How to implement a writable stream

Question

I want to pipe data from an amazon kinesis stream to a an s3 log or a bunyan log.

The sample works with a file write stream or stdout. How would I implmeny my own writable stream?

//this works
var file = fs.createWriteStream('my.log')
kinesisSource.pipe(file)

this doesn't work saying it has no method 'on'

var stream = {}; //process.stdout works however
stream.writable = true;
stream.write =function(data){
    console.log(data);
};
kinesisSource.pipe(stream);

what methods do I have to implement for my own custom writable stream, the docs seem to indicate I need to implement 'write' and not 'on'

Answer 1

To create your own writable stream, you have three possibilities.

Create your own class

For this you'll need:

1. To extend the Writable class.
2. To call the Writable constructor in your own constructor.
3. To define a _write() method in the prototype of your stream object.

Here's an example:

var stream = require('stream');
var util = require('util');

function EchoStream () { // step 2
  stream.Writable.call(this);
};
util.inherits(EchoStream, stream.Writable); // step 1
EchoStream.prototype._write = function (chunk, encoding, done) { // step 3
  console.log(chunk.toString());
  done();
}

var myStream = new EchoStream(); // instanciate your brand new stream
process.stdin.pipe(myStream);

Extend an empty Writable object

Instead of defining a new object type, you can instanciate an empty Writable object and implement the _write() method:

var stream = require('stream');
var echoStream = new stream.Writable();
echoStream._write = function (chunk, encoding, done) {
  console.log(chunk.toString());
  done();
};

process.stdin.pipe(echoStream);

Use the Simplified Constructor API

If you're using io.js, you can use the simplified constructor API:

var writable = new stream.Writable({
  write: function(chunk, encoding, next) {
    console.log(chunk.toString());
    next();
  }
});

Use an ES6 class in Node 4+

class EchoStream extends stream.Writable {
  _write(chunk, enc, next) {
    console.log(chunk.toString());
    next();
  }
}