如果数据库可以访问，如何快速检查？ （Qt，QML，C ++） - Linux

Question

I use qt with qml and c++. On my application i use a database. It all works, if the database is reachable.

My problem is, that i would like to check, if database is reachable (like ping).

I tried

    db.setDatabaseName(dsn);
    if(db.isValid())
    {

        if(db.open())
        {
            //std::cout <<"Offene Datenbank";
            connected=true;

        }
        else
        {
            connected=false;
        }


    }
    else
    {
        connected=false;
    }

and give the connected value as result. But that takes very long (maybe 30 seconds), if there is no connection. How i can check fast, if i have a database connection?

Is there maybe a way to break the command .open after 5 seconds not connected?

Answer 1

I have studied this question a bit. Here is what I found out.

The problem is in default db connection timeout - it is too long. Each db allows you to change it to an acceptable value, using their own API. In Qt there is one common db interface - QSqlDatabase . And it does not have such method. You can set connection settings by calling it's QSqlDatabase::setConnectOptions method, but it accepts only predefined list of options (which you can read in Qt's help).

For PostgreSQL there is an option connect_timeout , so you can write:

db.setConnectOptions("connect_timeout=5"); // Set to 5 seconds

For other databases there is no such parameter. Connection options of each db are parsed in it's 'driver' class, which derives QSqlDriver and is stored in a 'driver' library.

So, what you can do:

1. You can rewrite database's driver in order it to accept timeout option.
2. You can write separate code for each db, using it's native API.

UPDATE

Turns out, that ODBC has SQL_ATTR_CONNECTION_TIMEOUT option.

UPDATE 2

qsql_odbc.cpp:713

} else if (opt.toUpper() == QLatin1String("SQL_ATTR_CONNECTION_TIMEOUT")) {
    v = val.toUInt();
    r = SQLSetConnectAttr(hDbc, SQL_ATTR_CONNECTION_TIMEOUT, (SQLPOINTER) v, 0);

https://msdn.microsoft.com/en-us/library/ms713605(v=vs.85).aspx

SQL_ATTR_CONNECTION_TIMEOUT (ODBC 3.0)

An SQLUINTEGER value corresponding to the number of seconds to wait for any request on the connection to complete before returning to the application. The driver should return SQLSTATE HYT00 (Timeout expired) anytime that it is possible to time out in a situation not associated with query execution or login.

If ValuePtr is equal to 0 (the default), there is no timeout.

Should work fine...

Answer 2

I think one easy solution is to just check the ping of the database sever. You can use platform specific ways for pinging.

This would work on Linux :

int exitCode = QProcess::execute("ping", QStringList() << "-c 2" << serverIp);
if (exitCode==0) 
{
    // is reachable
} else 
{
    // is not reachable
}

Answer 3

I suggest having some separate thread/class where you check connection and emit signal after some timeout if nothing happens (with a check - knowConnection - if we found out already if its connected). This code is not tested and written from scratch on top of my head.. may contain some errors.

/// db connection validator in separate thread
void validator::doValidate() {
 this->knowConnection = false;
 db.setDatabaseName(dsn);
 if(db.isValid())
 {
     QTimer::singleShot(1000, [this]() { 
         if (!this->knowConnection) {
             emit connected(false);dm->connected=false;
         }
     });
     if(db.open())
     {
         //std::cout <<"Offene Datenbank";
         this->knowConnection = true;
         dm->connected=true;
         emit connected(true);
     }
     else
     {
         dm->connected=false;
         this->knowConnection = true;
         emit connected(false);
     }
 } 
 else
 {
     dm->connected=false;
     this->knowConnection = true;
     emit connected(false);
 }
}

/// db manager in different thread
void dm::someDbFunction() {
    if (connected) {
        /// db logic
    }
}

/// in gui or whatever
MainWindow::MainWindow() : whatever, val(new validator(..), .. {      
    connect(val, SIGNAL(connected(bool)), this, SLOT(statusSlot(bool));
    ....
}

void MainWindow::statusSlot(bool connected) {
    ui->statusBar->setText((connected?"Connected":"Disconnected"));
}