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diff --git a/nixos/doc/manual/from_md/development/unit-handling.section.xml b/nixos/doc/manual/from_md/development/unit-handling.section.xml new file mode 100644 index 00000000000..4c980e1213a --- /dev/null +++ b/nixos/doc/manual/from_md/development/unit-handling.section.xml @@ -0,0 +1,131 @@ +<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-unit-handling"> + <title>Unit handling</title> + <para> + To figure out what units need to be + started/stopped/restarted/reloaded, the script first checks the + current state of the system, similar to what + <literal>systemctl list-units</literal> shows. For each of the + units, the script goes through the following checks: + </para> + <itemizedlist> + <listitem> + <para> + Is the unit file still in the new system? If not, + <emphasis role="strong">stop</emphasis> the service unless it + sets <literal>X-StopOnRemoval</literal> in the + <literal>[Unit]</literal> section to <literal>false</literal>. + </para> + </listitem> + <listitem> + <para> + Is it a <literal>.target</literal> unit? If so, + <emphasis role="strong">start</emphasis> it unless it sets + <literal>RefuseManualStart</literal> in the + <literal>[Unit]</literal> section to <literal>true</literal> or + <literal>X-OnlyManualStart</literal> in the + <literal>[Unit]</literal> section to <literal>true</literal>. + Also <emphasis role="strong">stop</emphasis> the unit again + unless it sets <literal>X-StopOnReconfiguration</literal> to + <literal>false</literal>. + </para> + </listitem> + <listitem> + <para> + Are the contents of the unit files different? They are compared + by parsing them and comparing their contents. If they are + different but only <literal>X-Reload-Triggers</literal> in the + <literal>[Unit]</literal> section is changed, + <emphasis role="strong">reload</emphasis> the unit. The NixOS + module system allows setting these triggers with the option + <link linkend="opt-systemd.services">systemd.services.<name>.reloadTriggers</link>. + There are some additional keys in the <literal>[Unit]</literal> + section that are ignored as well. If the unit files differ in + any way, the following actions are performed: + </para> + <itemizedlist> + <listitem> + <para> + <literal>.path</literal> and <literal>.slice</literal> units + are ignored. There is no need to restart them since changes + in their values are applied by systemd when systemd is + reloaded. + </para> + </listitem> + <listitem> + <para> + <literal>.mount</literal> units are + <emphasis role="strong">reload</emphasis>ed. These mostly + come from the <literal>/etc/fstab</literal> parser. + </para> + </listitem> + <listitem> + <para> + <literal>.socket</literal> units are currently ignored. This + is to be fixed at a later point. + </para> + </listitem> + <listitem> + <para> + The rest of the units (mostly <literal>.service</literal> + units) are then <emphasis role="strong">reload</emphasis>ed + if <literal>X-ReloadIfChanged</literal> in the + <literal>[Service]</literal> section is set to + <literal>true</literal> (exposed via + <link linkend="opt-systemd.services">systemd.services.<name>.reloadIfChanged</link>). + A little exception is done for units that were deactivated + in the meantime, for example because they require a unit + that got stopped before. These are + <emphasis role="strong">start</emphasis>ed instead of + reloaded. + </para> + </listitem> + <listitem> + <para> + If the reload flag is not set, some more flags decide if the + unit is skipped. These flags are + <literal>X-RestartIfChanged</literal> in the + <literal>[Service]</literal> section (exposed via + <link linkend="opt-systemd.services">systemd.services.<name>.restartIfChanged</link>), + <literal>RefuseManualStop</literal> in the + <literal>[Unit]</literal> section, and + <literal>X-OnlyManualStart</literal> in the + <literal>[Unit]</literal> section. + </para> + </listitem> + <listitem> + <para> + Further behavior depends on the unit having + <literal>X-StopIfChanged</literal> in the + <literal>[Service]</literal> section set to + <literal>true</literal> (exposed via + <link linkend="opt-systemd.services">systemd.services.<name>.stopIfChanged</link>). + This is set to <literal>true</literal> by default and must + be explicitly turned off if not wanted. If the flag is + enabled, the unit is + <emphasis role="strong">stop</emphasis>ped and then + <emphasis role="strong">start</emphasis>ed. If not, the unit + is <emphasis role="strong">restart</emphasis>ed. The goal of + the flag is to make sure that the new unit never runs in the + old environment which is still in place before the + activation script is run. This behavior is different when + the service is socket-activated, as outlined in the + following steps. + </para> + </listitem> + <listitem> + <para> + The last thing that is taken into account is whether the + unit is a service and socket-activated. If + <literal>X-StopIfChanged</literal> is + <emphasis role="strong">not</emphasis> set, the service is + <emphasis role="strong">restart</emphasis>ed with the + others. If it is set, both the service and the socket are + <emphasis role="strong">stop</emphasis>ped and the socket is + <emphasis role="strong">start</emphasis>ed, leaving socket + activation to start the service when it’s needed. + </para> + </listitem> + </itemizedlist> + </listitem> + </itemizedlist> +</section> |