How can I update my DB with a new version without interference with clients (golang / sqlite3)?

Question

The database I am using is a global variable, initialized reading the file sql-repo.db:

const dbFile = "sql-repo.db"

var globalDB *LocalDB

type LocalDB struct {
    Path   string
    handle *sql.DB
}

func InitSqlDB(dbDir string) error {
    if globalDB != nil {
        return nil
    }
    db := LocalDB{Path: filepath.Join(dbDir, dbFile)}
    var err error
    db.handle, err = sql.Open("sqlite3", db.Path)
    if err != nil {
        return err
    }
    globalDB = &db
    return nil
}

From time to time, I will have an updated version of this database that I can download and store in dbDir.

Ideas that I have:

• Use ATTACH DATABASE sql-repo.db AS dbMain to attach a copy of the first database and use it by default.
  When I have my new .db file, I attach it as well ATTACH DATABASE sql-repo-new.db AS dbNew
  Then I detach dbMain and rename dbNew to dbMain
• Simply change the address my globalDB is pointing to:

const newDBFile = "sql-repo-new.db"

func PullNewDB(dbDir string) error {
    db := LocalDB{Path: filepath.Join(dbDir, newDBFile)}
    var err error
    db.handle, err = sql.Open("sqlite3", db.Path)
    if err != nil {
        return err
    }
    globalDB = &db
    return nil
}

How can I or how should I update my globalDB with the new version/file as I want to avoid any interference if the clients that I have in my code are connected to the DB and are querying it ?

Should I attach a sync.RWMutex to my LocalDB struct and then lock/unlock it when I do the update ?
Or should I use a channel to ask every client to stop querying the DB ?

Thank you for any help / advice / suggestion !

Answer 1

Or you can start a separate go routine or process that syncs your old database with the new file. Do an insert or update on all rows from new to old, then delete on missing rows. If it's all done in a single transaction, all queries will either read all old or new data without ever blocking.

An additional benefit is the separation of concerns, your application code doesn't get clustered with update logic, and in the case, the new file is corrupted, the update transaction errors out and no harm is done.