Qt: Writing to a database from multiple threads using one connection

Question

Here's the problem: let's say we have a lot of threads (>1000) receiving data via some protocol and I want to write this data in database (sql server).

For database connection, we use QSqlDatabase. According to the documentation of QSqlDatabase:

A connection can only be used from within the thread that created it. Moving connections between threads or creating queries from a different thread is not supported.

Considering we cannot create 1000 connections (which can cause performance problems), what are the options here?

What comes to mind is to create a separate thread for database writings handling.

But how to run this thread in background and wait for signals from data receiving threads? What are the other approaches that can be used in this situation?

"run this thread in background and wait for signals from data receiving threads" - What's wrong with this method? — TheDarkKnight, Feb 18 '15 at 14:35

score 2 · Answer 1 · answered Feb 18 '15 at 14:45

i used a database connection as a global logger, so you might be able to adapt it to fit your needs. the skeleton looks like:

    //Logger.h
class Logger : public QObject
{
    Q_OBJECT
    ...
public:
    void log(const QString& msg):

private:
    LoggerWorker* w;
    QThread t;
}


// Logger.cpp
Logger::Logger(QObject* p)
:QObject(p)
{
    w = new LoggerWorker;
    w->moveToThread(&t);
    connect(&t, SIGNAL(started()), w, SLOT(init()));
    t.start();
}

...
void 
Logger::log(const QString& msg)
{
    QMetaObject::invokeMethod(w, "log", Q_ARG(QString, msg));
}

// LoggerWorker.h
class LoggerWorker : public QObject
{
    Q_OBJECT
    ...
public slots:
    void init()
    {
        // create database and connect to it
    }
    void log(const QString& msg)
    {
        // insert to database
    }

private:
    QSqlDatabase db;
}

note: code above might contain nuts and bugs

cheers

score 1 · Answer 2 · answered Feb 18 '15 at 16:40

The best option is to have a class for working with database which resides in separate thread.You can perform all the database related operations in relevant slots. Different threads can interact with object of the class through connecting their signals to it's slots. Since the emitter and receiver exist in different threads, you will have a queued connection type. So different signals that are emitted from different threads would be queued in the database class to be processed.

Qt: Writing to a database from multiple threads using one connection

2 Answers2