在golang中的接口之间共享conn指针

Question

What I'm trying to accomplish is sharing a pointer of db.sqlx between multiple functions, except for posts saying pass along the pointer, which is fine but how to do that in an interface? I cannot find anything that illustrates the use of this anywhere. Basically what I have is an interface of type Datastore. I also have mysql & pgsql that implements the Datastore type. The interface by itself works fine however the issue is I'm trying to create a single connect function for *sqlx.DB to be shared across all functions within the implemented interface. I think the issue is I've confused myself on how to share the pointer between functions of the interface or even "where" to share it. The main interface looks like below:

var (
    storage Datastore
    db * sqlx.DB
)

type Datastore interface {
    Insert(db *sqlx.DB, table string, item DataItem) bool
    CheckEmpty(db *sqlx.DB, table string) bool
    FetchAll(db *sqlx.DB, table string) []DataItem
    DBInit(db *sqlx.DB)
    initDB()
}

Within my implemented interface (simplified mysql example) I have the initDB function which looks like this:

type MySQLDB struct {
    config *config.Configuration
}


func (my *MySQLDB) initDB() {
    log.Println("Getting DB Connection")
    tempdb, err := sqlx.Connect("mysql", my.config.Database.Dsn+"&parseTime=True")
    if err != nil {
        log.Println(err.Error())
    }
    db = tempdb
    defer db.Close()
}

func (my *MySQLDB) FetchAll(db *sqlx.DB, table string) []DataItem {
    dTable := []DataItem{}
    query := "SELECT foo, bar FROM " + table + " ORDER BY last_update ASC"
    err := db.Select(&dTable, query)
    if err != nil{
        panic(err)
    }
    return dTable
}

At this point I know the connection is initially opened but the next time a function is called I get db is closed error. So how do I properly share the db connection between functions, or do I really have to run a connection open in every function?

Answer 1

Don't call defer db.Close() in your initDB function. After that function executed, db will close too! So when you call your method you get the closed error.
Maybe you need to re-desgin your interface, for example:

type Datastore interface {
    Insert(table string, item DataItem) bool
    CheckEmpty(table string) bool
    FetchAll(table string) []DataItem
    Close() error // call this method when you want to close the connection
    initDB()
}

Your MySQLDB implement now look like:

type MySQLDB struct {
    config *config.Configuration
    db *sqlx.DB
}


func (my *MySQLDB) initDB() {
    log.Println("Getting DB Connection")
    tempdb, err := sqlx.Connect("mysql", my.config.Database.Dsn+"&parseTime=True")
    if err != nil {
        log.Println(err.Error())
    }
    my.db = tempdb
}

func (my *MySQLDB) Close() error {
    return my.db.Close()
}

func (my *MySQLDB) FetchAll(table string) []DataItem {
    dTable := []DataItem{}
    query := "SELECT foo, bar FROM " + table + " ORDER BY last_update ASC"
    err := my.db.Select(&dTable, query)
    if err != nil{
        panic(err)
    }
    return dTable
}