How do the two compare to each other?
connect;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 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 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.