Stopping systemd unit together with another. Starting works

Question

On a CentOS7 server I have an application stack composed by a Tomcat web server and a MySQL DB server, both installed on the same VM. I would like them to start and stop together in this order:

START: MySQL--> Tomcat
STOP: Tomcat-->MySQL

Reading systemd unit documentation, I managed to get them start together with Requires= directive, but when I stop tomcat with systemctl stop tomcat.service, MySQL keeps on running. On system logs I noticed that it does not even try to stop MySQL, so there must be something wrong with the systemd unit.

Here is my unit:

# Systemd unit file for tomcat
[Unit]
Description=Apache Tomcat Web Application Container
After=syslog.target network.target mysql.service
Requires=mysql.service

[Service]
Type=forking

Environment=JAVA_HOME=/opt/jdk
Environment=CATALINA_PID=/opt/tomcat/temp/tomcat.pid
Environment=CATALINA_HOME=/opt/tomcat
Environment=CATALINA_BASE=/opt/tomcat

ExecStart=/opt/tomcat/bin/startup.sh
ExecStop=/bin/kill -15 $MAINPID

User=tomcat
Group=tomcat

[Install]
WantedBy=multi-user.target

Answer 1

What you are seeing here is the expected systemd behavior. The Requires= dependency will make sure mysql.service is started whenever tomcat.service starts, but once started the two units are independent and one won't be stopped when the other is.

If you really want mysql.service to be stopped when tomcat.service is, you can use the PartOf= directive which links units on stop and restart.

For the example you described (having mysql.service stop whenever tomcat.service is stopped), what you need is to add PartOf=tomcat.service to the definition of mysql.service. Usually, the best way to do so is to use an override file, which you can do with systemctl edit mysql.service which will open a text editor with an empty file, then you can add this snippet to it:

[Unit]
PartOf=tomcat.service

This will get saved in a file /etc/systemd/system/mysql.service.d/override.conf which becomes part of mysql.service, you can check that with systemctl cat mysql.service.

After those changes and a systemctl daemon-reload, this should work as you expect...

Regarding ordering, everything should work as you expect with the single After=mysql.service you have in tomcat.service, since the dependencies are respected in the reverse order when stopping services. (Which means, in this case, tomcat.service will be stopped first, followed by mysql.service.)

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Stopping units this way might not be always a good idea... Perhaps a slightly better approach is to create a separate .target unit to group all services you want to control together. Perhaps something like webservices.target.

You would create that unit with contents such as:

[Unit]
Description=Web Services
Requires=tomcat.service mysql.service
After=tomcat.target mysql.service

[Install]
WantedBy=multi-user.target

And then have both tomcat.service and mysql.service set a PartOf=webservices.target, using the override mechanism described above.

Enable the target unit with systemctl enable webservices.target, and then you can start and stop both services together with systemctl start webservices.target or systemctl stop webservices.target.