What's the proper way to handle back-pressure in a node.js Transform stream?

Question

Intro

These are my first adventures in writing node.js server side. It\'s been fun so far but I\'m having some difficulty understanding the proper way to implement

Answer 1

I think Transform is suitable for this, but I would perform the inflate as a separate step in the pipeline.

Here's a quick and largely untested example:

var zlib        = require('zlib');
var stream      = require('stream');
var transformer = new stream.Transform();

// Properties used to keep internal state of transformer.
transformer._buffers    = [];
transformer._inputSize  = 0;
transformer._targetSize = 1024 * 38;

// Dump one 'output packet'
transformer._dump       = function(done) {
  // concatenate buffers and convert to binary string
  var buffer = Buffer.concat(this._buffers).toString('binary');

  // Take first 1024 packets.
  var packetBuffer = buffer.substring(0, this._targetSize);

  // Keep the rest and reset counter.
  this._buffers   = [ new Buffer(buffer.substring(this._targetSize)) ];
  this._inputSize = this._buffers[0].length;

  // output header
  this.push('HELLO WORLD');

  // output compressed packet buffer
  zlib.deflate(packetBuffer, function(err, compressed) {
    // TODO: handle `err`
    this.push(compressed);
    if (done) {
      done();
    }
  }.bind(this));
};

// Main transformer logic: buffer chunks and dump them once the
// target size has been met.
transformer._transform  = function(chunk, encoding, done) {
  this._buffers.push(chunk);
  this._inputSize += chunk.length;

  if (this._inputSize >= this._targetSize) {
    this._dump(done);
  } else {
    done();
  }
};

// Flush any remaining buffers.
transformer._flush = function() {
  this._dump();
};

// Example:
var fs = require('fs');
fs.createReadStream('depth_1000000')
  .pipe(zlib.createInflate())
  .pipe(transformer)
  .pipe(fs.createWriteStream('depth_1000000.out'));