This article will teach you how to run one or more Redis instances on a Linux server using systemd to spawn copies of a service. I’ll be using Redis as an example but you can also use PostgreSQL or any other service as long as you can configure the port.
The easiest way to install Redis in Linux is with your distributions package manager. Here is how you would do it on openSUSE:
sudo zypper install redis
In case your distribution doesn’t provide a Redis package, you can always follow these instructions to compile it from scratch.
Configuring a Redis instance
Once installed you will need to create a configuration file for each instance of the service you want to run.
Add a configuration file
Make a copy of the example/default file that is provided by the package
cd /etc/redis/ cp default.conf.example my_app.conf
Use a name that will help you recognize the purpose of the instance. For example if each instance will be mapped to a different application give it the name of the application. If each instance will be mapped to the same application use the port in which it will be running.
Change the ownership of the configuration file
The configuration file needs to be owned by “root” and belong to the “redis” group.
chown root.redis my_app.conf
Add a “pidfile”, a “logfile” and a “dir”
pidfile /var/run/redis/my_app.pid logfile /var/log/redis/my_app.log dir /var/lib/redis/my_app/
Each of these attributes has to match with the name of the configuration file without the extension.
Make sure the “daemonize” option is set to “no” (this is the default value)
If you set this option to yes Redis and systemd will interfere with each other when spawning the processes.
Define a “port” number.
Remember that each instance should be running on a different port.
Save the configuration file
Create the database directory at the location given in the configuration file
install -d -o redis -g redis -m 0750 /var/lib/redis/my_app
The database directory has to be owned by user “redis” and belong to the group “redis” and with permissions 750.
Repeat the steps in Configure a Redis instance for every instance you want to set up. In my case I set up a second instance called “my_other_app”
. ├── default.conf.example ├── my_app.conf └── my_other_app.conf
Adding Units to systemd for the Redis service
In order for systemd to know how to enable and start each instance individually
you will need to add a service unit inside the system configuration directory
/etc/systemd/system. For convenience you might also want to
start/stop all instances at once. For that you will need to add a target unit.
In case you installed Redis on openSUSE these two files will be already
provided for you under the sytem unit directory
Create the service unit file
redis@.servicewith the following contents:
[Unit] Description=Redis After=network.target PartOf=redis.target[Service] Type=simple User=redis Group=redis PrivateTmp=true PIDFile=/var/run/redis/%i.pid ExecStart=/usr/sbin/redis-server /etc/redis/%i.conf Restart=on-failure[Install] WantedBy=multi-user.target redis.target
The unit file is separated in sections. Each section consists of variables and the value assigned to them. In this example:
- After: when the Redis instance is enabled it will get started only after the network has been started.
- PartOf: this instance belongs to the redis.target and will get started/stopped as part of that group.
- Type: simple means the service process doesn’t fork.
- %i: a specifier that is expanded by systemd to the “my_app” instance.
Create the target unit file
redis.targetwith the following contents:
[Unit] Description=Redis target allowing to start/stop all redis@.service instances at once
Interacting with Redis
If everything went as expected you should be able to interact with the individual instances:
systemctl start redis@my_app systemctl enable redis@my_other_app
And also with all the instances at the same time:
systemctl restart redis.target systemctl stop redis.target
If things didn’t go as expected and you cannot start the instance make sure to check the instance’s status:
systemctl status redis@my_app
If the issue doesn’t show up there then check systemd’s journal:
journalctl -u redis@my_app
For example if you forgot to give the right permissions to the configuration file you’d see something like this inside the journal:
Apr 23 10:02:53 mxps redis-server: 26966:C 23 Apr 10:02:53.917 # Fatal error, can’t open config file ‘/etc/redis/my_app.conf’
Thanks to the openSUSE Redis package maintainers for creating such a nice package that you can learn from it.
The book How Linux Works provided the details on how systemd instances work.