This might be a stupid question but i cant find much info online about it. I would like to know if server sent events can be used between 2 servers. I think in theory this should be possible but everything i read about SSEs is about the client side and the EventSource object.
This is fine but my web API must also be able to stream data to clients that are not web browsers but most likely backend servers/ network apps. Can someone confirm that it can or cannot work and if it can, could you please add some example code of how a backend language would subscribe to it?
Reading from an event source (also called a Server Sent Event or SSE) can be done from the backend. This is an snippet from the golang repository (r3labs/sse) that illustrates how to create a backend client that reads from an event stream.
func main() {
client := sse.NewClient("http://server/events")
client.Subscribe("messages", func(msg *sse.Event) {
// Got some data!
fmt.Println(msg.Data)
})
}
It's also possible to do this with Python, as well as other frameworks such as NodeJS. On the off-chance that support isn't available in your language of choice, one could implement support by determining how to poll and read from the event stream when the HTTP header text/event-stream is present.
REFERENCES
Original comment
Related
I am wondering if there is a good way of making automated system testing for a Chromecast receiver application?
If you open the application URL in a Chrome browser, the cast_receiver library cannot find the websocket connection on:
ws://localhost:8008/v2/ipc
Since this handles the communication between the app and the Chromecast hardware, I am thinking of something like a Node.js websocket server that can talk to the chromecast receiver app. Is there such a system, or do anyone know if there are plans of google releasing something for this kind of testing?
Also, would there be other problems related to the difference between the chromecast browser and chrome browser? As I understand, the chromecast browser is just a subset of chrome, which makes me think it should work.
No, there is no easy way to do this.
DISCLAIMER: I haven't tried any of what I'm about to suggest. It's also probably a terribly idea as Google could change the protocol any time and in any fashion they desire since it isn't a public thing.
BIG DISCLAIMER: You may be in violation of the ToS by doing this as Section 3.2 (Developer Policies) states that you "may not ... develop a standalone technology ... any functionality of any Google Cast Receiver". Possibly, you'd be making a standalone piece of technology that replicated the IPC functionality. But I don't know. I'm not a lawyer.
If you want to go and do this, I'd suggest making a copy of the Google Cast Receiver SDK (www.gstatic.com/cast/sdk/libs/receiver/2.0.0/cast_receiver.js as of April 28, 2015) and altering it so that it logs out the messages that are being sent and received.
Luckily, it appears that we have logging messages to help us find the relevant code.
The receiving method has the string "Received message". I would guess that "a.message" is what is being received.
The sending method has the string "IPC message sent". I would guess that "a" is what is being sent.
Once you've instrumented your copy of the code, you need to publish it somewhere that your receiver app can see it and then you need to edit your receiver app to point to your new and improved SDK. Please please please make sure that you do this on a non-published app for testing purposes only.
Once that is done, you need to find some way to get your messages out of the code and into something that you can access. You have a few options.
Fiddle around with the code more and figure out how to get the Chromecast to log out the data you want;
Store the information in an array and read it using the debugger;
Open your own socket (or websocket) and send that data to a server that you control.
From here, you can run your app, interact with it, and then have a complete record of the IPC messages that were sent and received. Armed with this, you can create your own Fake-IPC server that listens for specific messages and spits out the stuff that is in your log.
It is quite easy to update the interface by sending jQuery ajax request and updating with new content. But I need something more specific.
I want to send the response to client without their having requested it and update the content when they have found something new on the server. No need to send an ajax request every time. When the server has new data it sends a response to every client.
Is there any way to do this using HTTP or some specific functionality inside the browser?
Websockets, Comet, HTTP long polling.
It has name server push (you can also find it under name Comet technology). Do search using these keywords and you will find bunch examples, tools and so on. No special protocol is required for that.
Aaah! You are trying to break the principles of the web :) You see if the web was pure MVC (model-view-controller) the 'server' could actually send messages to the client(s) and ask them to update. The issue is that the server could be load balanced and the same request could be sent to different servers. Now if you were to send a message back to the client you'll have to know who all are connected to the server. Let's say the site is quite popular and you have about 100,000 people connecting to it every day. You'll actually have to store the IPs of each of them to know where on the internet they are located and to be able to "push" them a message.
Caveats:
What if they are no longer browsing your website? You see currently there is no way to log out automatically if you close your browser. The server needs to check after a fixed timeout if you have logged out (or you send a new nonce with every response to prevent the server from doing that check)
What about a system restart/crash etc? You'd lose all the IPs that you were keeping track of and you are back to square one - you have people connected to you but until you receive new requests you can't really "send" them data when they may be expecting it as per your model.
Let's take an example of facebook's news feeds or "Most recent" link close to the top right - sometimes while you are browsing your wall you see the number next to most recent has gone up or a new 'feed' has come to the top of your wall post! It's the client sending periodic requests to the server to find out what was updated rather than the other way round
You see, it keeps it simple and restful. You may feel it's inefficient for the client to "poll" the server to pull the data and you'd prefer push, but the design of the server gets simplified :)
I suggest ajax-pulling is the best way to go - you are distributing computation to the client and keeping it simple (KIS principle :)
Of course you can get around it, the question is, is it worth it?
Hope this helps :)
RFC 6202 might be a good read.
In general chat application, client's browser always poll to server to check for new messages.
// the function to check new messages in server
function check(){
// but this question is less about jQuery.
$.ajax({
type: "POST",
url: "check.aspx",
data: "someparam=123",
success: function(msg){
// process msg here
// CHECK IT AGAIN, but sometimes we need to make delay here
check();
}
});
}
Then I read Nicholas Allen's blog about Keeping Connections Open in IIS.
It makes me think if it is possible to push data from my server to client's browser by transfer chunked HTTP (it means like streaming, right?) and keep the connection open.
while keeping the connection open, in server, I have idea to keep something run to check new messages. something like this, maybe
while(connectionStillOpen) {
// any new message?
if( AnyMessage() )
{
// send chunked data, can I?
SendMessageToBrowser();
// may be we need to make delay here
Sleep(forSomeTime);
}
}
that's a raw idea.
My Chat App created in ASP.net. With my less understanding of WCF and advanced IIS streaming module, I need your advice about to implement this idea.
yea, Impossible is probably the answer. But I need to know why if its still impossible.
UPDATE (3 years later):
This is exactly what I was looking for:
Microsoft ASP.NET SignalR
Yes, it's impossible to push data from server directly to your browser client.
But you can to check server for new messages every, let's say, 3 seconds and refresh your client interface.
Maybe you want to take a look on some Comet implementations
A server cannot initiate communication with the client. So the server cannot push data to the client. But you can achieve the push mechanism using "Reverse AJAX". The following article should shed more light.
Reverse AJAX
One method is there which is called Reverse AJAX. By using which server can transfer data to client without any request from the Client.
The current generation of JavaScript / Ajax libraries don't provide access to partial responses; you only receive notification when the entire request is complete.
If you're open to using Silverlight, you can use a raw TCP connection.
Comet is another option -- but that's basically just long polling that still originates from client-side script.
Its not possible to push the data from the server. Because HTTP respond only to the requests and cannot contact the client directly. But we have a workaround here called COMET or ReverseAJAX, by using this technique we can simulate the duplex calls.
Its nothing but the long living AJAX
calls, and will respond to the client
if there is a expected event happening
in server side, otherwise it stays
calm. This Comet (programming)
wikipedia article explains more about
the approach
I have answerd similar question here asp-net-chat-with-wcf. pls check out
This may be a dumb question - and the title may need to be improved... I think my requirement is pretty simple: I want to send a request for data from a client to a server program, and the server (not the client) should respond with something like "Received your request - working on it". The client then does other work. Then when the server has obtained the data, it should send an asynchronous message (a popup?) saying "I've got your data; click on ... (presumably a URL) to obtain data". I have been assuming that the server could be written in Java and that client is html and JavaScript. I haven't been able to come up with a clean solution - help would be appreciated.
Try to employ "Websocket Method" by using "SuperWebSocket" for server side, and "WebSocket4Net" for client side. It is working perfectly for my current project.
Most of the work invovles the server being asynchronous. To do this you must
Have an ajax call to the server that starts a job and returns a confirmation the job has been started.
A page on the server that will return whether or not any jobs are complete for a user.
Have an ajax widget on your client side that pings that page on teh server every so often to see if any jobs have been completed. And if so make a pop up.
This is the only way unless you use Flex data services.
Are you trying to do this on the HTTP protocol? It sounds like you're talking about a web application here, but it's not clear from the question. If so, then there are a variety of techniques for accomplishing this using AJAX which collectively go under the name "Comet". Depending on exactly what you're trying to accomplish, a number of different implementation, on both the client and server side, may be appropriate.
for pure java i suggest something like jgroups (client+server are java)
for html, you should use ajax - there you have a timer that checks every X seconds
Nowadays you have an alternative technique to use: Websockets. These are used for server->client communication without polling or ajax-style delayed responses.
I'm sure Wave doesn't poll the server every millisecond to find out if the other user has typed something... so how can I see what the other person is typing as they type? And without hogging the bandwidth.
Persistent HTTP, Comet
Keep your HTTP connection alive and send characters as they are typed
*Edit in 2014: also, take a look at WebSocket and HTTP/1.1 Upgrade header. Browsers started implementing this around 2010, so I'm adding this to original answer.
They probably use Web Sockets, aka server-sent events: http://www.w3.org/TR/websockets The underlying protocol can be found (as a draft) at the IETF.
Update: it doesn't seem WebSockets has any implementation yet; and a video from Google I/O (go to 11:00) talks about a long lived HTTP GET request.
Server Push in GWT
Server push is the Wait, Respond, Close, Re-Open paradigm:
Wait: When the GWT code makes a call
to your server for some data that you
don't have yet, freeze (wait)
Respond: Once the requested data is
available, respond with it
Close: Then, close the connection.
Re-Open: Once your GWT code receives the response, immediately open up a new connection to query for the next event.
See Video Google Wave: Powered by GWT around at minute 55 (near the end)
Q: How you implement the persistent Connections, the long living http connections
A: Future Plan: HTML5 Web Sockets. Longer term. That's what we use at the moment.
Q: Is there a platform or library for this we can download and play with?
A: Not sure. Don't think so
P.S.: That's what he said. To me it did not make much sense ("future plans" vs "using at the moment"). Any native english speaker might want to verify if I transcribed it correctly?
Pure speculation but could it be using the Server Side DOM events from the HTML 5 spec?
the entire reason for WebSockets is to have the browser keep a bi-directional socket open to a server so that real time communications can be used. When someone types on the other end, in a wave client, it triggers an event that is sent to the server and the server in turn looks to see who should also receive the event and pass them the event, in this case the typed letter.
WebSocket and Comet are different.
Granville
Probably comet for now websocket in the future. Because it works in Firefox 3.5 and from what I've read the websocket is only available in the nightly builds of FF... I could be wrong though... as it appears to not work in IE at all.
I spent some time reverse-engineering the Google Wave client code (shameless plug for http://antimatter15.com/misc/read/ which is a read-only public client for google wave for all public waves without need of robots or gadgets which was a lot more useful a month ago when Google didn't launch the upgrades).
Anyway, Google uses the GWT framework with certain aspects of the Google Closure library (which is actually open source and documented) and they use the goog.net.BrowserChannel library, which from the comments is also used for chat functionality within gmail.
http://closure-library.googlecode.com/svn/docs/closure_goog_net_browserchannel.js.html
I would assume that they use ajax requests. Do an XMLHttpRequest, which is asynchronous, and when the server has something to send your browser the javascript callback that was registered gets the data and does whatever with it. So basically the browser requests the next event, handles it, repeats indefinitely.