Init and Delete Device

Init and Delete Device#

The basic lifecycle of a Tango device can be simplified into two parts: initialisation and clean-up. This happens within the operating system process that is called the device server.

As you saw in the previous lesson there is an init_device() method. Your device can also have a delete_device() method. Both of these override the default implementation in the base class Device.

Python

main.py#

from tango.server import Device


class MegaCoffee3k(Device):

    def __init__(self, *args, **kwargs):
        print("MegaCoffee3k: __init__ start")
        super().__init__(*args, **kwargs)
        print("MegaCoffee3k: __init__ end")

    def init_device(self):
        super().init_device()
        print("MegaCoffee3k: init_device")

    def delete_device(self):
        print("MegaCoffee3k: delete_device")
        super().delete_device()


if __name__ == "__main__":
    MegaCoffee3k.run_server()

C++

Sorry, still TODO!

Java

Sorry, still TODO!

Here the init_device and delete_device methods simply print a message. The__init__ method is included with some print statements to show when it is called in the lifecycle.

Tip

It is important to still call the super class methods, as shown. init_device at the start of your method, and delete_device at the end of your method.

Bonus tip: __init__

Device methods that use the Tango “machinery” cannot be called before Device.__init__ has completed. E.g., the logging methods like info_stream, and things like push_change_event, which you’ll learn about later.

Run this example server, and you’ll see the output:

MegaCoffee3k: __init__ start
MegaCoffee3k: init_device
MegaCoffee3k: __init__ end
Ready to accept request
MegaCoffee3k started on port 8888 with properties {}
Device access: tango://127.0.0.1:8888/test/nodb/megacoffee3k#dbase=no
Server access: tango://127.0.0.1:8888/dserver/MegaCoffee3k/megacoffee3k#dbase=no

The “init_device” message is printed before the device is ready to accept requests. In this Python example, the init_device call came from the base class constructor, Device.__init__.

In a second terminal, use the device proxy client and send the built-in Init() command, which will re-initialise the device.

>>> dp.Init()
>>>

MegaCoffee3k: __init__ start
MegaCoffee3k: init_device
MegaCoffee3k: __init__ end
Ready to accept request
MegaCoffee3k started on port 8888 with properties {}
Device access: tango://127.0.0.1:8888/test/nodb/megacoffee3k#dbase=no
Server access: tango://127.0.0.1:8888/dserver/MegaCoffee3k/megacoffee3k#dbase=no
MegaCoffee3k: delete_device
MegaCoffee3k: init_device

To re-initialise the MegaCoffee3k device, Tango first calls the delete_device method, and then the init_device method. The constructor, __init__, is not called, so you still have the same Python object instance.

Finally, use the keyboard combination Ctrl+C to end the Tango device server application.

MegaCoffee3k: __init__ start
MegaCoffee3k: init_device
MegaCoffee3k: __init__ end
Ready to accept request
MegaCoffee3k started on port 8888 with properties {}
Device access: tango://127.0.0.1:8888/test/nodb/megacoffee3k#dbase=no
Server access: tango://127.0.0.1:8888/dserver/MegaCoffee3k/megacoffee3k#dbase=no
MegaCoffee3k: delete_device
MegaCoffee3k: init_device
^CMegaCoffee3k: delete_device
Done

You can see that delete_device method was called again during the graceful shutdown.

Tip

Variables needed for the device state are normally initialised in init_device rather than in __init__, since init_device gets called again when the Init() command is used.

Tip

If any system resources (memory, connections, file handles, etc.) need to be released when the device shuts down, this kind of clean-up is usually done in delete_device.

Restarting the device#

Using the admin device, you can restart the device completely, while the device server, i.e., operating system process, remains running. This is typically done less often than calling Init() on the device.

Start your example device server again.

Create another client, this time using the Server access Tango resource locator. Send the built-in DevRestart() command, with the name of the device: test/nodb/megacoffee3k.

>>> import tango
>>> ap = tango.DeviceProxy("tango://127.0.0.1:8888/dserver/MegaCoffee3k/megacoffee3k#dbase=no")
>>> ap.DevRestart("test/nodb/megacoffee3k")
>>>

You’ll see the following output:

MegaCoffee3k: __init__ start
MegaCoffee3k: init_device
MegaCoffee3k: __init__ end
Ready to accept request
MegaCoffee3k started on port 8888 with properties {}
Device access: tango://127.0.0.1:8888/test/nodb/megacoffee3k#dbase=no
Server access: tango://127.0.0.1:8888/dserver/MegaCoffee3k/megacoffee3k#dbase=no
MegaCoffee3k: delete_device
MegaCoffee3k: __init__ start
MegaCoffee3k: init_device
MegaCoffee3k: __init__ end

This time you notice a second call to __init__, so a new instance of the Python object representing the device has been created.

Bonus tip: RestartServer

The admin device also has a RestartServer() command than can be used to restart all the devices in a server, without naming a specific one. In this example ap.RestartServer().

Summary#

There are various ways of restarting/re-initialising a Tango device. From least to most drastic:

What?	How?	Process	Object
Re-initialise device	Send `Init()` command to device	Unchanged	Unchanged
Re-start device	Send `DevRestart()` or `RestartServer()` to admin device	Unchanged	New
Re-start device server	Terminate operating system process and run it again	New	New

Init and Delete Device

Contents

Init and Delete Device#

Restarting the device#

Summary#