Brad Brad - 1 year ago 122
Node.js Question

child_process stream backpressure

I'm using

with Node.js and piping that stream through a few other transform streams, eventually through a
stream to limit the data rate. The braking stream seems to have no effect, and in fact data eventually is lost at the end of a long chain.


What I think is happening is that the
stream (within
) is not pausing, and therefore buffers fill until data is lost.

streams behave this way? Is there any way to get backpressure to work properly with

Answer Source

I'm not familiar enough with exec-stream and node-brake to understand all the avenues for data-loss.

However, I did a little experiment to see if node-brake has a back-pressure effect, which you mentioned could be a potential area for data-loss.

File is also hosted on Gist.

Created for
Please pardon the CoffeeScript, but I couldn't stand to extend stream.Transform in native JavaScript.
fs = require("fs")
execStream = require("exec-stream")
brake = require("brake")

file = fs.createWriteStream("tmp.txt")

class Double extends require("stream").Transform
  _transform: (chunk, enc, cb) ->
    @_last ?=
    @_called ?= []
    @_called.push - @_last
    @_last =
    @push chunk.toString() + chunk.toString()

class UpperCase extends require("stream").Transform
  _transform: (chunk, enc, cb) ->
    @push chunk.toString().toUpperCase()

sum = (nums) ->
  o = 0
  o += i for i in nums

doTest = (size) ->

  transform1 = new Double()
  transform2 = new UpperCase()
  transform3 = new Double()

  execStream("dd", ["if=/dev/urandom", "bs=1024", "count=1"])

  file.on "finish", ->
    fs.stat "tmp.txt", (err, stats) ->
      throw err if err
      called1 = transform1._called
      averagePreBrake = sum(called1) / called1.length
      called2 = transform3._called
      averagePostBrake = sum(called2) / called2.length
      console.log """
        Generated with brake(#{size}): #{stats.size}
        Average time between transformations pre-brake:  #{averagePreBrake}ms
        Average time between transformations post-brake: #{averagePostBrake}ms

doTest 1024
doTest 256

Results of the process are below.

stdout from above Coffeescript

I notice that there are no gaps between transformations before the brake. However, the brake disrupts transformations later down the line. Given this data, I suspect that node-brake has no back-pressure effect.

[Screenshot of console output generated by my sh2png utility]

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