What is the difference between dnode and nowjs?

TL;DR

DNode

• provides RMI;
• remote functions can accept callbacks as arguments;
• which is nice, since it is fully asynchronous;
• runs stand-alone or through an existing http server;
• can have browser and Node clients;
• supports middleware, just like connect;
• has been around longer than NowJS.

NowJS

• goes beyond just RMI and implements a “shared scope” API. It’s like
  Dropbox, only with variables and functions instead of files;
• remote functions also accept callbacks (thanks to Sridatta and Eric from NowJS
  for the clarification);
• depends on a listening http server to work;
• can only have browser clients;
• became public very recently;
• is somewhat buggy right now.

Conclusion

NowJS is more of a toy right now — but keep a watch as it matures. For
serious stuff, maybe go with DNode. For a more detailed review of these
libraries, read along.

DNode

DNode provides a Remote Method Invocation framework. Both the client and server
can expose functions to each other.

// On the server

var server = DNode(function () {
    this.echo = function (message) {
        console.log(message)
    }
}).listen(9999)

// On the client

dnode.connect(9999, function (server) {
    server.echo('Hello, world!')
})

The function that is passed to DNode() is a handler not unlike the one passed to
http.createServer. It has two parameters: client can be used to access the
functions exported by the client and connection can be used to handle
connection-related events:

// On the server

var server = DNode(function (client, connection) {
    this.echo = function (message) {
        console.log(message)
        connection.on('end', function () {
            console.log('The connection %s ended.', conn.id)
        })
    }       
}).listen(9999)

The exported methods can be passed anything, including functions. They are properly
wrapped as proxies by DNode and can be called back at the other endpoint. This is
fundamental: DNode is fully asynchronous; it does not block while waiting
for a remote method to return:

// A contrived example, of course.
// On the server

var server = DNode(function (client) {
    this.echo = function (message) {
        console.log(message)
        return 'Hello you too.'
    }
}).listen(9999)

// On the client

dnode.connect(9999, function (server) {
    var ret = server.echo('Hello, world!')
    console.log(ret) // This won't work
})

Callbacks must be passed around in order to receive responses from the other
endpoint. Complicated conversations can become unreadable quite fast. This
question discusses possible solutions for this problem.

// On the server

var server = DNode(function (client, callback) {
    this.echo = function (message, callback) {
        console.log(message)
        callback('Hello you too.')
    }

    this.hello = function (callback) {
        callback('Hello, world!')
    }
}).listen(9999)

// On the client

dnode.connect(9999, function (server) {
    server.echo("I can't have enough nesting with DNode!", function (response) {
        console.log(response)
        server.hello(function (greeting) {
            console.log(greeting)
        })
    })
})

The DNode client can be a script running inside a Node instance or can be
embedded inside a webpage. In this case, it will only connect to the server that
served the webpage. Connect is of great assistance in this case. This scenario was tested with all modern browsers and with Internet Explorer 5.5 and 7.

DNode was started less than a year ago, on June 2010. It’s as mature as a Node
library can be. In my tests, I found no obvious issues.

NowJS

NowJS provides a kind of magic API that borders on being cute. The server has an
everyone.now scope. Everything that is put inside everyone.now becomes
visible to every client through their now scope.

This code, on the server, will share an echo function with every client that
writes a message to the server console:

// Server-side:

everyone.now.echo = function (message) {
    console.log(message)
}

// So, on the client, one can write:

now.echo('This will be printed on the server console.')

When a server-side “shared” function runs, this will have a now attribute
that is specific to the client that made that call.

// Client-side

now.receiveResponse = function (response) {
    console.log('The server said: %s')
}

// We just touched "now" above and it must be synchronized 
// with the server. Will things happen as we expect? Since 
// the code is not multithreaded and NowJS talks through TCP,
// the synchronizing message will get to the server first.
// I still feel nervous about it, though.

now.echo('This will be printed on the server console.')

// Server-side:

everyone.now.echo = function (message) {
    console.log(message)
    this.now.receiveResponse('Thank you for using the "echo" service.')
}

Functions in NowJS can have return values. To get them, a callback must be
passed:

// On the client

now.twice(10, function (r) { console.log(r) }

// On the server

everyone.now.twice = function(n) {
    return 2 * n
}

This has an implication if you want to pass a callback as an honest argument (not
to collect a return value) — one must always pass the return value collector, or
NowJS may get confused. According to the developers, this way of retrieving the
return value with an implicit callback will probably change in the future:

// On the client

now.crunchSomeNumbers('compute-primes', 

    /* This will be called when our prime numbers are ready to be used. */

    function (data) { /* process the data */ }, 

    /* This will be called when the server function returns. Even if we
    didn't care about our place in the queue, we'd have to add at least
    an empty function. */

    function (queueLength) { alert('You are number ' + queueLength + ' on the queue.') }
)

// On the server

everyone.now.crunchSomeNumbers = function(task, dataCallback) {
    superComputer.enqueueTask(task, dataCallback)
    return superComputer.queueLength
}

And this is it for the NowJS API. Well, actually there are 3 more functions that
can be used to detect client connection and disconnection. I don’t know why they
didn’t expose these features using EventEmitter, though.

Unlike DNode, NowJS requires that the client be a script running inside a web browser.
The page containing the script must be served by the same Node that is running
the server.

On the server side, NowJS also needs an http server listening. It must be passed
when initializing NowJS:

var server = http.createServer(function (req, response) {
    fs.readFile(__dirname + '/now-client.html', function (err, data) {
        response.writeHead(200, {'Content-Type':'text/html'})  
        response.write(data)
        response.end()
    })
})
server.listen(8080)
var everyone = now.initialize(server)

NowJS first commit is from a couple weeks ago (Mar 2011). As such, expect it to
be buggy. I found issues myself while writing this answer. Also expect its
API to change a lot.

On the positive side, the developers are very accessible — Eric even guided me
to making callbacks work. The source code is not documented, but is fortunately
simple and short and the user guide and examples are enough to get one started.