nixpkgs/nixos/modules/services/monitoring/prometheus/exporters.xml

<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="module-services-prometheus-exporters">
 <title>Prometheus exporters</title>
 <para>
  Prometheus exporters provide metrics for the <link xlink:href="https://prometheus.io">prometheus monitoring system</link>.
 </para>
 <section xml:id="module-services-prometheus-exporters-configuration">
  <title>Configuration</title>

  <para>
   One of the most common exporters is the <link xlink:href="https://github.com/prometheus/node_exporter">node exporter</link>, it provides hardware and OS metrics from the host it's running on. The exporter could be configured as follows:
<programlisting>
  services.prometheus.exporters.node = {
    enable = true;
    enabledCollectors = [
      "logind"
      "systemd"
    ];
    disabledCollectors = [
      "textfile"
    ];
    openFirewall = true;
    firewallFilter = "-i br0 -p tcp -m tcp --dport 9100";
  };
</programlisting>
   It should now serve all metrics from the collectors that are explicitly enabled and the ones that are <link xlink:href="https://github.com/prometheus/node_exporter#enabled-by-default">enabled by default</link>, via http under <literal>/metrics</literal>. In this example the firewall should just allow incoming connections to the exporter's port on the bridge interface <literal>br0</literal> (this would have to be configured seperately of course). For more information about configuration see <literal>man configuration.nix</literal> or search through the <link xlink:href="https://nixos.org/nixos/options.html#prometheus.exporters">available options</link>.
  </para>
 </section>
 <section xml:id="module-services-prometheus-exporters-new-exporter">
  <title>Adding a new exporter</title>

  <para>
   To add a new exporter, it has to be packaged first (see <literal>nixpkgs/pkgs/servers/monitoring/prometheus/</literal> for examples), then a module can be added. The postfix exporter is used in this example:
  </para>

  <itemizedlist>
   <listitem>
    <para>
     Some default options for all exporters are provided by <literal>nixpkgs/nixos/modules/services/monitoring/prometheus/exporters.nix</literal>:
    </para>
   </listitem>
   <listitem override='none'>
    <itemizedlist>
     <listitem>
      <para>
       <literal>enable</literal>
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>port</literal>
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>listenAddress</literal>
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>extraFlags</literal>
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>openFirewall</literal>
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>firewallFilter</literal>
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>user</literal>
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>group</literal>
      </para>
     </listitem>
    </itemizedlist>
   </listitem>
   <listitem>
    <para>
     As there is already a package available, the module can now be added. This is accomplished by adding a new file to the <literal>nixos/modules/services/monitoring/prometheus/exporters/</literal> directory, which will be called postfix.nix and contains all exporter specific options and configuration:
<programlisting>
# nixpgs/nixos/modules/services/prometheus/exporters/postfix.nix
{ config, lib, pkgs, options }:

with lib;

let
  # for convenience we define cfg here
  cfg = config.services.prometheus.exporters.postfix;
in
{
  port = 9154; # The postfix exporter listens on this port by default

  # `extraOpts` is an attribute set which contains additional options
  # (and optional overrides for default options).
  # Note that this attribute is optional.
  extraOpts = {
    telemetryPath = mkOption {
      type = types.str;
      default = "/metrics";
      description = ''
        Path under which to expose metrics.
      '';
    };
    logfilePath = mkOption {
      type = types.path;
      default = /var/log/postfix_exporter_input.log;
      example = /var/log/mail.log;
      description = ''
        Path where Postfix writes log entries.
        This file will be truncated by this exporter!
      '';
    };
    showqPath = mkOption {
      type = types.path;
      default = /var/spool/postfix/public/showq;
      example = /var/lib/postfix/queue/public/showq;
      description = ''
        Path at which Postfix places its showq socket.
      '';
    };
  };

  # `serviceOpts` is an attribute set which contains configuration
  # for the exporter's systemd service. One of
  # `serviceOpts.script` and `serviceOpts.serviceConfig.ExecStart`
  # has to be specified here. This will be merged with the default
  # service confiuration.
  # Note that by default 'DynamicUser' is 'true'.
  serviceOpts = {
    serviceConfig = {
      DynamicUser = false;
      ExecStart = ''
        ${pkgs.prometheus-postfix-exporter}/bin/postfix_exporter \
          --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
          --web.telemetry-path ${cfg.telemetryPath} \
          ${concatStringsSep " \\\n  " cfg.extraFlags}
      '';
    };
  };
}
</programlisting>
    </para>
   </listitem>
   <listitem>
    <para>
     This should already be enough for the postfix exporter. Additionally one could now add assertions and conditional default values. This can be done in the 'meta-module' that combines all exporter definitions and generates the submodules: <literal>nixpkgs/nixos/modules/services/prometheus/exporters.nix</literal>
    </para>
   </listitem>
  </itemizedlist>
 </section>
 <section xml:id="module-services-prometheus-exporters-update-exporter-module">
  <title>Updating an exporter module</title>

  <para>
   Should an exporter option change at some point, it is possible to add information about the change to the exporter definition similar to <literal>nixpkgs/nixos/modules/rename.nix</literal>:
<programlisting>
{ config, lib, pkgs, options }:

with lib;

let
  cfg = config.services.prometheus.exporters.nginx;
in
{
  port = 9113;
  extraOpts = {
    # additional module options
    # ...
  };
  serviceOpts = {
    # service configuration
    # ...
  };
  imports = [
    # 'services.prometheus.exporters.nginx.telemetryEndpoint' -> 'services.prometheus.exporters.nginx.telemetryPath'
    (mkRenamedOptionModule [ "telemetryEndpoint" ] [ "telemetryPath" ])

    # removed option 'services.prometheus.exporters.nginx.insecure'
    (mkRemovedOptionModule [ "insecure" ] ''
      This option was replaced by 'prometheus.exporters.nginx.sslVerify' which defaults to true.
    '')
    ({ options.warnings = options.warnings; })
  ];
}
</programlisting>
  </para>
 </section>
</chapter>