Background: I've got an C++/Qt-based application that communicates with servers on the user's LAN. It uses non-blocking TCP and UDP sockets, and the networking is implemented via calls to the BSD sockets API (i.e. socket()/send()/recv()/select()/etc). It all works well.
The other day, just for fun, I decided to recompile the application using emscripten, so that it could run as a WebAssembly app inside a web browser.
This worked surprisingly well -- within an hour or two, I had my app up and running inside Google Chrome. However, the app's usefulness in this configuration is severely limited by the fact that it isn't able to connect to any servers -- presumably this is because it is running in a restricted/sandboxed environment.
If I wanted to pursue this line of development beyond the clever-hack-demo stage and try to make it useful, I would need to find a way for my program to discover and connect to servers on the user's LAN.
My question is: is that functionality at all possible for a Emscripten/WebAssembly-based app to perform? If so, what steps would I need to take? (i.e. would it require upgrading the LAN's servers to handle WebSocket-based connections? Would it require adding some sort of proxy server to run on the web server that the web page was served from? Is UDP even a thing in a web-app context? Are there other hoops that would also have to be jumped through?)
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I'm writing a program in C++ that will run all the time in the background to do different tasks - a deamon.
Apart from it's usual tasks a web application should communicate with it (AJAX requests in doing a COMET pattern = lots of open but sleeping connections).
So the question is: how should I connect it to the web server (apache and lighttpd are relevant)?
Of what I've read FastCGI would be very interesting for that task, but all references I've read were talking about the web server starting the FastCGI application when necessary. This wouldn't work here as the deamon would already be running...
I've also read that the web server would talk via socets with the FastCGI application - so that could be an entry point for me, the deamon would "only" need to talk to such a socket.
But are there good libraries available for that?
Looking at the features of http://cppcms.com/wikipp/en/page/main it looks very interesting for me - but could that work in my case? And could it be stripped down - offering even a SQL connection is far too heavy for my case...
So what advice can you give me?
PS: Performance wise I recon a single threaded but asynchronous implementation would work for the deamon <-> web server glue.
FastCGI is the standard protocol to communicate with the web server. All mentioned web servers can communicate with the remote deamon application via fastcgi.
http://cppcms.com/wikipp/en/page/cppcms_1x_tut_web_server_config
Also if you are looking for Comet support, that what CppCMS provides you natively:
http://blog.cppcms.com/post/107
And could it be stripped down - offering even a SQL connection is far too heavy for my case...
CppCMS library is very small also it allows to reduce its size:
http://cppcms.com/wikipp/en/page/cppcms_1x_build#Build.Options
also SQL connectivity CppDB is independent part.
Performance wise I recon a single threaded but asynchronous implementation would work for the deamon <-> web server glue.
It is one of the standard CppCMS run mode - running asynchronous web applications.
Update:
Of what I've read FastCGI would be very interesting for that task, but all references I've read were talking about the web server starting the FastCGI application when necessary. This wouldn't work here as the deamon would already be running..
Indeed some web servers start the fast cgi applications but:
Lighttpd allows both to start FastCGI and SCGI application or connect to independent one
Cherokee (AFAIR) allows both to start FastCGI and SCGI application or connect to independent one
Apache:
mod_fascgi allows both to start FastCGI application or connect to independent one
mod_scgi connects to independent application - does not start application.
mod_fcgid always starts application - does not suite you
Also as general note, apache does not suit a pattern of working with many idle connections as it uses thread (or even process depending mpm) per connection.
Nginx - does not start applications at all, however for fastcgi currently not the best for comet streaming because of buffering, so SCGI would be better with nginx (assuming you are using latest nginx version)
Small note: SCGI is a protocol that is very similar to FastCGI but significantly simpler.
Is it possible to write C++ code to interface with a server, but to be executed on client side, but on the browser instead of native?
Like, for example, imagine using open source classes so that you produce a file.
But because you don't want all this work to be done on the server, you run it on the browser.
So that the client gives a file or two or more as inputs, then the code runs on his machine, the final result is produced, then this file is uploaded to database on the server.
please see google native client project. http://code.google.com/p/nativeclient/
This is strange question.
You can prepare binaries that do task that you want done on client side and make server send proper binary to client when asked for it. Client then runs this binary and returns results to server.
It is possible if you know configurations of client machines (binaries must work on them). Also it have to be some security layer implemented - you don't want to allow every binary run on client (imagine man-in-the-middle attack when some malicious code is run on client).
I think your request contradicts with the idea behind server-side programming. The main purpose in using server-side programs is to make use of infrastructural components like database, network, etc. in a controlled manner. (The most typical usage of server-side applications are web sites with server side coding like JSP and ASP.)
Since servers are machines that are to be kept secure, a remote application should not be permitted to make changes or access filesystem freely. If you want to do changes on a server like doing database operations or reading/writing files, you should use applications that run on the server or provide interfaces like web services or web sites to remote client applications.
So there are a couple solutions when if you want to do work on the browser, then have the results posted in a server database.
First of all, you must set up your server ready for database work. I have done this using the MEAN stack, set up a MongoDB and interfaced it with the Mongoose API.
Now, for the meat of the question, there are many examples of browsers doing intensive work. The majority of these applications thought is not C++, but it is Javascript.
If you really want to focus on C++ (like i did in the past, in the time i asked this question, wanting to make something big for college), then you could do one of the following:
*Use Google Native Client (NaCl). This is a sandbox for running compiled C and C++ code in the browser efficiently and securely, independent of the user’s operating system.
*Maybe you should want to check out Emscripten, which is a framwork for translating C and C++ code to jaascript. This way, you can have your C or C++ binaries that worked, and have them translated to Javascript, in order to have them work in the browser too.
What I'm trying to design is a system where-by a user can upload a compiled .hex file to a web site, and then that web server sends it on to another server which has a microprocessor connected to it via USB. The web server would then trigger a script to run on that sever which would load the .hex file to the micro.
So my question is: Is it possible for a web server to trigger a shell script or C/Java program on another (trusted) machine?
Yes it's possible. Webservers often support invocation of scripts, usually with the CGI interface. This scripts can perform the required interaction with the other server. How this has to be implemented depends on the webserver and its configuration.
If there is a microcontroller is involved and how this is connected to the second server is not related to this question. Therefore I suggest to remove the microcontroller tag. It's also doubful if it's somehow related to shell at all.
I'm building an app which upon login will connect you to certain ip addresses of which will also be running the same app.
The method of which i believe i should be using is direct tunnelling but as i say im a little new to c++, i have general coding skills, and i have sifted through a lot of forums and sites yet im still very unclear on what the best way forward is to achieve the requirement.
The reason for the connection will be to enable a secure chat, file transfer, and update software auto when connected to the program admin.
All those that have the app installed will once authorised, will be connected to admin client, then from that client all available ip's to connect to will become available to slave clients, this will increase the network size avilable to all users.
so the app needs to be able to handle ports but not via a server, instead it would be direct.
The connections also must ideally be encrypted.
Im kind of looking for what the application RetroShare does, but in text app.
(This is using C++ within Dev C++)
so just to recap, What method should i use to achieve the above?
I would take a look at SDL net to start with, its really simple to learn if you have never done any socket programming before.
for a secure connection you will probably want to start with TCP and then once you get the hang of network programming, start looking at other protocols.
Hope this helped! and good luck.
My application runs in Windows and is implemented using C++/Qt.
The application will invoke another application deployed in the Linux server which in turn will invoke some third party tools. The Linux server application will send some status updates based on the running of third party tools. Usually the third party application will run for hours and the updates will be sent at various stages. The Linux server may also has to send some files in addition to the status updates and the Windows client will also send some files required for the running of those third party tools.
I planned to implement this in libssh2 since file transfers can be done and applications can be executed as well using libssh2_channel_exec(). Updates can be sent and received through non-blocking socket transfers. Also the transfers must be secured and they are password authenticated, so I thought SSH will conform my requirements.
I also looked into Qpid of apache which implements the AMQP. The messaging seems to be a more appropriate one for my status updates since the updates are less frequent. But I am not so sure about the secured connection, password authentication and also the application invocation.
So, which one can I choose between these two? Or is there any other better option available? I am not quite used to network programming so any pointers, links regarding this are welcome..
Have you considered some web-based solutions like XML-RPC, REST, SOAP or other? Note that you can either have constant network connection and stream updates or just make your client ask for update as often as it needs.
Also, I think that building solution based on some of these protocols will give you easier coding - no need for some low-level solutions when you have great libraries. As for security part, I would consider SSL that is part of HTTPS protocol to be secure enough. Of course you can also do it hybrid style, for example SSH tunel to secure server and use SSH key authorization.
But if you are sure youwant SSH or AMQP then use first one - I think it has better security. Also, try not using username/passowrd. Instead use mentioned above keys.
Start with SSH, and then consider layering other protocols on top. You can use SSH port forwarding to create a VPN connection to a server, and maybe that will make it easier to use something like AMQP or 0MQ.