Add a collector to gather metrics on WAL size

Signed-off-by: Tom Hughes <tom@compton.nu>

collector/pg_wal.go

@@ -0,0 +1,84 @@
+// Copyright 2023 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package collector
+
+import (
+	"context"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const walSubsystem = "wal"
+
+func init() {
+	registerCollector(walSubsystem, defaultEnabled, NewPGWALCollector)
+}
+
+type PGWALCollector struct {
+}
+
+func NewPGWALCollector(config collectorConfig) (Collector, error) {
+	return &PGWALCollector{}, nil
+}
+
+var (
+	pgWALSegments = prometheus.NewDesc(
+		prometheus.BuildFQName(
+			namespace,
+			walSubsystem,
+			"segments",
+		),
+		"Number of WAL segments",
+		[]string{}, nil,
+	)
+	pgWALSize = prometheus.NewDesc(
+		prometheus.BuildFQName(
+			namespace,
+			walSubsystem,
+			"size_bytes",
+		),
+		"Total size of WAL segments",
+		[]string{}, nil,
+	)
+
+	pgWALQuery = `
+		SELECT
+			COUNT(*) AS segments,
+			SUM(size) AS size
+		FROM pg_ls_waldir()
+		WHERE name ~ '^[0-9A-F]{24}$'`
+)
+
+func (c PGWALCollector) Update(ctx context.Context, instance *instance, ch chan<- prometheus.Metric) error {
+	db := instance.getDB()
+	row := db.QueryRowContext(ctx,
+		pgWALQuery,
+	)
+
+	var segments uint64
+	var size uint64
+	err := row.Scan(&segments, &size)
+	if err != nil {
+		return err
+	}
+	ch <- prometheus.MustNewConstMetric(
+		pgWALSegments,
+		prometheus.GaugeValue, float64(segments),
+	)
+	ch <- prometheus.MustNewConstMetric(
+		pgWALSize,
+		prometheus.GaugeValue, float64(size),
+	)
+	return nil
+}

collector/pg_wal_test.go

@@ -0,0 +1,63 @@
+// Copyright 2023 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package collector
+
+import (
+	"context"
+	"testing"
+
+	"github.com/DATA-DOG/go-sqlmock"
+	"github.com/prometheus/client_golang/prometheus"
+	dto "github.com/prometheus/client_model/go"
+	"github.com/smartystreets/goconvey/convey"
+)
+
+func TestPgWALCollector(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("Error opening a stub db connection: %s", err)
+	}
+	defer db.Close()
+
+	inst := &instance{db: db}
+
+	columns := []string{"segments", "size"}
+	rows := sqlmock.NewRows(columns).
+		AddRow(47, 788529152)
+	mock.ExpectQuery(sanitizeQuery(pgWALQuery)).WillReturnRows(rows)
+
+	ch := make(chan prometheus.Metric)
+	go func() {
+		defer close(ch)
+		c := PGWALCollector{}
+
+		if err := c.Update(context.Background(), inst, ch); err != nil {
+			t.Errorf("Error calling PGWALCollector.Update: %s", err)
+		}
+	}()
+
+	expected := []MetricResult{
+		{labels: labelMap{}, value: 47, metricType: dto.MetricType_GAUGE},
+		{labels: labelMap{}, value: 788529152, metricType: dto.MetricType_GAUGE},
+	}
+
+	convey.Convey("Metrics comparison", t, func() {
+		for _, expect := range expected {
+			m := readMetric(<-ch)
+			convey.So(expect, convey.ShouldResemble, m)
+		}
+	})
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("there were unfulfilled exceptions: %s", err)
+	}
+}