Using jackc/pgx to automatically send queries to the primary server

.env

DATABASE_URL="host=db001,db002 port=5432 database=mydatabase user=myuser password=mypassword"
# The two `host` entries above are a primary and a replica in a cluster
# Change the order of the `host` entries and you will see that it always uses the primary

main.go

package main

import (
	"context"
	"errors"
	"fmt"
	"os"

	"github.com/jackc/pgconn"
	"github.com/jackc/pgx/v4/pgxpool"
	_ "github.com/joho/godotenv/autoload"
)

func main() {
	dbConfig, err := pgxpool.ParseConfig(os.Getenv("DATABASE_URL"))
	if err != nil {
		fmt.Fprintf(os.Stderr, "Unable to parse DATABASE_URL: %v\n", err)
		os.Exit(1)
	}

	dbConfig.ConnConfig.Config.ValidateConnect = func(ctx context.Context, pgconn *pgconn.PgConn) error {
		query := pgconn.Exec(context.Background(), "SELECT pg_is_in_recovery()")
		result, err := query.ReadAll()
		if err != nil {
			return err
		}

		row := result[0].Rows[0]
		field := row[0]

		if rune(field[0]) == 't' {
			return errors.New("Server is in recovery mode")
		}
		return nil
	}

	dbPool, err := pgxpool.ConnectConfig(context.Background(), dbConfig)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Unable to connect to database: %v\n", err)
		os.Exit(1)
	}
	defer dbPool.Close()

	var status string
	err = dbPool.QueryRow(context.Background(), `select 'this server is ' || (CASE pg_is_in_recovery() WHEN TRUE THEN 'replica' ELSE 'primary' END)`).Scan(&status)
	if err != nil {
		fmt.Fprintf(os.Stderr, "QueryRow failed: %v\n", err)
		os.Exit(1)
	}

	fmt.Println(status)
}