Process Management on Linux (Media Engineering Architecture & Deployment)

# Process Management on Linux

Learn about process management on Linux and how to do it with [systemd][systemd].

This material is part of [architecture & deployment course](https://github.com/MediaComem/comem-archidep) for [Media Engineering](https://heig-vd.ch/formations/bachelor/filieres/ingenierie-des-medias).

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## What is process management?

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Many processes must run for a computer to do its job.

Which processes to run and when to run them depends on the use case:

* On a desktop computer, you need processes to be launched to display the graphical user interface (GUI), for example.
* On a web-facing server, you probably need to run a database and a web server at least.

Some processes may have **dependencies**:
i.e. they might need other processes to be already running in order to run properly themselves.
For example, a PHP application might need a MySQL database to store some data.

Processes also need to be **managed**, i.e. it should be easy to start them, stop them, restart them,
or to configure them to automatically restart if they crash.

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### Process managers

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**Process managers** are programs that fulfill this role:
i.e. make sure that the correct processes are run in the correct order and are managed correctly thereafter.

Most operating systems have a process manager built in:

> Some of these do more than just process management,
> like systemd which also manages the Linux init process to bootstrap the operating system.

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### Lightweight process managers

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Simpler process managers also exist,
meant to be used to run and manage one application.
Some of them are generic while others are specific to a programming language:

They are easier to use but have fewer features.
For example, none of those can handle dependencies between processes.

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## Systemd

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### What is systemd?

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[Systemd][systemd] provides not only process management,
but also the fundamental building blocks of a [Linux][linux] operating system,
such as an [init system][init].

> Systemd is a replacement for the [Unix System V][unix-system-v] init system
> and has been the de facto standard for most Linux distributions since 2015.

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### Unit files

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Systemd records the instructions on how to launch and manage a process in a configuration file referred to as a [**unit file**][systemd-unit].
It supports many unit file types such as `service`, `socket`, `timer`, etc.

The start of a unit file usually looks like this:

```
[Unit]
Description=Something to run

[Install]
WantedBy=something-else.target

...
```

`[Unit]` is one section in the configuration file.
There will be other sections depending on the type of unit file.

`[Install]` is an optional section that can be used to express dependencies between units.
Here, the `WantedBy` option specifies that this unit will be started after the `something-else` unit has been started.

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### Service unit file

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The unit file type that interests us is [`service`][systemd-service],
which will configure, launch and manage a long-running process such as a database or application.

A service unit file will have a `[Service]` section.
Common options are:

And [many more][systemd-service-options].

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### Location of unit files

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Unit files are named after their type.
For example, a `service` unit file will be named `something.service`.

Systemd will look in several directories for unit files:

Location              | Description
:---                  | :---
`/lib/systemd/system` | Unit files installed by the operating system or by packages (e.g. a database)
`/etc/systemd/system` | Unit files installed by the system administrator

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### The `systemctl` command

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The `systemctl` or **system** **c**on**t**ro**l** command can be used to enable/disable and start/stop units once their configuration file is in place:

Command                          | Description
:------------------------------- | :-----------------------------------------------------------------------------------------------
`sudo systemctl status <unit>`   | Display the current status of a unit.
`sudo systemctl enable <unit>`   | Enable a new unit file. This will enable it to start on boot if it has the correct dependencies.
`sudo systemctl start <unit>`    | Start a unit.
`sudo systemctl stop <unit>`     | Stop a unit.
`sudo systemctl restart <unit>`  | Restart (stop and start) a unit.
`sudo systemctl reenable <unit>` | Re-enable an existing unit file after its dependencies have been modified.
`sudo systemctl daemon-reload`   | Reload unit files after they have been modified.

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### The `journalctl` command

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Services started with systemd have their standard output and standard error streams collected and logged by [journald][systemd-journald],
one of the default services provided by systemd itself.
The `systemctl status` command sometimes shows you an excerpt of these logs.

The `journalctl` command allows you to read the full logs:

Command                        | Description
:---                           | :---
`sudo journalctl -u <unit>`    | Display a unit's logs.
`sudo journalctl -f -u <unit>` | Display and follow a unit's logs in real time.

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## References

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* [Systemd Unit Configuration][systemd-unit]
* [Systemd Service Configuration][systemd-service]
* [Systemd Execution Environment Configuration][systemd-exec]
* [Red Hat Enterprise Linux - System Administrator's Guide - Managing Services With Systemd](https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/chap-managing_services_with_systemd)
* [Systemd journald Service][systemd-journald]

[god]: http://godrb.com/
[init]: https://en.wikipedia.org/wiki/Init
[launchd]: https://en.wikipedia.org/wiki/Launchd
[linux]: https://en.wikipedia.org/wiki/Linux
[macos]: https://en.wikipedia.org/wiki/MacOS
[node]: https://nodejs.org/en/
[pm2]: http://pm2.keymetrics.io/
[php]: http://www.php.net/
[php-fpm]: http://php.net/manual/en/install.fpm.php
[ruby]: https://www.ruby-lang.org/
[python]: https://www.python.org/
[supervisord]: http://supervisord.org/
[systemd]: https://en.wikipedia.org/wiki/Systemd
[systemd-exec]: https://www.freedesktop.org/software/systemd/man/systemd.exec.html
[systemd-journald]: https://www.freedesktop.org/software/systemd/man/systemd-journald.service.html
[systemd-service]: https://www.freedesktop.org/software/systemd/man/systemd.service.html
[systemd-service-options]: https://www.freedesktop.org/software/systemd/man/systemd.service.html#Options
[systemd-unit]: https://www.freedesktop.org/software/systemd/man/systemd.unit.html
[unix]: https://en.wikipedia.org/wiki/Unix
[unix-system-v]: https://en.wikipedia.org/wiki/UNIX_System_V
[windows]: https://en.wikipedia.org/wiki/Microsoft_Windows
[windows-scm]: https://en.wikipedia.org/wiki/Service_Control_Manager