Add support for using an adapter with read replica

main.go

@@ -48,7 +48,6 @@ type config struct {
 	telemetryPath      string
 	pgPrometheusConfig pgprometheus.Config
 	logLevel           string
-	readOnly           bool
 	haGroupLockId      int
 	restElection       bool
 	prometheusTimeout  time.Duration
@@ -150,11 +149,11 @@ func parseFlags() *config {
 	flag.StringVar(&cfg.listenAddr, "web.listen-address", ":9201", "Address to listen on for web endpoints.")
 	flag.StringVar(&cfg.telemetryPath, "web.telemetry-path", "/metrics", "Address to listen on for web endpoints.")
 	flag.StringVar(&cfg.logLevel, "log.level", "debug", "The log level to use [ \"error\", \"warn\", \"info\", \"debug\" ].")
-	flag.BoolVar(&cfg.readOnly, "read.only", false, "Read-only mode. Don't write to database.")
 	flag.IntVar(&cfg.haGroupLockId, "leader-election.pg-advisory-lock-id", 0, "Unique advisory lock id per adapter high-availability group. Set it if you want to use leader election implementation based on PostgreSQL advisory lock.")
 	flag.DurationVar(&cfg.prometheusTimeout, "leader-election.pg-advisory-lock.prometheus-timeout", -1, "Adapter will resign if there are no requests from Prometheus within a given timeout (0 means no timeout). "+
 		"Note: make sure that only one Prometheus instance talks to the adapter. Timeout value should be co-related with Prometheus scrape interval but add enough `slack` to prevent random flips.")
 	flag.BoolVar(&cfg.restElection, "leader-election.rest", false, "Enable REST interface for the leader election")
+
 	flag.Parse()
 
 	return cfg
@@ -184,7 +183,7 @@ type reader interface {
 
 func buildClients(cfg *config) (writer, reader) {
 	pgClient := pgprometheus.NewClient(&cfg.pgPrometheusConfig)
-	if cfg.readOnly {
+	if pgClient.ReadOnly() {
 		return &noOpWriter{}, pgClient
 	}
 	return pgClient, pgClient

postgresql/client.go

@@ -39,6 +39,7 @@ type Config struct {
 	pgPrometheusChunkInterval time.Duration
 	useTimescaleDb            bool
 	dbConnectRetries          int
+	readOnly                  bool
 }
 
 // ParseFlags parses the configuration flags specific to PostgreSQL and TimescaleDB
@@ -59,6 +60,7 @@ func ParseFlags(cfg *Config) *Config {
 	flag.DurationVar(&cfg.pgPrometheusChunkInterval, "pg.prometheus-chunk-interval", time.Hour*12, "The size of a time-partition chunk in TimescaleDB")
 	flag.BoolVar(&cfg.useTimescaleDb, "pg.use-timescaledb", true, "Use timescaleDB")
 	flag.IntVar(&cfg.dbConnectRetries, "pg.db-connect-retries", 0, "How many times to retry connecting to the database")
+	flag.BoolVar(&cfg.readOnly, "pg.read-only", false, "Read-only mode. Don't write to database. Useful when pointing adapter to read replica")
 	return cfg
 }
 
@@ -105,18 +107,23 @@ func NewClient(cfg *Config) *Client {
 		cfg: cfg,
 	}
 
-	err = client.setupPgPrometheus()
+	if !cfg.readOnly {
+		err = client.setupPgPrometheus()
 
-	if err != nil {
-		log.Error("err", err)
-		os.Exit(1)
-	}
+		if err != nil {
+			log.Error("err", err)
+			os.Exit(1)
+		}
 
-	createTmpTableStmt, err = db.Prepare(fmt.Sprintf(sqlCreateTmpTable, cfg.table))
-	if err != nil {
-		log.Error("msg", "Error on preparing create tmp table statement", "err", err)
-		os.Exit(1)
+		createTmpTableStmt, err = db.Prepare(fmt.Sprintf(sqlCreateTmpTable, cfg.table))
+		if err != nil {
+			log.Error("msg", "Error on preparing create tmp table statement", "err", err)
+			os.Exit(1)
+		}
+	} else {
+		log.Info("msg", "Running in read-only mode. Skipping schema/extension setup (should already be present)")
 	}
+
 	return client
 }
 
@@ -165,6 +172,10 @@ func (c *Client) setupPgPrometheus() error {
 	return nil
 }
 
+func (c *Client) ReadOnly() bool {
+	return c.cfg.readOnly
+}
+
 func metricString(m model.Metric) string {
 	metricName, hasName := m[model.MetricNameLabel]
 	numLabels := len(m) - 1