141 lines
6.5 KiB
XML
141 lines
6.5 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-systemctl">
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<title>Service Management</title>
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<para>
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In NixOS, all system services are started and monitored using the
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systemd program. systemd is the <quote>init</quote> process of the
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system (i.e. PID 1), the parent of all other processes. It manages a
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set of so-called <quote>units</quote>, which can be things like
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system services (programs), but also mount points, swap files,
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devices, targets (groups of units) and more. Units can have complex
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dependencies; for instance, one unit can require that another unit
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must be successfully started before the first unit can be started.
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When the system boots, it starts a unit named
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<literal>default.target</literal>; the dependencies of this unit
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cause all system services to be started, file systems to be mounted,
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swap files to be activated, and so on.
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</para>
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<section xml:id="sect-nixos-systemd-general">
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<title>Interacting with a running systemd</title>
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<para>
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The command <literal>systemctl</literal> is the main way to
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interact with <literal>systemd</literal>. The following paragraphs
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demonstrate ways to interact with any OS running systemd as init
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system. NixOS is of no exception. The
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<link linkend="sect-nixos-systemd-nixos">next section </link>
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explains NixOS specific things worth knowing.
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</para>
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<para>
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Without any arguments, <literal>systemctl</literal> the status of
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active units:
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</para>
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<programlisting>
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$ systemctl
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-.mount loaded active mounted /
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swapfile.swap loaded active active /swapfile
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sshd.service loaded active running SSH Daemon
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graphical.target loaded active active Graphical Interface
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...
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</programlisting>
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<para>
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You can ask for detailed status information about a unit, for
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instance, the PostgreSQL database service:
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</para>
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<programlisting>
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$ systemctl status postgresql.service
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postgresql.service - PostgreSQL Server
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Loaded: loaded (/nix/store/pn3q73mvh75gsrl8w7fdlfk3fq5qm5mw-unit/postgresql.service)
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Active: active (running) since Mon, 2013-01-07 15:55:57 CET; 9h ago
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Main PID: 2390 (postgres)
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CGroup: name=systemd:/system/postgresql.service
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├─2390 postgres
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├─2418 postgres: writer process
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├─2419 postgres: wal writer process
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├─2420 postgres: autovacuum launcher process
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├─2421 postgres: stats collector process
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└─2498 postgres: zabbix zabbix [local] idle
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Jan 07 15:55:55 hagbard postgres[2394]: [1-1] LOG: database system was shut down at 2013-01-07 15:55:05 CET
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Jan 07 15:55:57 hagbard postgres[2390]: [1-1] LOG: database system is ready to accept connections
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Jan 07 15:55:57 hagbard postgres[2420]: [1-1] LOG: autovacuum launcher started
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Jan 07 15:55:57 hagbard systemd[1]: Started PostgreSQL Server.
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</programlisting>
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<para>
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Note that this shows the status of the unit (active and running),
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all the processes belonging to the service, as well as the most
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recent log messages from the service.
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</para>
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<para>
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Units can be stopped, started or restarted:
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</para>
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<programlisting>
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# systemctl stop postgresql.service
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# systemctl start postgresql.service
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# systemctl restart postgresql.service
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</programlisting>
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<para>
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These operations are synchronous: they wait until the service has
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finished starting or stopping (or has failed). Starting a unit
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will cause the dependencies of that unit to be started as well (if
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necessary).
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</para>
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</section>
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<section xml:id="sect-nixos-systemd-nixos">
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<title>systemd in NixOS</title>
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<para>
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Packages in Nixpkgs sometimes provide systemd units with them,
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usually in e.g <literal>#pkg-out#/lib/systemd/</literal>. Putting
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such a package in <literal>environment.systemPackages</literal>
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doesn't make the service available to users or the system.
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</para>
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<para>
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In order to enable a systemd <emphasis>system</emphasis> service
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with provided upstream package, use (e.g):
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</para>
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<programlisting language="bash">
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systemd.packages = [ pkgs.packagekit ];
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</programlisting>
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<para>
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Usually NixOS modules written by the community do the above, plus
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take care of other details. If a module was written for a service
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you are interested in, you'd probably need only to use
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<literal>services.#name#.enable = true;</literal>. These services
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are defined in Nixpkgs'
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<link xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/modules">
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<literal>nixos/modules/</literal> directory </link>. In case the
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service is simple enough, the above method should work, and start
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the service on boot.
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</para>
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<para>
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<emphasis>User</emphasis> systemd services on the other hand,
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should be treated differently. Given a package that has a systemd
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unit file at <literal>#pkg-out#/lib/systemd/user/</literal>, using
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<xref linkend="opt-systemd.packages" /> will make you able to
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start the service via <literal>systemctl --user start</literal>,
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but it won't start automatically on login. However, You can
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imperatively enable it by adding the package's attribute to
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<xref linkend="opt-systemd.packages" /> and then do this (e.g):
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</para>
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<programlisting>
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$ mkdir -p ~/.config/systemd/user/default.target.wants
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$ ln -s /run/current-system/sw/lib/systemd/user/syncthing.service ~/.config/systemd/user/default.target.wants/
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$ systemctl --user daemon-reload
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$ systemctl --user enable syncthing.service
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</programlisting>
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<para>
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If you are interested in a timer file, use
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<literal>timers.target.wants</literal> instead of
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<literal>default.target.wants</literal> in the 1st and 2nd
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command.
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</para>
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<para>
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Using <literal>systemctl --user enable syncthing.service</literal>
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instead of the above, will work, but it'll use the absolute path
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of <literal>syncthing.service</literal> for the symlink, and this
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path is in <literal>/nix/store/.../lib/systemd/user/</literal>.
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Hence <link linkend="sec-nix-gc">garbage collection</link> will
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remove that file and you will wind up with a broken symlink in
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your systemd configuration, which in turn will not make the
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service / timer start on login.
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</para>
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</section>
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</chapter>
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