There's a syscall called `writev` that lets you write an array (ie,
"Vector") of buffers of data rather than a single buffer.
I'd like to support something like this for Streams in Node, mostly
because it will allow us to save a lot of TCP write() calls, without
having to copy data around, especially for chunked encoding writes.
(We write a lot of tiny buffers for HTTP, it's kind of a nightmare,
actually.)
Fedor Indutny has already done basically all of the legwork to
implement this. Where we're stuck is the API surface, and here are
some options. Node is not a democracy, but your vote counts anyway,
especially if it's a really good vote with some really good argument
behind it :)
Goals:
1. Make http more good.
2. Don't break existing streams.
3. Don't make things hard.
4. Don't be un-node-ish
For all of these, batched writes will only be available if the
Writable stream implements a `_writev()` method. No _writev, no
batched writes. Any bulk writes will just be passed to _write(chunk,
encoding, callback) one at a time in the order received.
In all cases, any queued writes will be passed to _writev if that
function is implemented, even if they're just backed up from a slow
connection.
Ideas:
A) stream.bulk(function() { stream.write('hello');
stream.write('world'); stream.end('!\n') })
Any writes done in the function passed to `stream.bulk()` will be
batched into a single writev.
Upside:
- Easier to not fuck up and stay frozen forever. There is basically
zero chance that you'll leave the stream in a corked state. (Same
reason why domain.run() is better than enter()/exit().)
Downsides:
- easier to fuck up and not actually batch things. eg,
s.bulk(function(){setTimeout(...)})
- bulk is a weird name. "batch" maybe? Nothing else really seems
appropriate either.
- somewhat inflexible, since all writes have to be done in the same
function call
B) stream.cork(); stream.write('hello'); stream.write('world');
stream.end('!\n'); stream.uncork();
Any writes done while corked will be flushed to _writev() when uncorked.
Upside:
- Easy to implement
- Strictly more flexible than stream.bulk(writer). (Can trivially
implement a bulk function using cork/uncork)
- Useful for cases outside of writev (like corking a http request
until the connection is established)
Downsides:
- Easy to fuck up and stay corked forever.
- Two functions instead of just one (double the surface area increase)
C) stream.writev([chunks,...], [encodings,...], callback)
That is, implement a first-class top-level function called writev()
which you can call with an array of chunks and an array of encodings.
Upside:
- No unnecessary surface area increase
- NOW IT'S YOUR PROBLEM, NOT MINE, HAHA! (Seriously, though, it's
less magical, simpler stream.Writable implementation, etc.)
Downside:
- A little bit tricky when you don't already have a list of chunks to
send. (For example, with cork, you could write a bunch of stuff into
it, and then uncork all at the end, and do one writev, even if it took
a few ms to get it all.)
- parallel arrays, ew.
D) stream.writev([ {chunk:buf, encoding: blerg}, ...], callback)
That is, same as C, but with an array of {chunk,encoding} objects
instead of the parallel arrays.
Same +/- as C, except the parallel array bit. This is probably how
we'd call the implementation's stream._writev() anyway, so it'd be a
bit simpler.
Which of these seems like it makes the most sense to you?
Is there another approach that you'd like to see here? (Note: "save
all writes until end of tick always" and "copy into one big buffer"
approaches are not feasible for obvious performance reasons.)
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