Running Multiple Instances of a Service

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.

Installing Redis

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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 the upstream instructions to compile it from scratch.

Configuring a Redis instance

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1. 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.

2. Change the ownership of the newly created configuration file to user “root” and group “redis”

   chown root.redis my_app.conf
   

3. Configuration

   Add a “pidfile”, a “logfile” and a “dir” to the .conf file.

   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.

   daemonize no
   

   Define a “port” number and remember that each instance should be running on a different port.

   port 6379
   

4. 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 group “redis” and with permissions 750.

Repeat these steps 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

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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 located at /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 system unit directory /usr/lib/systemd/system.

1. Create the service unit file “redis@.service” with 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.

2. Create the target unit file “redis.target” with the following contents:

   [Unit]
   Description=Redis target allowing to start/stop all redis@.service instances at once
   

Interacting with Redis

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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

Troubleshooting

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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]: 26966:C 23 Apr 10:02:53.917
# Fatal error, can’t open config file ‘/etc/redis/my_app.conf’

Acknowledgments

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• 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.

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