Understanding Systemd

[Nytro]

Understanding Systemd

 

Linux distributions are adopting or planning to adopt the systemd init system fast. 

systemd is a suite of system management daemons, libraries, and utilities designed as a central management and configuration platform for the Linux computer operating system. Described by its authors as a “basic building block” for an operating system, systemd primarily aims to replace the Linux init system (the first process executed in user space during the Linux startup process) inherited from UNIX System V and Berkeley Software Distribution (BSD). The name systemd adheres to the Unix convention of making daemons easier to distinguish by having the letter d as the last letter of the filename.

 

systemd is designed for Linux and programmed exclusively for the Linux API. It is published as free and open-source software under the terms of the GNU Lesser General Public License (LGPL) version 2.1 or later.

The design of systemd generated significant controversy within the free software community, leading the critics to argue that systemd’s architecture violates the Unix philosophy and that it will eventually form a system of interlocking dependencies. However, as of 2015 most major Linux distributions have adopted it as their default init system.

 

Lennart Poettering and Kay Sievers, software engineers that initially developed systemd, sought to surpass the efficiency of the init daemon in several ways. They wanted to improve the software framework for expressing dependencies, to allow more processing to be done concurrently or in parallel during system booting, and to reduce the computational overhead of the shell.

Poettering describes systemd development as “never finished, never complete, but tracking progress of technology”. In May 2014, Poettering further defined systemd as aiming to unify “pointless differences between distributions”, by providing the following three general functions:

A system and service manager (manages both the system, as by applying various configurations, and its services)

A software platform (serves as a basis for developing other software)

 

The glue between applications and the kernel (provides various interfaces that expose functionalities provided by the kernel)

systemd is not just the name of the init daemon but also refers to the entire software bundle around it, which, in addition to the systemd init daemon, includes the daemons journald, logind and networkd, and many other low-level components. In January 2013, Poettering described systemd not as one program, but rather a large software suite that includes 69 individual binaries. As an integrated software suite, systemd replaces the startup sequences and runlevels controlled by the traditional init daemon, along with the shell scripts executed under its control. systemd also integrates many other services that are common on Linux systems by handling user logins, the system console, device hotplugging, scheduled execution (replacing cron) logging, hostnames and locales.

 

Like the init daemon, systemd is a daemon that manages other daemons, which, including systemd itself, are background processes. systemd is the first daemon to start during booting and the last daemon to terminate during shutdown. The systemd daemon serves as the root of the user space’s process tree; the first process (pid 1) has a special role on Unix systems, as it receives a SIGCHLD signal when a daemon process (which has detached from its parent) terminates. Therefore, the first process is particularly well suited for the purpose of monitoring daemons; systemd attempts to improve in that particular area over the traditional approach, which would usually not restart daemons automatically but only launch them once without further monitoring.

 

systemd executes elements of its startup sequence in parallel, which is faster than the traditional startup sequence’s sequential approach. For inter-process communication (IPC), systemd makes Unix domain sockets and D-Bus available to the running daemons. The state of systemd itself can also be preserved in a snapshot for future recall.

systemd records initialization instructions for each daemon in a configuration file (referred to as a “unit file”) that uses a declarative language, replacing the traditionally used per-daemon startup shell scripts. Unit file types include service, socket, device, mount, automount, swap, target, path, timer (which can be used as a cron-like job scheduler), snapshot, slice and scope.

 

Articol complet: https://n0where.net/understanding-systemd/