Connecting with a front-end

In graphical mode, according to the front-end technology that is used (i.e. desktop client, mobile client, web server client), there are different solutions to establish the connection between the runtime system and the front-end.

This topic describes the development context case, where programs are executed directly with fglrun. In a production environment, programs will typically be started with another technology, since the execution of programs will be triggered by the end user interacting with the front-end. Read front-end specific documentation for more details.

From the point of view of the runtime system, the front-end acts as a graphical server and thus the programs must connect to that GUI server in order to display forms and get user input.

The runtime system will try to connect to the front-end only when the first interactive instruction like MENU or INPUT is reached.

For the runtime system, the front end is identified by the FGLSERVER environment variable. This variable defines the host name of the machine where the front end resides, and the number of the front end instance to be used.

The syntax for FGLSERVER is:

{hostname|ip-address}[:servernum]

For example:

$ FGLSERVER=fox:1
$ fglrun myprog

The servernum parameter is a whole number that defines the instance of the front-end. It is actually defining a TCP port number the front-end is listening to, starting from 6400. For example, if servernum equals 2, the TCP port number used is 6402 (6400+2).

This is the standard/basic connection technique, but you can set up different types of configurations. For example, you can have the front end connect to an application server via ssh, to pass through firewalls over the internet. Refer to the front end documentation for more details.

There can an exception to the standard FGLSERVER specification, if the front-end is denied to listen to a port. If you need to revert the connection principle in this particular case, use the --gui-listen option of fglrun. With this option, the runtime system will listen to the specified port, so the front-end can bind to the program and start to use the GUI protocol. The procedure to work in such configuration is the following:

Start the program with:
```
 fglrun --gui-listen tcp-port prog-name
```
Connection from the front-end, for example, with an URL with the following format:
```
fgl://host-name:tcp-port
```