docs/architecture/openbmc-systemd.md

1 # OpenBMC & Systemd
3 OpenBMC uses [systemd](https://www.freedesktop.org/wiki/Software/systemd/) to
5 which processes are started. There is a lot of documentation on systemd and to
9 [Unit](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#) -
10 Units are the basic framework of all systemd work.
11 [Service](https://www.freedesktop.org/software/systemd/man/systemd.service.html) -
13 [Target](https://www.freedesktop.org/software/systemd/man/systemd.target.html) -
19 On an OpenBMC system, you can go to /lib/systemd/system/ and see all of the
20 systemd units on the system. You can easily cat these files to start looking at
22 /etc/systemd/system and /run/systemd/system as well.
24 [systemctl](https://www.freedesktop.org/software/systemd/man/systemctl.html) is
25 the main tool you use to interact with systemd and its units.
49 ls -1 /lib/systemd/system/obmc-host-start@0.target.requires/
67 ls -1 /lib/systemd/system/obmc-host-startmin@0.target.requires/
80 ls -1 /lib/systemd/system/obmc-chassis-poweron@0.target.requires/
85 If you run `systemctl start obmc-host-start@0.target` then systemd will start
134 ## Systemd Control in OpenBMC
136 There are a collection of services within OpenBMC that interact with systemd and
138 OpenBMC system and systemd. See the
149 Underneath the covers, this is calling systemd with the server power on target.
151 ## Systemd Services or Monitoring Applications
154 some logic in the context of systemd targets is whether they should be triggered
155 by systemd services or by monitoring for the appropriate D-Bus signal indicating
158 The basic guidelines for when to create a systemd service are the following:
160 - If your application logic depends on other systemd based services then make it
161   a systemd service and utilize the Wants/After/Before service capabilities.
163   systemd service.
165   services that run after it, then it should be a systemd service. This ensures
168 ## Error Handling of Systemd
179 There are two main failure scenarios when it comes to OpenBMC and systemd usage:
194 2. A failure outside of a normal systemd target/service (host watchdog expires,
198   appropriate error, and instructing systemd to go to the appropriate target
223   to call systemd with the `obmc-quiesce-host@.target` on failures
238 There are some tools available out there to visualize systemd service and target
239 dependencies (systemd-analyze) but due to the complexity of our design, they do