kick off a systemd service just before shutdown

Question

I am developing a systemd shutdown script (unit file) for a postgresql database and I followed the below two posts but could not get it to work.

How to run a script with systemd right before shutdown?

https://superuser.com/questions/1016827/how-do-i-run-a-script-before-everything-else-on-shutdown-with-systemd

Here is what I am doing with my postgresql installation: I have written a startup script that starts the postgresql database cluster at boot time but the shutdown script shown below does not seem to work at shutdown time. While it works fine if I execute it with systemctl start service_name.

When I looked at the database log, I could see a message "LOG: received smart shutdown request" which is the message when database cluster goes down in normal mode which is what the shutting down operating system does when I shutdown the host with a "shutdown -h now". What I want to see is "LOG: received fast shutdown request" which is a fast (immediate mode) shutdown which occurs when I run the script with a systemctl start service_name

Here is the unit file definition

[Unit]
Description=Database Shutdown
Before=shutdown.target reboot.target halt.target
DefaultDependencies=no
[Service]
ExecStart=/usr/bin/su - postgres -c "/opt/postgres-95/bin/pg_ctl -D /database/inst1/data -w stop &"
Type=oneshot
RemainAfterExit=yes
[Install]
WantedBy=shutdown.target

I also tried to ditch the [Install] section but the systemctl would not let me enable the service without the [Install] section. could someone please point me in the right direction.

Answer 1

systemd is designed to cleanly handle this case. You can do so by creating a small file specifies a command you want to run as part of stopping the PostgreSQL service. First, create a directory named:

/etc/systemd/system/postgresql.d/

In it, create a file with any name that ends in .conf, like custom-shutdown.conf. Add to that file a couple lines like this:

[Service]
ExecStop=/usr/bin/su - postgres -c "/opt/postgres-95/bin/pg_ctl -D /database/inst1/data -w stop &" 

You may wish to use ExecStopPost= depending your case.

See official docs for Extending the Default Configuration and also official docs for the ExecStop= and ExecStopPost=.