Go demo code lazy initialization with sync.OnceValue

main.go

/*
This code is released under the MIT license - https://opensource.org/license/mit
*/

package main

import (
	"database/sql"
	"log"
	"sync"

	_ "github.com/mattn/go-sqlite3"
)

type tDB struct {
	dsn     string
	db      func() *sql.DB
	stmt    map[string]func() *sql.Stmt
}

func newServer(dbFn string) *tDB {
	dsn := dbFn 
	return &tDB{
		dsn: dsn,
		db: sync.OnceValue(func() *sql.DB {
			db, err := sql.Open("sqlite3", dsn)
			if err != nil {
				log.Fatal(err)
			}
			log.Println("Database connection opened")
			return db
		}),
		stmt:    make(map[string]func() *sql.Stmt),
	}
}

func (d *tDB) createSchema() {
	sql := "CREATE TABLE IF NOT EXISTS user (id text NOT NULL, name text NOT NULL);"
	_, err := d.db().Exec(sql)
	if err != nil {
		log.Fatalf("%+v", err)
	}
	log.Println("Schema created")
}

func (d *tDB) prepareStmtOnce(name string, s string) func() *sql.Stmt {
	return sync.OnceValue(func() *sql.Stmt {
		stmt, err := d.db().Prepare(s)
		if err != nil {
			log.Fatal(err)
		}
		log.Printf("Prepared Statement Created with name: %s", name)
		return stmt 
	})
}

func (d *tDB) loadPrepStmtSql(name string, s string) {
	d.stmt[name] = d.prepareStmtOnce(name, s)
}

func (d *tDB) preparedStmt(name string) *sql.Stmt {
	return d.stmt[name]()
}

func main() {
	dbFn := ":memory:" 
	d := newServer(dbFn)
	d.createSchema() // Opens database connection via d.db
	d.loadPrepStmtSql("selectUser", "SELECT * FROM user")
	for i := 0; i < 10; i++ {
		log.Println(i)
		db := d.db() // log msg not printed since database was already open
		_ = db
		stmt := d.preparedStmt("selectUser") // log msg only printed on first time through
		_ = stmt
		_ = 1
	}
	log.Println("Finished")
}