How do I tell why a systemd service was started?

I am trying to figure out why a specific service is started on a system, and I wonder if there is a command to tell why a specific service was started?

When asking for status on the service, systemctl claims that it is disabled, yet it is running and I haven't explicitly asked to start it, AFAICT. Output below (slightly anonymized)

# systemctl status myservice
● myservice.service - My Service
   Loaded: loaded (/usr/lib/systemd/system/myservice.service; disabled; vendor preset: disabled)
   Active: active (running) since mån 2017-02-27 13:57:15 CET; 30min ago
     Docs: http://www.example.com/
 Main PID: 4680 (ewe)
   CGroup: /system.slice/myservice.service
           ├─4680 /opt/myservice/vbc/bin/myservice
           └─4944 /opt/myservice/vbc/bin/myservice

I am starting a service that has set Before=myservice.service in its .service file (no Requires), so I am suspecting that, but I can't tell for sure.

This is running on a CentOS 7.3 system.

EDITED: I have been able to work around the issue by making sure that a service that the above service has a Requires=, After= and Wants= relationship to, and which is started by a transient run-once service, is explicitly stopped. When doing this, the service is not started. I am not closer to figuring out why it was started in the first place, however.

EDITED: It seems that my service file is started whenever one of the services it has a Requires relationship to is restarted. I did not expect that to happen, I assumed that it would only mean that my service would start the other service when started, not that it would also start mine. Removing Requires fixes the phantom restarts.

Solution 1:

Running sudo systemctl status might help, as might systemctl list-dependencies.

I had heard of services being started on install on (I think) Ubuntu, but never on CentOS. However, IF your service has a socket unit defined, then traffic to a given socket/port could be causing your service to start automatically.

I have such a setup for TFTP, for example:

sudo systemctl list-dependencies | grep socket

│ ├─sockets.target
│ │ ├─dbus.socket
<snip>
│ │ └─tftp.socket

Config for it says to start the service if UDP/69 traffic is received:

# /usr/lib/systemd/system/tftp.socket
[Unit]
Description=Tftp Server Activation Socket

[Socket]
ListenDatagram=69

[Install]
WantedBy=sockets.target