[systemd-devel] systemd service start/stop conditions

[Mario Schuknecht]

I have the following problem:
There are 3 (or more) systemd targets. Each of the three targets starts a
number of systemd services.
It can be switched back and forth between the systemd targets.

As a limitation, it is necessary that the services of a target start only
if the services of actual target are already stopped. Because there have
resources to be released, such as a serial port.

The problem is solved with Unit paramter Conflicts. Starting a target will
stop the other.

The following systemd units demonstrate the problem.
3 systemd targets, 3 services and a dummy-program
================================
mode-1.target
--------------------
[Unit]
Description=mode 1

Conflicts=mode-2.target
Conflicts=mode-3.target

[Install]
WantedBy=multi-user.target
================================
mode-2.target
--------------------
[Unit]
Description=mode 2

Conflicts=mode-1.target
Conflicts=mode-3.target

After=mode-1.target

[Install]
WantedBy=multi-user.target
================================
mode-3.target
--------------------
[Unit]
Description=mode 3

Conflicts=mode-1.target
Conflicts=mode-2.target

After=mode-1.target
After=mode-2.target

[Install]
WantedBy=multi-user.target
================================
unit-a-resource-1.service
-----------------------------------
[Unit]
Description=Unit A use resource 1

After=mode-1.target

Conflicts=mode-2.target
Conflicts=mode-3.target

[Service]
ExecStart=/usr/bin/dummy_unit.sh "Unit A use resource 1"
TimeoutSec=10

[Install]
WantedBy=mode-1.target
================================
unit-b-resource-1.service
-----------------------------------
[Unit]
Description=Unit B use resource 1

After=mode-2.target

Conflicts=mode-1.target
Conflicts=mode-3.target

[Service]
ExecStart=/usr/bin/dummy_unit.sh "Unit B use resource 1"


[Install]
WantedBy=mode-2.target
================================
unit-c-resource-2.service
------------------------------------
[Unit]
Description=Unit C use resource 2

After=mode-3.target

Conflicts=mode-1.target
Conflicts=mode-2.target

[Service]
ExecStart=/usr/bin/dummy_unit.sh "Unit C use resource 2"


[Install]
WantedBy=mode-3.target
================================
dummy_unit.sh
----------------------
#!/bin/bash

running=true
trap 'echo got signal TERMinated; running=false' TERM

echo "$1"

# do something
while $running
do
        sleep 1
done

# simulate longer shutdown
sleep 1
sleep 1
sleep 1
sleep 1
sleep 1

echo "$1 done"
================================
#####################################

Switch from mode-2.target to mode-1.target
systemctl start mode-1.target

systemd[1]: Stopping Unit B use resource 1...
dummy_unit.sh[3303]: Terminated
dummy_unit.sh[3303]: got signal TERMinated
dummy_unit.sh[3303]: Terminated
systemd[1]: Stopped Unit B use resource 1.
dummy_unit.sh[3303]: Unit B use resource 1 done
systemd[1]: Stopping mode 2.
systemd[1]: Stopped target mode 2.
systemd[1]: Starting mode 1.
systemd[1]: Reached target mode 1.
systemd[1]: Starting Unit A use resource 1...
systemd[1]: Started Unit A use resource 1.
dummy_unit.sh[3322]: Unit A use resource 1

systemd service service unit-b-resource-1.service is stopped an than
service unit-a-resource-1.service starts.

This all works only if the line

After=mode-1.target

exist in mode-2.target and the lines

After=mode-1.target
After=mode-2.target

exists in mode-3.target.
=======================================
Without these lines, it looks like this:

Switch from mode-2.target to mode-1.target
systemctl start mode-1.target

systemd[1]: Stopping Unit B use resource 1...
dummy_unit.sh[1009]: Terminated
dummy_unit.sh[1009]: got signal TERMinated
systemd[1]: Starting mode 1.
systemd[1]: Reached target mode 1.
systemd[1]: Starting Unit A use resource 1...
dummy_unit.sh[1009]: Terminated
systemd[1]: Started Unit A use resource 1.
dummy_unit.sh[2873]: Unit A use resource 1
dummy_unit.sh[1009]: Unit B use resource 1 done
systemd[1]: Stopped Unit B use resource 1.
systemd[1]: Stopping mode 2.
systemd[1]: Stopped target mode 2.

systemd service unit-a-resource-1.service starts before the other service
unit-b-resource-1.service is stopped.

Can anyone explain this behavior? (The parameter "After=" in mode-2.target
and mode-3.target but not in mode-1.target)
Can someone give a hint or a reference to the appropriate documentation?
Or is this all working by change? I use systemd version 211.
Is there a better solution?

I hope I could describe the problem understandable.

Regards,

Mario
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