switch liquid designate to use prometheus queries for usage

Author: wagnerd3

docs/liquids/designate.md

@@ -13,7 +13,11 @@ This liquid provides support for the DNS service Designate.
 
 ## Service-specific configuration
 
-None.
+| Field | Type | Description |
+| ----- | ---- | ----------- |
+| `prometheus_config.api` | [`promquery.Config`](https://pkg.go.dev/github.com/sapcc/go-bits@v0.0.0-20260306103145-db6bddb66aec/promquery#Config) | Configuration for the Prometheus connection from which usage data is queried by the liquid. |
+| `prometheus_config.queries.zones` | [`text/template`](https://pkg.go.dev/text/template) compatible string | Prometheus query for scraping the number of zones per project. The template should contain a filter string `{{.ProjectUUID}}` to be filled with the UUID of the project to be queried for usages. |
+| `prometheus_config.queries.recordsets_per_zone` | [`text/template`](https://pkg.go.dev/text/template) compatible string | Prometheus query for scraping the maximum number of recordsets across all zones of this project. The template should contain a filter string `{{.ProjectUUID}}` to be filled with the UUID of the project to be queried for usages. |
 
 ## Resources
 

internal/liquids/designate/liquid.go

@@ -4,18 +4,31 @@
 package designate
 
 import (
+	"bytes"
 	"context"
 	"errors"
+	"fmt"
+	"math"
 	"net/http"
+	"text/template"
 
 	"github.com/gophercloud/gophercloud/v2"
 	. "github.com/majewsky/gg/option"
 	"github.com/sapcc/go-api-declarations/liquid"
+	"github.com/sapcc/go-bits/promquery"
 	"github.com/sapcc/go-bits/respondwith"
 )
 
 // Logic implements the liquidapi.Logic interface for Designate.
 type Logic struct {
+	// configuration
+	PrometheusConfig struct {
+		APIConfig promquery.Config `json:"api"`
+		Queries   struct {
+			Zones             string `json:"zones"`
+			RecordsetsPerZone string `json:"recordsets_per_zone"`
+		} `json:"queries"`
+	} `json:"prometheus_config"`
 	// connections
 	DesignateV2 *Client `json:"-"`
 }
@@ -62,24 +75,43 @@ func (l *Logic) ScanUsage(ctx context.Context, projectUUID string, req liquid.Se
 		return liquid.ServiceUsageReport{}, err
 	}
 
-	// to query usage, start by listing all zones
-	zoneIDs, err := l.DesignateV2.listZoneIDs(ctx, projectUUID)
-	if err != nil {
-		return liquid.ServiceUsageReport{}, err
-	}
-
-	// query "recordsets per zone" usage by counting recordsets in each zone
-	// individually (we could count all recordsets over the all project at once,
-	// but that won't help since the quota applies per individual zone)
-	maxRecordsetsPerZone := uint64(0)
-	for _, zoneID := range zoneIDs {
-		count, err := l.DesignateV2.countZoneRecordsets(ctx, projectUUID, zoneID)
+	// note: The following data is available via designate API, but we transitioned to use
+	// Prometheus queries because of the faster response times for more frequent scraping.
+	client, err := l.PrometheusConfig.APIConfig.Connect()
+	scrapeUsageMetric := func(query string) (uint64, error) {
+		if query == "" {
+			return 0, errors.New("config for this metric is missing")
+		}
+		tmpl, err := template.New("query").Parse(query)
 		if err != nil {
-			return liquid.ServiceUsageReport{}, err
+			return 0, fmt.Errorf("error while parsing the template: %w", err)
+		}
+		data := map[string]any{
+			"ProjectUUID": projectUUID,
 		}
-		if maxRecordsetsPerZone < count {
-			maxRecordsetsPerZone = count
+		var templated bytes.Buffer
+		err = tmpl.Execute(&templated, data)
+		if err != nil {
+			return 0, fmt.Errorf("error while filling the template: %w", err)
 		}
+		defaultValue := float64(0)
+		value, err := client.GetSingleValue(ctx, templated.String(), &defaultValue)
+		if err != nil {
+			return 0, fmt.Errorf("error while retrieving prometheus value: %w", err)
+		}
+		if value < 0 || math.IsNaN(value) || math.IsInf(value, 0) {
+			return 0, fmt.Errorf("unexpected value: %f", value)
+		}
+		return uint64(math.Round(value)), nil
+	}
+
+	zones, err := scrapeUsageMetric(l.PrometheusConfig.Queries.Zones)
+	if err != nil {
+		return liquid.ServiceUsageReport{}, fmt.Errorf("while scraping zones usage: %w", err)
+	}
+	maxRecordsetsPerZone, err := scrapeUsageMetric(l.PrometheusConfig.Queries.RecordsetsPerZone)
+	if err != nil {
+		return liquid.ServiceUsageReport{}, fmt.Errorf("while scraping recordsets per zone usage: %w", err)
 	}
 
 	return liquid.ServiceUsageReport{
@@ -88,7 +120,7 @@ func (l *Logic) ScanUsage(ctx context.Context, projectUUID string, req liquid.Se
 			"zones": {
 				Quota: Some(quotas.Zones),
 				PerAZ: liquid.InAnyAZ(liquid.AZResourceUsageReport{
-					Usage: uint64(len(zoneIDs)),
+					Usage: zones,
 				}),
 			},
 			"recordsets_per_zone": {

main.go

@@ -88,6 +88,7 @@ func main() {
 			opts.ServiceInfoRefreshInterval = 0
 			must.Succeed(liquidapi.Run(ctx, &cronus.Logic{}, opts))
 		case "designate":
+			opts.TakesConfiguration = true
 			opts.ServiceInfoRefreshInterval = 0
 			must.Succeed(liquidapi.Run(ctx, &designate.Logic{}, opts))
 		case "ironic":