jschan  

jschan is a JavaScript port of libchan based around node streams

Status

The jschan API should be stable at this point, but libchan is a developing standard and there may be breaking changes in future.

When used over the standard SPDY transport, jschan is compatible with the current Go reference implementation.

We also have a websocket transport to allow us to run jschan on the browser. This feature is not covered by the spec, and is not compatible with other implementations.

Transports

The jschan API has support for swappable transports, some of which are included and others which need to be installed.

• memory (built-in)
• binary streams (built-in)
• WebSockets
• SPDY

Install

npm install jschan --save

Example

This example exposes a service over SPDY. It is built to be interoperable with the original libchan version rexec.

Server

The server opens up a jschan server to accept new sessions, and then execute the requests that comes through the channel.

'use strict';
 
var spdy = require('jschan-spdy');
var childProcess = require('child_process');
var server = spdy.server();
server.listen(9323);
 
function handleReq(req) {
  var child = childProcess.spawn(
    req.Cmd,
    req.Args,
    {
      stdio: [
        'pipe',
        'pipe',
        'pipe'
      ]
    }
  );
 
  req.Stdin.pipe(child.stdin);
  child.stdout.pipe(req.Stdout);
  child.stderr.pipe(req.Stderr);
 
  child.on('exit', function(status) {
    req.StatusChan.write({ Status: status });
  });
}
 
function handleChannel(channel) {
  channel.on('data', handleReq);
}
 
function handleSession(session) {
  session.on('channel', handleChannel);
}
 
server.on('session', handleSession);

Client

'use strict';
 
var usage = process.argv[0] + ' ' + process.argv[1] + ' command <args..>';
 
if (!process.argv[2]) {
  console.log(usage)
  process.exit(1)
}
 
var spdy = require('jschan-spdy');
var session = spdy.clientSession({ port: 9323 });
var sender = session.WriteChannel();
 
var cmd = {
  Args: process.argv.slice(3),
  Cmd: process.argv[2],
  StatusChan: sender.ReadChannel(),
  Stderr: process.stderr,
  Stdout: process.stdout,
  Stdin: process.stdin
};
 
sender.write(cmd);
 
cmd.StatusChan.on('data', function(data) {
  sender.end();
  setTimeout(function() {
    console.log('ended with status', data.Status);
    process.exit(data.Status);
  }, 500);
})

What can we write as a message?

You can write:

• Any plain JS object, string, number, ecc that can be serialized by msgpack5
• Any channels, created from the Channel interface
• Any binary node streams, these will automatically be piped to jschan bytestreams, e.g. you can send a fs.createReadStream() as it is. Duplex works too, so you can send a TCP connection, too.
• objectMode: true streams. If it's a Transform (see through2) then it must be already piped with their source/destination.

What is left out?

• Your custom objects, we do not want you to go through that route, we are already doing too much on that side with jsChan.

API

• Session Interface
• session.WriteChannel()
• session.close()
• session.destroy()
• Channel Interface
• channel.ReadChannel()
• channel.WriteChannel()
• channel.BinaryStream()
• channel.destroy()
• jschan.memorySession()
• jschan.streamSession()

Session Interface

A session identifies an exchange of channels between two parties: an initiator and a recipient. Top-level channels can only be created by the initiator in 'write' mode, with WriteChannel().

Channels are unidirectional, but they can be nested (more on that later).

session.WriteChannel()

Creates a Channel in 'write mode', e.g. a streams.Writable. The channel follows the interface defined in Channel Interface. The stream is in objectMode with an highWaterMark of 16.

session.close([callback])

Close the current session, but let any Channel to finish cleanly. Callback is called once all channels have been closed.

session.destroy([callback])

Terminate the current session, forcing to close all the involved channels. Callback is called once all channels have been closed.

Event: 'channel'

function (channel) { }

Emitted each time there is a new Channel. The channel will always be a Readable stream.

Channel Interface

A Channel is a Stream and can be a Readable or Writable depending on which side of the communication you are. A Channel is never a duplex.

In order to send messages through a Channel, you can use standards streams methods. Moreover, you can nest channels by including them in a message, like so:

var chan = session.WriteChannel();
var ret  = chan.ReadChannel();
 
ret.on('data', function(res) {
  console.log('response', res);
});
 
chan.write({ returnChannel: ret });

Each channel has two properties to indicate its direction:

• isReadChannel, is true when you can read from the channel, e.g. chan.pipe(something)
• isWriteChannel, is true when you can write to the channel, e.g. something.pipe(chan)

channel.ReadChannel()

Returns a nested read channel, this channel will wait for data from the other party.

channel.WriteChannel()

Returns a nested write channel, this channel will buffer data up until is received by the other party. It fully respect backpressure.

channel.BinaryStream()

Returns a nested duplex binary stream. It fully respect backpressure.

channel.destroy([callback])

Close the channel now.

jschan.memorySession()

Returns a session that works only through the current node process memory.

This is an examples that uses the in memory session:

'use strict';
 
var jschan  = require('jschan');
var session = jschan.memorySession();
var assert  = require('assert');
 
session.on('channel', function server(chan) {
  // chan is a Readable stream
  chan.on('data', function(msg) {
    var returnChannel  = msg.returnChannel;
 
    returnChannel.write({ hello: 'world' });
  });
});
 
function client() {
  // chan is a Writable stream
  var chan = session.WriteChannel();
  var ret  = chan.ReadChannel();
  var called = false;
 
  ret.on('data', function(res) {
    called = true;
    console.log('response', res);
  });
 
  chan.write({ returnChannel: ret });
 
  setTimeout(function() {
    assert(called, 'no response');
  }, 200);
}
 
client();

jschan.streamSession(readable, writable, opts)

Returns a session that works over any pair of readable and writable streams. This session encodes all messages in msgpack, and sends them over. It can work on top of TCP, websocket or other transports.

streamSession is not compatible with libchan.

Supported options:

• header: true or false (default true), specifies if we want to prefix every msgPack message with its length. This is not needed if the underlining streams have their own framing.
• server: true or false (default false), specifies if this is the server component or the client component.

About LibChan

It's most unique characteristic is that it replicates the semantics of go channels across network connections, while allowing for nested channels to be transferred in messages. This would let you to do things like attach a reference to a remote file on an HTTP response, that could be opened on the client side for reading or writing.

The protocol uses SPDY as it's default transport with MSGPACK as it's default serialization format. Both are able to be switched out, with http1+websockets and protobuf fallbacks planned. SPDY is encrypted over TLS by default.

While the RequestResponse pattern is the primary focus, Asynchronous Message Passing is still possible, due to the low level nature of the protocol.

The Graft project is formed to explore the possibilities of a web where servers and clients are able to communicate freely through a microservices architecture.

"instead of pretending everything is a local function even over the network (which turned out to be a bad idea), what if we did it the other way around? Pretend your components are communicating over a network even when they aren't." Solomon Hykes (of Docker fame) on LibChan - [link]

Find out more about Graft

Contributors

• Adrian Rossouw
• Peter Elger
• Matteo Collina

License

MIT