Centos7 / RHEL7 Services with SystemD + Systemctl For Dummies – Presto Example

History – SystemV & Init.d

Historically in Centos and RHEL, you would use system-v to run a service.  Basically an application (e.g. Spring Boot) would provide an init-d script and you would either place it in /etc/init.d or place a symbolic link from there to your script.

The scripts would have functions for start/stop/restart/status and they would follow some general conventions.  Then you could use “chkconfig” to turn the services on so they would start with the sysem when it rebooted.

SystemD and SystemCTL

Things have moved on a bit and now you can use SystemD instead.  It is a very nice alternative.  Basically, you put a “unit” file in /etc/systemd/system/.service.  This unit file has basic information on what type of application you are trying to run and how it works.  You can specify the working directory, etc as well.

Here is an example UNIT file for Facebook’s Presto application.  We would place this at /etc/systemd/system/presto.service.

[Unit]
Description=Presto
After=syslog.target network.target

[Service]
User=your-user-here
Type=forking
ExecStart=/opt/presto/current/bin/launcher start
ExecStop=/opt/presto/current/bin/launcher stop
WorkingDirectory=/opt/presto/current/bin/
Restart=always

[Install]
WantedBy=multi-user.target

Here are the important things to note about this:

  1. You specify the user the service will run as – it should have access to the actual program location.
  2. Type can be “forking” or “simple”.  Forking implies that you have specific start and stop commands to manage the service (i.e. it kind of manages itself).  Simple implies that you’re just running something like a bash script or a Java JAR that runs forever (so SystemD will just make sure to start it with the command you give and restart it if it fails).
  3. Restart=always will make sure that, as long as you had it started in the first place, it starts whenever it does.  Try it; just kill -9 your application and it will come back.
  4. The install section is critical if you want the application to start up when the computer reboots.  You can not enable it for restart without this.

Useful Commands

  • sudo systemctl status presto (or your app name) –> current status.
  • sudo systemctl stop presto
  • sudo systemctl start presto
  • sudo systemctl restart presto
  • sudo systemctl enable presto -> enable for starting on reboot of server.
  • sudo systemctl disable presto -> don’t start on reboot of server.
  • sudo systemctl is-enabled presto; echo $? –> show if it is currently enabled for start-on-boot.

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