I am writing a debug/admin node server that allows users to execute a long-running process on the machine. I want to stream the output of the child process to the form they began the action from.
I can do this with sockets, but I have to have the client subscribe to a channel, and I have to post messages to the whole channel when they only have to do with the one client.
I'd prefer to be able to stream the http body down to the client. I can do this fairly easily with node: just keep writing to the request's socket, call end when I'm done.
Is there any way to use XhrHttpRequest to call a web service, have it fire events whenever new data is available, and a final event when it closes? Possible with jQuery?
Note that this isn't really the same use case as normal real-time updates, for which sockets are a good choice. This is a single request. I just want to get the response in pieces.
What I was hoping isn't possible: you can't make an xhr http request and keep it open, parsing chunks at a time.
Here is a summary of people's suggestions
Use socket.io anyway, and change your architecture to support pushing events.
Use socket.io, but make requests through it, as if you were hitting urls. Make a little url router on the server side of socket.io and stream stuff down all you want.
Keep the initial html page open and parse it as you go (not feasible for my implementation)
(3), but in a hidden iframe.
I went with 2.
As an update to this question, nowadays, you can use Sever-sent events (SSE). That way, you don't need to do anything particularly special on the server side, or setup websockets, which is overkill when you don't need full duplex. And XHR will keep the entire data in memory, which is non-ideal for large files. I had the same question, and I answered it here:
How to process streaming HTTP GET data?
some years ago i used "javascript" streaming over open http response. (years before ajax appeared)
the idea here : write chunks of
<script type="text/javascript">do js stuff here</script>
for each step of the process you want the client to react on.
it may still work.
Related
I've seen how to make a post request from JavaScript to get data from the server, but how would I do this flipped. I want to trigger a function in the flask server that will then dynamically update the variable on the JavaScript side to display. Is there a way of doing this in a efficient manner that does not involve a periodic iteration. I'm using an api and I only want to the api to be called once to update.
There are three basic options for you:
Polling - With this method, you would periodically send a request to the server (maybe every 5 seconds for example) and ask for an update. The upside is that it is easy to implement. The downside is that many requests will be unnecessary. It sounds like this isn't a great option for you.
Long Polling - This method means you would open a request up with the server and leave the request open for a long period of time. When the server gets new information it will send a response and close the request - after which the client will immediately open up a new "long poll" request. This eliminates some of the unnecessary requests with regular polling, but it is a bit of a hack as HTTP was meant for a reasonably short request response cycle. Some PaaS providers only allow a 30 second window for this to occur for example.
Web Sockets - This is somewhat harder to setup, but ultimately is the best solution for real time server to client (and vice versa) communication. A socket connection is opened between the server and client and data is passed back and forth whenever either party would like to do so. Javascript has full web socket support now and Flask has some extensions that can help you get this working. There are even great third party managed solutions like Pusher.com that can give you a working concept very quickly.
I just started learning Node.js and as I was learning about the fs.watchFile() method, I was wondering if a chat website could be efficiently built with it (and fs.writeFile()), against for example Socket.IO which is stable, but I believe not 100% stable (several fallbacks, including flash).
Using fs.watchFile could perhaps also be used to keep histories of the chats quite simply (as JSON would be used on the spot).
The chat files could be formatted in JSON in such a way that only the last chatter's message is brought up to the DOM (or whatever to make it efficient to 'fetch' messages when the file gets updated).
I haven't tried it yet as I still need to learn more about Node, and even more to be able to compare it with Socket.IO, but what's your opinion about it? Could it be an efficient/stable way of doing chats?
fs.watchFile() can be used to watch changes to the file in the local filesystem (on the server). This will not solve your need to update all clients chat messages in their browsers. You'll still need web sockets, AJAX or Flash for that (or socket.io, which handles all of those).
What you could typically do in the client is to try to use Web Sockets. If browser does not support them, try to use XMLHttpRequest. If that fails, fallback to Flash. It's a lot of programming to do, and it has to be handled by node.js server as well. Socket.io does that for you.
Also, socket.io is pretty stable. Fallback to Flash is not due to it's instability but due to lack of browser support for better solutions (like Web Sockets).
Storing chat files in flatfile JSON is not a good idea, because if you are going to manipulating the files, you would have to parse and serialize entire JSON objects, which would become very slow as the size of the JSON object increased. The watch methods for the filesystem module also don't work on all operating systems.
You also can't compare Node.js to Socket.IO because they are entirely different things. Socket.IO is a Node module for realtime transport between the browser and the server. What you need is dependent on what you're doing. If you need chat history, then you should be using a database such as MongoDB or MySQL. Watching files for changes is not an efficient way and you should just send messages as they received.
In conclusion no, using fs.watchFile() and fs.writeFile() is a very bad idea, because race conditions would occur due to concurrent file writes, besides that fs.watchFile() uses polling to check if a file has changed. You should instead use Socket.IO and push messages to other clients / store them in a database as they are received.
You can use long pooling method using javascript setTimeout and setInterval
long pooling
basically long pooling working on Ajax reqest and server responce time.
server will respond after a certain time (like after 50 seconds ) if there is not notification or message else it will respond with data and from client side when client gets response client javascript makes another request for new update and wait till response this process is endless until server is running
I'm setting up a stock-ticker like web-page, written in JavaScript. I am also writing the server that the page talks to, in C++.
I would like to make the web page efficient, such that it makes sends single subscription message to the app server, and then holds a keep-alive connection open, constantly receiving inbound data pushed from the app server.
At the moment, I have to re-issue the web clients' subscription call every time I receive data from the server. The problem is that each time the XHR object hits readyState(4), the call is effectively completed. Any data arriving at the web page after this is ignored. The web-client can resend data and that resets the object, but the send is unnecessary, and is only being used to reset the XHR object.
I would like to know if it is possible to somehow reset the existing XHR object, and put it into a state where it expects more inbound data, so that when more data is pushed to the web page, the web page responds and processes it.
Thanks in advance for any help you can give. Note: Not using JQuery on this project.
I highly recommend looking into Websockets, especially a library like socket.io, which encapsulates various browser's implementations of Websocket transports into a single API (WS, JSON, JSONP, Flash and Long Polling).
Socket.io client libraries should be now be supported by all major browsers. Your only challenge might be locating a C++ specific server implementation. Hopefully this SO question might be of some help
Otherwise, your only other option is long-polling or comet on the client with a suitable server-side implementation that would scale (i.e. something like an event driven server like NginX as opposed to a thread-per-connection architecture.)
I do appreciate that you are committed to C++ but my humble advise would be to investigate Node.js as it can and does provide a very performant solution with very little effort.
HTH and all the best.
HTTP is a one-off protocol: one request, one response, and you're done. If you want to keep a connection open, you can use Websockets (MDN reference page, client-side code example). However consider it won't be supported on older browsers (IE, for example, just started supporting Websockets on version 10), so you'll probably need to implement a fallback using XHR and long-polling.
I found an interesting library that allows DOM manipulation from the server, this allows much of the logic to be hidden from the browser, so all the browser gets to see is what happens when you check the box, it does not actually know what to do when you check the box.
Seeing that it responds so quickly, I took a look at the network activity, and found an item with HTTP 101 (switching protocols) and it says the connection is still open.
It seems there is a way in JavaScript to receive streaming data.
How can this be? The code is somewhat large and the googling I did indicated it would be called Comet, but there is much more information about the highly cross-browser "long polling", but that does not seem to be what is happening.
What is a (simple) example for how to achieve fast results like this?
The HTTP/1.1 101 response header is the Websocket protocol handshake.
I have found socket.io to be the best ready rolled library (both client and server), especially when working with JavaScript. Socket.io will drop to flash sockets if possible then last resort long polling in older browsers that do not support Websockets.
Long Polling is a push technology. Before web sockets(HTML 5), the web suffered from class client server problem. The server can not push until client requests. So it was not possible to push data to clients. Long Polling is one of the way to do it.
It works in the simple way. The client would make a request to server, the server if has anything new to server would serve immediately or would wait till new content comes. When ever a new content comes, the server would send the response. On receiving the response, the client would again re-issue a request to the server. There are other ways of implementing Push Technology. Read this : wiki
I've just started learning Node.js and was very interested in its real-time capabilities, especially with Socket.io. Since then, I've written a very basic script to connect to Twitter's streaming server and broadcast tweets to all connected clients.
To build that, I used http.createClient to connect to stream.twitter.com and added in the relevant response and data event handlers. Everything works quite well.
Obviously, Twitter's Streaming API pretty much outputs an infinitely loading webpage and what why using a data event handler works fairly well with it. However, is it possible to make other types of websites 'streamable'?
For example, if a client (browser) updates a website periodically using an XMLHTTPRequest, would it be possible to track the output of those requests using the HTTP API of Node.js? Or similar Node.js extension?
Thanks.
websites do not periodically use XMLHTTPRequest. Clients periodically send XMLHTTPRequests to an URL.
A simple call to http.request(options, callback) with the correct headers should emulate XHR's. Most of these servers will also accept normal POST or GET requests.
If you want node.js to connect to a server and simulate a browser then something like zombie would serve you well. It claims to support XMLHTTPRequest.
The best case for you would be to use web-sockets between your dashboard and node server. This way node will be notified immediately that something has updated at your dashboard ( I am assuming that you can modify your dashboard a bit to accept such connections, won't be difficult as long as you have access).
Then you can use long polling at client-end i.e. send a request to the node server and wait. Node will receive the request and then register an event to it. The moment it receives the updates from dashboard, it'll fire the event which will send the response to all the clients one by one waiting.
I would recommend take a look at http://github.com/andrewdavey/vogue . It does something similar but the functionality is ofcourse different.