I have a robot that I'm controlling via a browser. A page with buttons to go forward, reverse, etc is written in PHP hosted on an onboard computer. The PHP is just sending ASCII characters over a serial connection to a microcontroller. Anyway, I need to implement a failsafe so that when the person driving it gets disconnected, the robot will stop. The only thing I can think to do is to ping the person on the web page or something, but I'm sure there is a better way than that. The robot is connected either via an ad hoc network or a regular wireless network that is connected to the internet. Obviously if I go with the ping method then there will have to be a delay between the actual time disconnected and when it realizes it's been disconnected. I'd like this delay to be a small as possible, whatever the method used. I'd appreciate any ideas on how to do this.
Pinging a web client is somewhat unreliable, for you have to take into account, that the client ip might change.
On the other hand, you could emulate a "dead-man-button" via Ajax. Let the webpage send a defined command every now and then (maybe every 5 to 10 seconds). If the robot doesn't receive the message for some time, it can stop. The Ajax script could run in the background so the controlling user won't even notice anything.
This would of course mean, that your robot needs to have a counter which is incremented every second and reset when the message is received. The moment the timer variable is too high, FULL STOP
May I suggest you use a simple flash object embedded in the web browser to open a socket connection to a server on the robot? The server can be written in any suitable language - even PHP (cough).
Then it is a simple matter to detect immediately when the connection goes down, and implement your fail-safe approach.
HTTP is not a ideal protocol for robot control.
Good luck!
All I can think of is to include ajax code in your HTML that "pings" your server every X second. I believe that's what Facebook Chat does to know whether or not you are still online.
HTML 5 Web sockets might be the solution you are looking for but you have to consider that it won't be implemented by most of your users' browsers.
You might find this article interesting: http://www.infoq.com/news/2008/12/websockets-vs-comet-ajax.
Related
I have a client device that request a web page.
I trying to send data to a client when a database table entry is changed.
Problems: the client is not a "browser" ie client side scripting wont do me any good here.(Its a micro controller)
Attempts at first I was thinking of using php and the flush command. I could ever so often output waiting to the client while still in a loop thats checking the database for changes. This to me is a stretch of a method for I don't think my server supports the function and I dont really like it for it seems "dirty" :) ...
Next thought have the php constantly poll the database for changes using a loop. the client should wait until the server finishes and thus I will have a stable connection for "as long as it takes for a change to happen :) optimistic I know". Taking into account that if the connection does time out I can have the client reconnect.
Now a bit of a silly stretch is server side JavaScript a thing lol yes i asked...maybe there is something i don't know about...
I'm hoping someone here can help on this quest of knowledge
Thanks JT
My client is currently:
Opening a socket using tcp connection on port 8090... Then opening a connection to my web site using my socket number and the server address and server port number(80)...I not sure how to correlate this type of socket to the type i would need to stream data very sparingly to the client.
If you need to stick with the HTTP protocol (see comments on other possible methods), read about the meta refresh HTML header. This does what you asked without client side scripting.
Another possible thing would be to setup your db updates as an RSS feed.
It does feel better design not to use HTTP.
Non HTTP based notes:
1) could you just do your current HTTP request, sleep abit, then repeat the same process?
Nothing special, or original; but this fundamentally is what other options are doing.
2) using the same socket, blocking reads would allow you to get more data as soon as its available
the webserver may need to have the config adjusted to act as streaming media server.
38951
As discussion, not a solution, have a look at streaming media
I am writing a JavaScript for an in-browser IM client for the sake of practicing and learning JavaScript and AJAX.
I need to be able to check for a change in the file size of a text file that is being used as a temporary storage for 40-80 SQL entries that contain messages so that it can update the display.
At the moment I am using a setInterval function to periodically check for a change in file size using short PHP script, but this can cause issues, if the interval is to long, messages are delayed, if it is shorter, it means a lot of php scripts running very quickly, which takes up server resources.
What is the best way to do this if the main concern is to reduce server resource usage?
(I am running my server off of a rather low tech PC I've scraped together(2gb ram, 2.8ghz AMD seperon processor))
Preferably, I would want to do this using an AJAX event triggered by someone sending a message, I.E. When user B triggers the event that edits the file by pressing enter, that triggers a function on user A's side that updates the HTML file
Any ideas? I am open to any solution to this particular problem. I gave specific examples of what I want to happen in the specific languages in order to give a better idea of what it is I am attempting to do.
If there is a way to do this that isn't JavaScript/PHP, I'd also be open to exploring that as an option.
Doing this with PHP can be a bit cumbersome. You could try doing something like long polling where you keep the HTTP request open until the server has new data to send to the user. If messages are sent frequently, this might not be ideal. You might want to consider using event-driven web technologies like node.js with something like Socket.IO.
In any case, you'll likely want to maintain a connection with the server if you want to get the message in near real-time. There are ways to use WebSockets with PHP as well, but PHP isn't really the best for this because it's not designed to keep scripts running for long periods (also see What exactly entails setting up a PHP Websocket Server?).
Browsers & HTTP/ AJAX generally work by a "pull" model. The browser/ or AJAX sends the server a request, then the server answers a response.
There isn't generally much provision for the server to contact the browser, to "push" an event. This can however be simulated by a long-running request, to which the server writes data when the event/ or events occur.
For example, this could be a request that answers "empty" after a timeout of 10-30 seconds.. or the server returns & answers immediately, if there are event(s) in its queue.
With a Java server this is easy to do, and I've used this successfully for event notification in a major integration project a few years back.
However I'm not sure in PHP how much ability there is (probably very near zero) to maintain an overall server state, coordinate or communicate between threads/requests, or maintain event queues.
You could look into something like a Java webapp running on Tomcat. All you need is a basic web.xml and one Servlet class, and you can build just about anything from there.
I've been working on sockets, generally in PHP for a while. Currently I have a PHP client for connecting to a chat server, and output every each data sent from server it's connected to.
To explain that in a wider matter, I accomplished this using flush() function in PHP to write out every each buffer waiting in the loop. Buffer reader is withing a while where the condition is the status of the connection socket. But this matters less.
Now to what I want to accomplish. I want to keep socket handling to server side and data from server outputted to client, via AJAX/jQuery. So far, my researches always returned me HTML5 WebSocket and node.js, however, I "have to" be real picky about this, as for users of this, my minimal dependency might be:
WinXP IE6 users(Already disables jQuery, even)
Users without JAVA/Flash installed
So I have to think of possibilities in this, which is why I can't use a Flash/Java backend or a new technology like WebSockets, and neither I want to handle server stuff in the client. I really hate to be stuck in old technology but for this it's a must.
As I was searching around, I found this one being as similiar to my needs.
Is PHP socket a viable option for making PHP jQuery based chat?
And to quick review the answers, they all point to one direction, PHP multi-process and memory eating. I know this is a minus, but it's the best I can take for now. But yet still, there'll be timeout disconnects for inactive connections within a certain delay, and extension of the delay if wanted. So I'm not much onto this one.
Secondly, the last answer pointing to "Ajax Chat Application Tutorial", I made an overall review but whoa, writing each line into an html file and re-including it each time, that is which I could do without using an extra file but, is it really necessary? Plus re-reading the file from server side, and re-importing the whole read file into document every each time, isn't that just worse for "both sides"?
Either ways that's about it, I wasn't able to come to a conclusion for a while, and it happened, here I am again. (:P) Waiting for your answers/suggestions/ideas, thanks by now.
Regards.
There is server software available that specializes in such matters. Is called a push server/service. There's for example APE (http://www.ape-project.org/); according to their website, it's compatible with all web browsers and they even got a demo chat there. I'd suggest you to go for that solution.
The challenge I'm facing is simulating a communication channel between two users of a website (e.g. gaming site), by using solely the technologies mentioned in the title.
Recently I've developed an online chess website where, the idea behind which was to give users the chance to play "live" matches too, not just lengthy games where you would make a move, then come back in 1-15 days to see if your opponent has responded. And the way this game engine works implies sending asynchronous requests to the server, both to update the info related to the game (in case you make a move), but also to verify if anything has changed (if you are waiting for the opponent to move).
To better explain this... the player whose turn it is (the browser of course) sends an async. request to update the game info, exactly when he makes his move. Meanwhile, the opponent sends PERIODIC requests, "asking" if anything has changed. If anything does change, the roles switch after the board updates.
Timers are behind the functionality of the engine, so my question to you is this: How would you go about simulating a com channel between two players, while trying not to put too much stress on the server, but also having games update as fast as possible, in order to maintain the "live" feeling to it. This is most important in 1 minute games (one of the available categories). Because in that case, requests NEED to take place VERY often (at least 1 second). But server responses could delay, there would be much stress on the server when having hundreds of games available at the same time etc... so you see my problem.
I look forward to hearing from you guys and picking your brain if you have any good ideas :)
Cheers!
Andrei
PS: In case you want to try it, the name is e-chess960.com
you would want to builds a socket server. node.js would be a good javascript based library to use for a socket server. each client would create a socket connection to the socket server, then when a client sends a message to the socket server, the socket server could immediately send the message back out to its subscribers without having to store it anywhere.
socket servers require socket access, which usually requires a vps instead of a shared server.
to make the solution work, you would require that the clients have the ability to create socket connections to your server. some browsers already can as part of html5, but not all yet. you could also use Flash as a way to create a socket connection.
What you actually want is not pulling the server every second, but instead keep a connection open and use the observer-pattern to distribute updates to the specific clients (in other words: push instead of pull). That way, your server resources will not get swamped by a new connection for every second and every client. A library to accomplish that is the Comet Library. Check out this nice tutorial for a sample application.
Am not sure if what I am doing is absolutely correct. But here goes:
User logins into chat via web-based interface
User is informed of updates via Comet
User enters details which goto a PHP file which further connects to a Jabber server
Now the problem is that when the user wants to send a message, it's simple, run php in which i connect to jabber server and send the message. The problem arises when I am waiting for a message. Cause if I login and check messages and disconnect, on the other users end I will show up as disconnected.
Am I approaching this problem in a wrong way? Should I directly connect to the Jabber server (via javascript) instead of a PHP layer in between? How to recieve messages via PHP?
I haven't tried it out, but you might want to look at xmpphp. Secondly, you might want to consider keeping the user logged in to the XMPP server (aka a Jabber server) for as long as they're logged in to your website. You probably want to have a timeout of some kind in case they leave your website and don't come back.
As for whether or not you should connect via JavaScript, I don't see why you couldn't. I would suggest that you go for whatever seems the simplest to you. You might want to check out Strophe, which I hear good things about, for that case.
The only XMPP library that I've used extensively though is headstock, but that requires using python and Kamaelia.
this is an inherent problem (or feature) with http - there are no lasting connections (not really). you need a workaround, there is no real solution.
you could do it with java or flash, but that's not really nice (javascript 4tw!).
the other possibility would be to create an intermediate client what translates connections between the browser and the webserver to connections between the webserver and the jabber server. messy, but possible.
or maybe there is an API that helps with this.
directly connecting to the jabber server via javascript
i possibly slept through the latest ajax-inventions, but afaik you can only communicate with the host the source-html file comes from (ignoring greasmonkey and addons). no different domains, no different ports, period. unless you're going to teach your jabber server how to serve your chatpage-html to the browser, this will get problematic. moreover, staying connected doesn't even work, because that would require multipart-responses. those are only supported by mozilla, and this is why the ugly duckling COMET even exists in the first place. comet itself is a workaround to avoid the inability to hold connections while transfering data.
So the issue, as far as I can tell, is that when the Jabber user on the other end responds. The problem there, at least in part, is that the user is responding to another user on the Jabber server, yet you want the php script to be aware that this response has taken place without holding the connection open (which makes sense since the script is no longer running, probably).
One option, albeit a really silly one, is:
Have a php script that can broker a connection to the Jabber server for both sending and receiving for the user on your page,
Use AJAX to send messages for the user (the AJAX would point to the above script, the script would send the message.)
Have a Javascript infinite loop that pings the same script ever 10 seconds or so, checking in to see if there are messages. If there are, they get passed back to the client and output to the user.
There are only two issues with the above:
1) If the user isn't connected when the message is transmitted, will the php script still see/get the message?
2) A client side loop that makes ajax requests every 3 seconds would probably be a huge drain.
Solution 2:
OpenFire jabber server. It comes with a web chat client built in, and it has an addon called Fastpath, which is meant to handle HTML-based chats on the client end (like the "chat with an agent now!" feature on too many support pages.)
We use this at work and it is very customizable, can be integrated with other scripts (for instance, if you want a script that fills in the user details from their login, or adds some custom avatar, or whatever), and it (OpenFire) has tons of other extensions and addons that, if this isn't what you want, they probably have what you are looking for.