Asynchronous Database Access with Qt 4.x
Listing 6. You must register any custom data types in order to share them.
// first, make the object system aware qRegisterMetaType< QList<QSqlRecord> >("QList<QSqlRecord>"); // now set up the queued connection connect( m_worker, SIGNAL( results( const QList<QSqlRecord>& ) ), this, SIGNAL( queryFinished( const QList<QSqlRecord>& ) ) );
The sample application provided with this article implements the strategy outlined above, in which queries are executing in parallel with the rest of the application [the application is available for download from the Linux Journal FTP site, ftp.linuxjournal.com/pub/lj/listings/issue158/9602.tgz]. The UI is not disturbed while the queries are underway. A queued connection is created between the QueryThread interface and the encapsulated worker thread after the appropriate types are registered with the Qt metaobject system. This allows the separate threads to communicate safely with one another, with minimal overhead and code complexity. The sample application was tested with SQLite and PostgreSQL; however, it will work with any database connection supported by Qt that enforces the same connection-per-thread limitation.
The following points should be kept in mind when designing asynchronous database applications with Qt:
Create a database connection per thread. Use the name parameter of the thread-safe QSqlDatabase::addDatabase() method in order to distinguish various database connections.
Encapsulate the database connection within worker thread objects as much as possible. Never share a database connection with another thread. Never use a database connection from any thread other than the one that created it.
Manage communication between threads using the tools provided by Qt. In addition to QMutex, QSemaphore and QWaitCondition, Qt provides much more direct mechanisms: events and signals/slots. The implementation of signals/slots across thread boundaries relies on events; therefore, ensure that your threads start their own event loop using QThread::exec().
Register unknown types with the Qt metaobject system. Any unknown types cannot be marshaled properly without first invoking qRegisterMetaType(). This enables a queued connection to invoke a slot in a separate thread within that thread's context using new types.
Utilize queued connections to communicate between the application and the database threads. The queued connection provides all the advantages for dealing with asynchronous database connections, but maintains a simple and familiar interface using QObject::connect().
Dave Berton is a professional programmer working for Eventide, Inc. He welcomes your comments at email@example.com.
|Nativ Disc||Sep 23, 2016|
|Android Browser Security--What You Haven't Been Told||Sep 22, 2016|
|The Many Paths to a Solution||Sep 21, 2016|
|Synopsys' Coverity||Sep 20, 2016|
|Naztech's Roadstar 5 Car Charger||Sep 16, 2016|
|RPi-Powered pi-topCEED Makes the Case as a Low-Cost Modular Learning Desktop||Sep 15, 2016|
- Android Browser Security--What You Haven't Been Told
- Nativ Disc
- Download "Linux Management with Red Hat Satellite: Measuring Business Impact and ROI"
- The Many Paths to a Solution
- Naztech's Roadstar 5 Car Charger
- Synopsys' Coverity
- Securing the Programmer
- RPi-Powered pi-topCEED Makes the Case as a Low-Cost Modular Learning Desktop
- Glass Padding
- Jose Dieguez Castro's Introduction to Linux Distros (Apress)
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide