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Server Generator and the Client python programs

Overview

The Server Generator program produces a template python server program. The server program may blindly broadcast data to the client application or supply data to the client on command from the client.

This server program should be modified to suite your requirements. Focus on having the server program capture and process data. The server template program is easily adapted to dispatch that data to a client.

The inter-process communication (IPC) between the server and the client is performed over the System D-Bus (as opposed to the Session D-Bus).

The Linux operating systems systemd auto-starts the server application. The systemctl utility may be used to manage the server application, while the journalctl will report the status of the server.

Practical Example

For a practical example a Raspberry Pi computer has General Purpose Input/Output (GPIO) ports. A sensor in a machine monitors a rotating wheel. The sensor sends pulses into a GPIO port. The server program counts the pulses. Every second the server processes the number of pulses counted. It then publishes onto the system D-Bus this processed data. For example the data may be the calculated speed of the circumference of a wheel. As long as the Raspberry Pi is booted this data is published. The server program may also retains the value of the maximum speed.

When a client application is launched it subscribes and receives the regularly published data from the server. The client is then responsible for displaying this data. If the user wants the value for the maximum speed, then the client performs a "method call" to the server and the server returns this value.

If the client application is no longer required it can be shut-down and restarted at another time. Meanwhile the server will be unaware of the status of the client and the server continues performing its task.

The server application that is generated and the client applications supplied allow the developer to focus on the task the server should perform, without having to devote too much time toward understanding and implementing the D-Bus inter-process data transfer.

These programs were written to demonstrate:

  • The use of pydbus library for python3 to provide system D_Bus signal emission publishing and subscription and D-Bus method calls.

  • The use of systemd to launch a local python server program on booting.

  • The use of systemd's systemctl to start a local python server program. In the GUI client this is performed with subprocess.run(). It may also be performed at the command line using systemctl start.

  • A systemd .service file is provided to support systemd services to the local server template file.

  • A D-Bus configuration file is provided to modify the standard system D-Bus configuration so that permissions are correct.

  • The locally located python server template file will run as a server and interacts with the client programs via the system D-Bus.

  • The python server template file that is created, should then be modified to create a server program that meets the developers specifications.

Installation and Operation

Prerequisites are:

  • Python 3.5 or higher in order to support subprocess.run() function.

  • pydbus module installed system-wide. pydbus module requires GLib at version 2.46 or higher. Recent distro's support this with:

    $ sudo apt-get install python3-pydbus.

    Otherwise to install from PyPI enter:

    $ sudo pip3 install pydbus --target=/usr/lib/python3/dist-packages

Installation:

Unzip the files into a local folder. E.g. ~/server_generator

The folders contents will be:

client_1

client_2

client_3

client_4

client_5

help_client_full.md

help_client.md

help_full_server_generator.md

help_server_generator.md

README.md

server_generator

Use root priv to run the server_generator. E.g.

$ sudo server_generator

If you accept the defaults supplied in the installation script and also request the starting of the server program, then the program in ~/project_1/server_1/app_1 will be launched.

app_1 is the server template application. It contains code that makes it run for 100 seconds:

    if counter < 100:
        return True
    else:
        loop.quit()
        sys.exit()
        return False

Edit this code to make this program run for longer, or remove it to make it continuously run.

The client programs may be launched and each one adds a little more complexity with regard to its interaction over the D-Bus with the server app_1.

Removing template server application

To remove the template application server and its folders, the systemd service, and the D-Bus configuration file:

$ sudo server_generator --remove

The removal script will use systemctl to stop, disable and delete the systemd service that runs the server application. A choice is provided as to whether or not to remove the server application and its folders, and the D-Bus configuration file.

Presentation

These programs were demonstrated at the Waikato Linux User Group meeting on 27 June 2017. A slide show was also delivered as an introduction. This slide show is the included file Client Server presentation.