Connecting with a front-end

In graphical mode, depending on the front-end technology that is used (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 instances to be used.

The syntax for FGLSERVER is:

{hostname|ip-address}[:servernum]

$ 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.