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