nixos/graphite: add seyren service, graphite alerting dashboard

nixos/modules/services/monitoring/graphite.nix

···
     ${cfg.api.extraConfig}
   '';
 
+  seyrenConfig = {
+    SEYREN_URL = cfg.seyren.seyrenUrl;
+    MONGO_URL = cfg.seyren.mongoUrl;
+    GRAPHITE_URL = cfg.seyren.graphiteUrl;
+  } // cfg.seyren.extraConfig;
+
   configDir = pkgs.buildEnv {
     name = "graphite-config";
     paths = lists.filter (el: el != null) [
···
         '';
       };
     };
+
+    seyren = {
+      enable = mkOption {
+        description = "Whether to enable seyren service.";
+        default = false;
+        type = types.uniq types.bool;
+      };
+
+      port = mkOption {
+        description = "Seyren listening port.";
+        default = 8081;
+        type = types.int;
+      };
+
+      seyrenUrl = mkOption {
+        default = "http://localhost:${toString cfg.seyren.port}/";
+        description = "Host where seyren is accessible.";
+        type = types.str;
+      };
+
+      graphiteUrl = mkOption {
+        default = "http://${cfg.web.host}:${toString cfg.web.port}";
+        description = "Host where graphite service runs.";
+        type = types.str;
+      };
+
+      mongoUrl = mkOption {
+        default = "mongodb://${config.services.mongodb.bind_ip}:27017/seyren";
+        description = "Mongodb connection string.";
+        type = types.str;
+      };
+
+      extraConfig = mkOption {
+        default = {};
+        description = ''
+          Extra seyren configuration. See
+          <link xlink:href='https://github.com/scobal/seyren#config' />
+        '';
+        type = types.attrsOf types.str;
+        example = literalExample ''
+          {
+            GRAPHITE_USERNAME = "user";
+            GRAPHITE_PASSWORD = "pass"; 
+          }
+        '';
+      };
+    };
   };
 
   ###### implementation
 
-  config = mkIf (cfg.carbon.enableAggregator || cfg.carbon.enableCache || cfg.carbon.enableRelay || cfg.web.enable || cfg.api.enable) {
+  config = mkIf (
+    cfg.carbon.enableAggregator ||
+    cfg.carbon.enableCache ||
+    cfg.carbon.enableRelay ||
+    cfg.web.enable ||
+    cfg.api.enable ||
+    cfg.seyren.enable
+  ) {
     systemd.services.carbonCache = {
       enable = cfg.carbon.enableCache;
       description = "Graphite Data Storage Backend";
···
         fi
       '';
     };
+
+    systemd.services.seyren = {
+      enable = cfg.seyren.enable;
+      description = "Graphite Alerting Dashboard";
+      wantedBy = [ "multi-user.target" ];
+      after = [ "network-interfaces.target" "mongodb.service" ];
+      environment = seyrenConfig;
+      serviceConfig = {
+        ExecStart = "${pkgs.seyren}/bin/seyren -httpPort ${toString cfg.seyren.port}";
+        WorkingDirectory = dataDir;
+        User = "graphite";
+        Group = "graphite"; 
+      };
+      preStart = ''
+        if ! test -e ${dataDir}/db-created; then
+          mkdir -p ${dataDir}
+          chown -R graphite:graphite ${dataDir}
+        fi
+      '';
+    };
+
+    services.mongodb.enable = mkDefault cfg.seyren.enable;
 
     environment.systemPackages = [
       pkgs.pythonPackages.carbon