Talk Preparation: systemd’s D-Bus Implementation, And Its Python asyncio Binding

• Slides: systemd’s D-Bus Implementation, And Its Python asyncio Binding
  • TL;DR
  • Show
  • A Big Picture
  • Sample Client (sdbus, Blocking)
  • Defining Interfaces, Pythonically
  • History/Implementations/Bindings ⟶ sdbus
  • Concrete Use Case: jf-irrigation
  • Irrigation Client: Enter asyncio
  • First Step: Create Proxy
  • Naive try: Use Async Definition To Block
  • Fix: Async Machinery
  • Create Irrigator Proxies
  • Print Statistics
  • D-Bus Signals
  • And Parallelism?
  • Introduce asyncio.TaskGroup
  • Keep In Mind …
  • More asyncio

Old Stuff, Random Notes

D-Bus Show

• Monitoring/Debugging

  $ dbus-monitor --session
  

D-Bus Spec, Implementations

• What is D-Bus: Blah protocol and type system https://dbus.freedesktop.org/doc/dbus-specification.html

  jjj draw sketch jf-irrigation

• Very good D-Bus intro: Poettering: The new sd-bus API of systemd

• D-Bus reference implementation https://www.freedesktop.org/wiki/Software/dbus/

  • Python binding https://dbus.freedesktop.org/doc/dbus-python/

  • Really old and quirky, cannot remember

• Glib’s D-Bus implementation

  • Source: https://gitlab.gnome.org/GNOME/glib

  • gdbus commandline tool, https://gnome.pages.gitlab.gnome.org/libsoup/gio/gdbus.html

  • Python binding LEW21/pydbus

  • Interfaces defined in XML, pretty weird. Show jfasch/openheating

• SD-Bus

  • https://www.freedesktop.org/software/systemd/man/latest/sd-bus.html

  • Python binding https://python-sdbus.readthedocs.io/en/latest/

• What else? bring this at the end

  • Errors: https://python-sdbus.readthedocs.io/en/latest/exceptions.html#list-of-error-exception

  • https://dbus.freedesktop.org/doc/dbus-specification.html

Client Implementation

• Bus name (service name): me.faschingbauer.IrrigationService

• Object path: /me/faschingbauer/IrrigationSystem

• Interface name: me.faschingbauer.IrrigationSystem, as parameter of DBusIrrigationSystem (see jfasch/jf-irrigation)

• Irrigation system

  • Simple: blocking operations

    • Structure/objects ⟶ sketch

  • Challenge: expose objects via D-Bus. Continue irrigation in the “background” while serving D-Bus operations. No-go: create a thread.

• Read doc: https://python-sdbus.readthedocs.io/en/latest/examples.html

Gotchas, Weirdnesses

• D-Bus methods (and properties) are always BiCapitalized; if you name a method some_method, SDBus will translate it to SomeMethod

• DBus socket is watched somehow magically. in main(), I’d expect I have to create a task for DBus traffic - but no. Must be done magically inside some of the routines that we use to export object and register names.

• complain about confusion: busname, path, and iface name

What Else?

Irrigation Outlook

• Hardware, finally

• User interface with textual

• Add logging, error reporting, watchdog