The right way to use QtSqlDatabase across multiple threads
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Hello,
It's being two days that I am trying to make a multithreaded application that need to collect data from several databases.
I tried to make a QRunnable + QThreadPool but I get an first chance access violation when calling QSqlDatabase::open() on a db created in the QRunnable::run(). After this failed I tried to use Worker-Object+QThread this option work, but I didn't figure out how to put the thread on sleep when data collection is done (I collect data every 30m).
I need to know about the correct and the right way to use QSqlDatabase across multiple threads. -
Hi,
To add to @VRonin, one thing you have to do is to name your connection on a per thread basis. Otherwise you'll have threads starting more or less at the "same time" thus trying to use the default connection and therefore shooting at each other.
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I agree with you on naming connection which I did, every thread is using his unique connection name.
Worker object:class AgencyMasterInfoFetcher: public QObject { Q_OBJECT QSqlDatabase* _db; bool _cant_fetch; public: explicit AgencyMasterInfoFetcher(QSqlDatabase* db, QObject* parent = nullptr); public slots: void fetch(); void stop(); signals: void closing(); void fetched(AgencyMaster); void error(QString); };Implementation
void AgencyMasterInfoFetcher::fetch() { if(_cant_fetch) return; if(!_db->isOpen()) if(!_db->open()) { emit error(QString{"Couldn't connect to the database"}); qDebug() << "Done fetching"; return; } _cant_fetch = true; // fetching some data emit fetched(agency); _cant_fetch = false; } void AgencyMasterInfoFetcher::stop() { _db->close(); emit closing(); }starting threads from main.cpp
int main() { QVector<QThread*> threads {}; for (int i = 0; i < dbs.size(); i++) threads << new QThread(); QVector<AgencyMasterInfoFetcher*> workers{}; for (int i = 0; i < dbs.size(); i++) { workers << new AgencyMasterInfoFetcher(&dbs[i]); workers[i]->moveToThread(threads[i]); QObject::connect(agencies[i],SIGNAL(needData()),workers[i],SLOT(fetch()),Qt::QueuedConnection); QObject::connect(workers[i],SIGNAL(fetched(AgencyMaster)),agencies[i],SLOT(setData(AgencyMaster)),Qt::QueuedConnection); QObject::connect(workers[i],SIGNAL(error(QString)),agencies[i],SLOT(error_fetching(QString)),Qt::QueuedConnection); QObject::connect(workers[i],SIGNAL(closing()),threads[i],SLOT(quit()),Qt::QueuedConnection); QObject::connect(threads[i],SIGNAL(finished()),workers[i],SLOT(deleteLater()),Qt::QueuedConnection); QObject::connect(threads[i],SIGNAL(finished()),threads[i],SLOT(deleteLater()),Qt::QueuedConnection); threads[i]->start(); } }
