status checks…
e8d1197 
the exact context of invocation of _writev API is not well known
also, the choice between _write and _writev is not well known.
add a description to make it explicit.

Fixes: nodejs#28408
@HarshithaKP HarshithaKP mentioned this issue Jan 14, 2020
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3 of 4 tasks complete
HarshithaKP added a commit to HarshithaKP/node that referenced this issue Jan 16, 2020
@HarshithaKP @tyof45
doc: explain _writev() API
Loading status checks…
e2f3c96 
the exact context of invocation of _writev API is not well known
also, the choice between _write and _writev is not well known.
add a description to make it explicit.

Fixes: nodejs#28408
Refs: nodejs#28690

Co-authored-by: Parker Bjur <bjur.parker45@gmail.com>
@Trott Trott closed this in e47a1eb Jan 16, 2020
codebytere added a commit that referenced this issue Feb 17, 2020
@HarshithaKP @codebytere
doc: explain _writev() API
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codebytere Shelley Vohr
GPG key ID: F13993A75599653C Learn about signing commits
49c5b4f 
The exact context of invocation of _writev API is not well known.
Also, the choice between _write and _writev is not well known.
Add a description to make it explicit.

Fixes: #28408
Refs: #28690

Co-authored-by: Parker Bjur <bjur.parker45@gmail.com>

PR-URL: #31356
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
codebytere added a commit that referenced this issue Feb 17, 2020
@HarshithaKP @codebytere
doc: explain _writev() API
Verified
This commit was signed with a verified signature.
codebytere Shelley Vohr
GPG key ID: F13993A75599653C Learn about signing commits
bd54abe 
The exact context of invocation of _writev API is not well known.
Also, the choice between _write and _writev is not well known.
Add a description to make it explicit.

Fixes: #28408
Refs: #28690

Co-authored-by: Parker Bjur <bjur.parker45@gmail.com>

PR-URL: #31356
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
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Labels
doc good first issue stream
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@roman-io @lpinca @addaleax @asbillings07 @ronag @Trott @Fishrock123 @trivikr
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