CODEWITHSUNDEEP
node.js
Dmitry Minkovsky
Dmitry Minkovsky

Reputation: 38143

Why doesn't this program output any data?

I'm messing with the Node.js 0.10 Stream classes to try and figure out how to use them. I'm not sure why this experiment isn't working. It's supposed to output letters of the alphabet to an HTTP response object, but does not. I've annotated the source with a few comments.

Thank you!

var Readable = require('stream').Readable
  , inherits = require('util').inherits
  , http = require('http');

/**
 * A stream that streams the English alphabet
 */

function AlphabetStream() {
  Readable.call(this);
  this.code = this.offset = 'a'.charCodeAt(0);
  this.last = 'z'.charCodeAt(0);
}

inherits(AlphabetStream, Readable);

AlphabetStream.prototype._read = function(size) {
  for (var i = 0; i < size; i++)
    this.push(this.next_char());
  this.push(null);
};

AlphabetStream.prototype.next_char = function() {
  var cycle = this.last+1;
  return String.fromCharCode((++this.code % cycle) + this.offset);
};


/**
 * An HTTP server, prints the first n letters of the English alphabet
 */

var server = http.createServer(function(req, res) {
  // $ curl localhost:3001/?size=11

  var size = require('url').parse(req.url, true).query.size;

  if (size) {
    var rs = new AlphabetStream;
    rs.pipe(res); // This calls `_read()` with `size` 16kb
    rs.read(parseInt(size)); // This also calls `_read()` with `size` 16kb
  }

  res.end(''); // Nothing gets printed, despite the pipe and the reading.
});

server.listen(3001, function() {
  console.log('Listening on 3001');
});

Upvotes: 0

Views: 63

Answers (1)

freakish
freakish

Reputation: 56467

Have a look at this piece of code:

if (size) {
  var rs = new AlphabetStream;
  rs.pipe(res); // This calls `_read()` with `size` 16kb
  rs.read(parseInt(size)); // This also calls `_read()` with `size` 16kb
}

res.end(''); // Nothing gets printed, despite the pipe and the reading.

You end the response (last line) before the actual piping can take place (this happens because .pipe is asynchronous). What you should do is something like that:

if (size) {
  var rs = new AlphabetStream;
  rs.pipe(res);
  rs.read(parseInt(size));
} else {
  // NOTE THE ELSE STATEMENT
  res.end('');
}

.pipe function will take care of ending the destination stream (i.e. the response) unless explicitely stated otherwise, see the docs:

http://nodejs.org/api/stream.html#stream_readable_pipe_destination_options

EDIT As for why 16kb? Well, I had to do some tests and it seems that this is the default behaviour of .pipe (and I'm not sure how to change that to be honest). First of all note that this line:

  rs.read(parseInt(size));

is totally useless (you can remove it). .pipe will take care of reading the data. Now the default behaviour is to read chunks of 16kb of data. So in order to do what you are trying to do you should probably pass size to the constructor of AlphabetStream, like this:

function AlphabetStream(size) {
  Readable.call(this);
  this.code = this.offset = 'a'.charCodeAt(0);
  this.last = 'z'.charCodeAt(0);
  this.size = size; // <--- store size here
}

inherits(AlphabetStream, Readable);

AlphabetStream.prototype._read = function(size) {
  // this allows the stream to be a true stream
  // it reads only as much data as it can
  // but .read can be called multiple times without issues
  // with a predefined limit

  var chunk = Math.min(size, this.size);
  this.size -= chunk;
  for (var i = 0; i < chunk; i++) {
    this.push(this.next_char());
  }
  if (!this.size) {
    // end the stream only when the limit is reached
    this.push(null);
  }
};

after all a stream should not depend on how much data you read. Then you do:

if (size) {
  var rs = new AlphabetStream(parseInt(size));
  rs.pipe(res);
} else {
  res.end('');
}

Upvotes: 3

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