Add all bool/integer/real runtime variables

Use the `pg_settings` view to retrieve runtime variables:
https://www.postgresql.org/docs/current/static/view-pg-settings.html

This replaces the use of `SHOW` to retrieve runtime variables.

In PostgreSQL 9.6, this adds 189 metrics, which use the `short_desc`
field as a description.

Only runtime variables with a `vartype` of `bool`, `real`, or `integer`
are currently supported.

Example metrics:

    # HELP pg_settings_allow_system_table_mods Allows modifications of the structure of system tables.
    # TYPE pg_settings_allow_system_table_mods gauge
    pg_settings_allow_system_table_mods 0
    # HELP pg_settings_archive_timeout_seconds Forces a switch to the next xlog file if a new file has not been started within N seconds. [Units converted to seconds.]
    # TYPE pg_settings_archive_timeout_seconds gauge
    pg_settings_archive_timeout_seconds 0
    # HELP pg_settings_array_nulls Enable input of NULL elements in arrays.
    # TYPE pg_settings_array_nulls gauge
    pg_settings_array_nulls 1
    # HELP pg_settings_authentication_timeout_seconds Sets the maximum allowed time to complete client authentication. [Units converted to seconds.]
    # TYPE pg_settings_authentication_timeout_seconds gauge
    pg_settings_authentication_timeout_seconds 60
    # HELP pg_settings_autovacuum Starts the autovacuum subprocess.
    # TYPE pg_settings_autovacuum gauge
    pg_settings_autovacuum 1
    # HELP pg_settings_autovacuum_analyze_scale_factor Number of tuple inserts, updates, or deletes prior to analyze as a fraction of reltuples.
    # TYPE pg_settings_autovacuum_analyze_scale_factor gauge
    pg_settings_autovacuum_analyze_scale_factor 0.1
    # HELP pg_settings_autovacuum_analyze_threshold Minimum number of tuple inserts, updates, or deletes prior to analyze.
    # TYPE pg_settings_autovacuum_analyze_threshold gauge
    pg_settings_autovacuum_analyze_threshold 50
    # HELP pg_settings_autovacuum_freeze_max_age Age at which to autovacuum a table to prevent transaction ID wraparound.
    # TYPE pg_settings_autovacuum_freeze_max_age gauge
    pg_settings_autovacuum_freeze_max_age 2e+08
    # HELP pg_settings_autovacuum_max_workers Sets the maximum number of simultaneously running autovacuum worker processes.
    # TYPE pg_settings_autovacuum_max_workers gauge
    pg_settings_autovacuum_max_workers 3
    # HELP pg_settings_autovacuum_multixact_freeze_max_age Multixact age at which to autovacuum a table to prevent multixact wraparound.
    # TYPE pg_settings_autovacuum_multixact_freeze_max_age gauge
    pg_settings_autovacuum_multixact_freeze_max_age 4e+08
    # HELP pg_settings_autovacuum_naptime_seconds Time to sleep between autovacuum runs. [Units converted to seconds.]
    # TYPE pg_settings_autovacuum_naptime_seconds gauge
    pg_settings_autovacuum_naptime_seconds 60
    # HELP pg_settings_autovacuum_vacuum_cost_delay_seconds Vacuum cost delay in milliseconds, for autovacuum. [Units converted to seconds.]
    # TYPE pg_settings_autovacuum_vacuum_cost_delay_seconds gauge
    pg_settings_autovacuum_vacuum_cost_delay_seconds 0.02
    # HELP pg_settings_autovacuum_vacuum_cost_limit Vacuum cost amount available before napping, for autovacuum.
    # TYPE pg_settings_autovacuum_vacuum_cost_limit gauge
    pg_settings_autovacuum_vacuum_cost_limit -1
    # HELP pg_settings_autovacuum_vacuum_scale_factor Number of tuple updates or deletes prior to vacuum as a fraction of reltuples.
    # TYPE pg_settings_autovacuum_vacuum_scale_factor gauge
    pg_settings_autovacuum_vacuum_scale_factor 0.2
    # HELP pg_settings_autovacuum_vacuum_threshold Minimum number of tuple updates or deletes prior to vacuum.
    # TYPE pg_settings_autovacuum_vacuum_threshold gauge
    pg_settings_autovacuum_vacuum_threshold 50
    # HELP pg_settings_autovacuum_work_mem_bytes Sets the maximum memory to be used by each autovacuum worker process. [Units converted to bytes.]
    # TYPE pg_settings_autovacuum_work_mem_bytes gauge
    pg_settings_autovacuum_work_mem_bytes -1

Author: mattbostock

pg_setting.go

@@ -0,0 +1,132 @@
+package main
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+	"math"
+	"strconv"
+
+	"code.cfops.it/go/log"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Query the pg_settings view containing runtime variables
+func querySettings(ch chan<- prometheus.Metric, db *sql.DB) error {
+	log.Debugln("Querying pg_setting view")
+
+	// pg_settings docs: https://www.postgresql.org/docs/current/static/view-pg-settings.html
+	//
+	// NOTE: If you add more vartypes here, you must update the supported
+	// types in normaliseUnit() below
+	query := "SELECT name, setting, COALESCE(unit, ''), short_desc, vartype FROM pg_settings WHERE vartype IN ('bool', 'integer', 'real');"
+
+	rows, err := db.Query(query)
+	if err != nil {
+		return errors.New(fmt.Sprintln("Error running query on database: ", namespace, err))
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		s := &pgSetting{}
+		err = rows.Scan(&s.name, &s.setting, &s.unit, &s.shortDesc, &s.vartype)
+		if err != nil {
+			return errors.New(fmt.Sprintln("Error retrieving rows:", namespace, err))
+		}
+
+		ch <- s.metric()
+	}
+
+	return nil
+}
+
+// pgSetting is represents a PostgreSQL runtime variable as returned by the
+// pg_settings view.
+type pgSetting struct {
+	name, setting, unit, shortDesc, vartype string
+}
+
+func (s *pgSetting) metric() prometheus.Metric {
+	var (
+		err       error
+		name      = s.name
+		unit      = s.unit
+		shortDesc = s.shortDesc
+		subsystem = "settings"
+		val       float64
+	)
+
+	switch s.vartype {
+	case "bool":
+		if s.setting == "on" {
+			val = 1
+		}
+	case "integer", "real":
+		if val, unit, err = s.normaliseUnit(); err != nil {
+			// Panic, since we should recognise all units
+			// and don't want to silently exlude metrics
+			panic(err)
+		}
+
+		if len(unit) > 0 {
+			name = fmt.Sprintf("%s_%s", name, unit)
+			shortDesc = fmt.Sprintf("%s [Units converted to %s.]", shortDesc, unit)
+		}
+	default:
+		// Panic because we got a type we didn't ask for
+		panic(fmt.Sprintf("Unsupported vartype %q", s.vartype))
+	}
+
+	desc := newDesc(subsystem, name, shortDesc)
+	return prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, val)
+}
+
+func (s *pgSetting) normaliseUnit() (val float64, unit string, err error) {
+	val, err = strconv.ParseFloat(s.setting, 64)
+	if err != nil {
+		return val, unit, errors.New(fmt.Sprintf("Error converting setting %q value %q to float: %s", s.name, s.setting, err))
+	}
+
+	// Units defined in: https://www.postgresql.org/docs/current/static/config-setting.html
+	switch s.unit {
+	case "":
+		return
+	case "ms", "s", "min", "h", "d":
+		unit = "seconds"
+	case "kB", "MB", "GB", "TB", "8kB", "16MB":
+		unit = "bytes"
+	default:
+		err = errors.New(fmt.Sprintf("Unknown unit for runtime variable: %q", s.unit))
+		return
+	}
+
+	// -1 is special, don't modify the value
+	if val == -1 {
+		return
+	}
+
+	switch s.unit {
+	case "ms":
+		val /= 1000
+	case "min":
+		val *= 60
+	case "h":
+		val *= 60 * 60
+	case "d":
+		val *= 60 * 60 * 24
+	case "kB":
+		val *= math.Pow(2, 10)
+	case "MB":
+		val *= math.Pow(2, 20)
+	case "GB":
+		val *= math.Pow(2, 30)
+	case "TB":
+		val *= math.Pow(2, 40)
+	case "8kB":
+		val *= math.Pow(2, 13)
+	case "16MB":
+		val *= math.Pow(2, 24)
+	}
+
+	return
+}

pg_setting_test.go

@@ -0,0 +1,190 @@
+// +build !integration
+package main
+
+import (
+	dto "github.com/prometheus/client_model/go"
+	. "gopkg.in/check.v1"
+)
+
+type PgSettingSuite struct{}
+
+var _ = Suite(&PgSettingSuite{})
+
+var fixtures = []fixture{
+	fixture{
+		p: pgSetting{
+			name:      "seconds_fixture_metric",
+			setting:   "5",
+			unit:      "s",
+			shortDesc: "Foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  5,
+			unit: "seconds",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_seconds_fixture_metric_seconds\", help: \"Foo foo foo [Units converted to seconds.]\", constLabels: {}, variableLabels: []}",
+		v: 5,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "milliseconds_fixture_metric",
+			setting:   "5000",
+			unit:      "ms",
+			shortDesc: "Foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  5,
+			unit: "seconds",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_milliseconds_fixture_metric_seconds\", help: \"Foo foo foo [Units converted to seconds.]\", constLabels: {}, variableLabels: []}",
+		v: 5,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "eight_kb_fixture_metric",
+			setting:   "17",
+			unit:      "8kB",
+			shortDesc: "Foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  139264,
+			unit: "bytes",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_eight_kb_fixture_metric_bytes\", help: \"Foo foo foo [Units converted to bytes.]\", constLabels: {}, variableLabels: []}",
+		v: 139264,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "16_mb_real_fixture_metric",
+			setting:   "3.0",
+			unit:      "16MB",
+			shortDesc: "Foo foo foo",
+			vartype:   "real",
+		},
+		n: normalised{
+			val:  5.0331648e+07,
+			unit: "bytes",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_16_mb_real_fixture_metric_bytes\", help: \"Foo foo foo [Units converted to bytes.]\", constLabels: {}, variableLabels: []}",
+		v: 5.0331648e+07,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "bool_on_fixture_metric",
+			setting:   "on",
+			unit:      "",
+			shortDesc: "Foo foo foo",
+			vartype:   "bool",
+		},
+		n: normalised{
+			val:  1,
+			unit: "",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_bool_on_fixture_metric\", help: \"Foo foo foo\", constLabels: {}, variableLabels: []}",
+		v: 1,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "bool_off_fixture_metric",
+			setting:   "off",
+			unit:      "",
+			shortDesc: "Foo foo foo",
+			vartype:   "bool",
+		},
+		n: normalised{
+			val:  0,
+			unit: "",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_bool_off_fixture_metric\", help: \"Foo foo foo\", constLabels: {}, variableLabels: []}",
+		v: 0,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "special_minus_one_value",
+			setting:   "-1",
+			unit:      "d",
+			shortDesc: "foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  -1,
+			unit: "seconds",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_special_minus_one_value_seconds\", help: \"foo foo foo [Units converted to seconds.]\", constLabels: {}, variableLabels: []}",
+		v: -1,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "unknown_unit",
+			setting:   "10",
+			unit:      "nonexistent",
+			shortDesc: "foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  10,
+			unit: "",
+			err:  `Unknown unit for runtime variable: "nonexistent"`,
+		},
+	},
+}
+
+func (s *PgSettingSuite) TestNormaliseUnit(c *C) {
+	for _, f := range fixtures {
+		switch f.p.vartype {
+		case "integer", "real":
+			val, unit, err := f.p.normaliseUnit()
+
+			c.Check(val, Equals, f.n.val)
+			c.Check(unit, Equals, f.n.unit)
+
+			if err == nil {
+				c.Check("", Equals, f.n.err)
+			} else {
+				c.Check(err.Error(), Equals, f.n.err)
+			}
+		}
+	}
+}
+
+func (s *PgSettingSuite) TestMetric(c *C) {
+	defer func() {
+		if r := recover(); r != nil {
+			if r.(error).Error() != `Unknown unit for runtime variable: "nonexistent"` {
+				panic(r)
+			}
+		}
+	}()
+
+	for _, f := range fixtures {
+		d := &dto.Metric{}
+		m := f.p.metric()
+		m.Write(d)
+
+		c.Check(m.Desc().String(), Equals, f.d)
+		c.Check(d.GetGauge().GetValue(), Equals, f.v)
+	}
+}
+
+type normalised struct {
+	val  float64
+	unit string
+	err  string
+}
+
+type fixture struct {
+	p pgSetting
+	n normalised
+	d string
+	v float64
+}