Instruct to execute an unit after completing another unit successfully

Question

I use cloud-config to install and configure DCOS cluster.

Normally agentinstall.service service takes 5 minutes to complete.

Is it possible to instruct to systemd to execute agentconfigure.service only after agentinstall.service completed?

#cloud-config
coreos:
  units:
    - name: "agentinstall.service"
      command: "start"
      content: |
        [Unit]
        Description=agent_setup
        After=network.target

        [Service]
        Type=simple
        User=root
        WorkingDirectory=/tmp
        ExecStartPre=/bin/curl -o /tmp/dcos_install.sh  http://bootstapnode-0.dev.myztro.internal:9090/dcos_install.sh
        ExecStartPre=/bin/chmod 755 dcos_install.sh
        ExecStart=/bin/bash dcos_install.sh slave

        [Install]
        WantedBy=multi-user.target
    - name: "agentconfigure.service"
      command: "start"
      content: |
        [Unit]
        Description=agent_config
        After=agentinstall.service

        [Service]
        Type=simple
        User=root
        WorkingDirectory=/opt/mesosphere/etc/
        ExecStartPre=/bin/echo "MESOS_ATTRIBUTES=cluster:uploader" >> /opt/mesosphere/etc/mesos-slave-common
        ExecStartPre=/bin/rm -f /var/lib/mesos/slave/meta/slaves/latest
        ExecStart=/usr/bin/systemctl restart dcos-mesos-slave

        [Install]
        WantedBy=multi-user.target

Thank you.

Answer 1

I am not aware of a way to get this done with systemd, as I think it is only concerned about starting and running of units. As in, you can use After= to force a unit to start only AFTER the specified one has started, or both start in parallel.

Wants= will cause the specified units to be started in parallel (if the wanted units are not yet started/active), not what you want.

Requires= will, if used in conjunction with After= on agentconfigure.service, ensure that agentconfigure.service is started AFTER agentinstall.service is active ("busy"). Now, you could have a loop in agentconfigure.service that waits, say max 5 minutes, and regularly checks for agentinstall.service to complete before it proceeds with its actual work. The only option I see for this.

EDIT: Another option I think is even better ... agentinstall.service starts agentconfigure.service when installation succeeds, then exits.

Answer 2

You want to run after the /bin/bash dcos_install.sh slave command?

Change the service with that command to Type=oneshot. Also then I don't think you need to use ExecStartPre=, just use ExecStart= for everything.

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@JdeBP asks me to point out that your echo command is attempting to using shell redirection which will not work.

Answer 3

systemd 249, released in July 2021, added support for new dependency type OnSuccess, which does exactly that - allows you to specify what units should be run after this unit completes.

So if your host has systemd 249 or newer, agentinstall.service needs:

[Unit]
Description=agent_setup
After=network.target
OnSuccess=agentconfigure.service

Now you can do systemctl start agentinstall.service, and whenever it finishes, agentconfigure.service will be started automatically.

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For completion - there's also OnFailure= dependency which does the same, but triggers only when unit has failed. It has been introduced in systemd v201, released sometime back in 2013.