Faster Transform stream #33397
Description
Some thoughts on how to make Transform streams faster.
Part of the overhead of Transform (and PassThrough) is that it is actually 2 streams, one Writable and one Readable, both with buffering and state management, which are connected together.
We could try to skip this and implement Transform as a Readable which implements the Writable interface and proxies the naming.
e.g.
class FastTransform extends Readable {
constructor(options) {
super(options)
this._writableState = {
length: 0,
needDrain: false,
ended: false,
finished: false
}
}
get writableEnded () {
return this._writableState.ended
}
get writableFinished () {
return this._writableState.finished
}
_read () {
const rState = this._readableState
const wState = this._writableState
if (!wState.needDrain) {
return
}
if (wState.length + rState.length > rState.highWaterMark) {
return
}
wState.needDrain = false
this.emit('drain')
}
write (chunk) {
const rState = this._readableState
const wState = this._writableState
const len = chunk.length
wState.length += len
this._transform(chunk, null, (err, data) => {
wState.length -= len
if (err) {
this.destroy(err)
} else if (data != null) {
this.push(data)
}
this._read()
})
wState.needDrain = wState.length + rState.length > rState.highWaterMark
return wState.needDrain
}
end () {
const wState = this._writableState
wState.ended = true
if (this._flush) {
this._flush(chunk, (err, data) => {
const wState = this._writableState
if (err) {
this.destroy(err)
} else {
if (data != null) {
this.push(data)
}
this.push(null)
wState.finished = true
this.emit('finish')
}
})
} else {
this.push(null)
wState.finished = true
this.emit('finish')
}
}
}
// TODO: Make Writable[Symbol.hasInstance] recognize `FastTransform`.Making this fully backwards compatible with Transform might be difficult.