|
You need an architect level person to design your application.
This is one of the far too numerous reasons AGILE is a false
methodology. Developing even a simple application without a SAG
(Systems Architecture Guide) opens an 8-lane highway for problems
like these.
Why wouldn't your database be open?
You didn't write a custom table or replace any of the default
I/O. As long as a QSqlTableModel is in existence it will keep the
database open. You put it on a main window which won't go out of
scope until the application is exited.
The "default" ease of use classes aren't for production systems
connecting with a database. They all (as far as I can tell) take
the lazy approach of opening the database once and keeping it open
until the destructor is called. If you want to have an application
which stays open, you have to write your own table class and
provide all of your own I/O so it only connects to the database
when data is needed. If you want a properly sized scroll bar,
every I/O operation will need to perform a count(*) to get the
maximum record count.
In a production quality system with a SAG, the SAG will state no
database connection exists longer than a single transaction. Each
I/O operation must
open
start transaction
perform I/O
commit transaction
close
Everyone of them. It's been a very long time since I looked at
the code behind QSqlTableModel, but, it never used to do that.
From your test it still doesn't. Part of this has to do with
scrolling. They never wanted to figure out where they were in the
database. One big select() creates a cursor in the database engine
(or database itself) which lives as long as the
transaction/connection. In order to dance around in that cursor,
the connection must stay alive.
Putting scroll bars on a table necessitates either writing a lot
of code yourself, or bad design. Most touch screen applications
seem to rely on them rather than putting <Prev> <Next>
buttons on the screen or enabling PageUp/PageDown. When your
design uses this interface you still have to write a bit of code,
but you can have a limited life connection. You only retrieve what
fits on the screen bounded by the contents of either the first or
last record.
Here's the real killer for you.
Almost every example you find on-line, including your test, is
wrong. All I/O is performed in the GUI event loop instead of being
threaded off. Oh sure, on a fast system with only a few thousand
records in your database, like most examples, you don't notice the
screen hang/stutter. If you are logging data points from sensors
every few seconds, your database will have many hundreds of
thousands of records after a few days. Something simple like
changing the sort order of the display will cause the GUI to just
hang until the I/O completes. If your database resides on a first
generation (or something well below Class 10) MicroSD card, you
won't even need a 100,000 records to see your GUI starting to
hang.
On 08/21/2018 04:35 PM, Israel Brewster
wrote:
Ok, so as a test, I threw together
a basic project, consisting of just the default main window and
boiler plate code created by Qt creator. I then modified
main.cpp to be the following:
#include "mainwindow.h"
#include <QApplication>
#include <QSqlDatabase>
bool setupTestConnection(){
QSqlDatabase db=QSqlDatabase::addDatabase("QSQLITE");
db.setDatabaseName("/tmp/testDb.db");
bool result=db.open();
db.close();
return result;
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
bool canOpen=setupTestConnection();
//TODO: if can't open, give appropriate error and
terminate.
qDebug("Call Result: %i, DB is STILL open: %i",canOpen,QSqlDatabase::database(QSqlDatabase::defaultConnection,false).isOpen());
MainWindow w;
w.show();
return a.exec();
}
And as expected (since I called close()), the
debug line did show it was closed. Then in the main window I
added a QSqlTableModel member (which I called testTable), and
put the following in the QMainWindow constructor to set up the
table model:
testTable.setTable("test");
testTable.select();
So keeping it as basic as I can. Note that I never
need to call QSqlDatabase::database(), so that particular link
you sent doesn't help. When running this new application, lsof
shows that the database is open at all times - so apparently
the table model opened the connection and kept it open. So how
is this one constructed wrong? I'm only opening the DB in main
to make sure I can (which is kinda important), and I make sure
to close it when I am done checking.
And BTW, I *have* read the manual. Repeatedly
over the years. Obviously I have managed to miss the
solutions you are seeing.
-----------------------------------------------
5245 Airport Industrial Rd
-----------------------------------------------
![]() ![]()
You opened the database in main. The
function you called is so small it most likely was
placed in-line by the compiler. This would be
especially true if you did nothing with dbOpen.
You have no reason to open it there.
Again, read the fine manual.
http://doc.qt.io/qt-5/qsqldatabase.html#database
Every time you call
QSqlDatabase::database( "someConnectionName") it
will open the database if it is not currently
open. You should never open the database in main.
Just set up all of the values for it.
On 08/21/2018 03:54
PM, Israel Brewster wrote:
Ok, maybe you can
give me a pointer on what I'm doing wrong then.
My code is structured like the following:
bool openDatabase(){
QSqlDatabase
db=QSqlDatabase::addDatabase('QSQLITE');
db.setDatabaseName(<path_to_my_database_file>); return db.open();
}
int main(int argc, char *argv[])
{
QApplication
a(argc, argv);
...
<various
initialization code>
...
bool
dbOpen=openDatabase();
...
<more
initilization>
return
a.exec();
}
And if I put a breakpoint at any
point after the call to OpenDatabase, the
database is open - it does not close, even
though that QSqlDatabase object went out of
scope. At no point in my application do I keep
a copy of QSqlDatabase around in any of my
classes - every call to QSqlDatabase is as a
function local member, and as such goes out of
scope (thereby theoretically calling the
destructor) as soon as the function exits.
So what am I missing here?
-----------------------------------------------
5245 Airport
Industrial Rd
-----------------------------------------------
<image001.jpg>
<image002.jpg>
Why are you creating yet
another class instead of properly
structuring your application?
Take a gander at
http://www.logikalsolutions.com/wordpress/information-technology/qt-and-usb-pt-4/
for some ideas. You
probably also want to find a copy
of this book
http://www.theminimumyouneedtoknow.com/qt_book.html
Most importantly you
need to read the fine manual.
http://doc.qt.io/qt-5/qsqldatabase.html#dtor.QSqlDatabase
QSqlDatabase::~QSqlDatabase()
Destroys the object and
frees any allocated resources.
Note: When
the last connection is destroyed,
the destructor implicitly calls
close()
to release the database
connection.
See also close().
If your database is open
for the life of the application
then the application has a failed
architecture. Database connections
aren't supposed to have actual
life spans.
On
08/21/2018 03:08 PM, Israel
Brewster wrote:
Ideally, what I'd have is a system where you set up the connection, and then it automatically opens/closes the connection when needed (i.e. when doing a SELECT, INSERT, UPDATE, etc). With my app, this would generally only be for a) periodic updates or b) in direct response to user input, so most of the time the connection could remain closed. To the best of my knowledge (correct me if I am wrong), this is not possible with the existing QSqlDatabase/ QSQLITE database driver classes.
--
Roland Hughes, President
Logikal Solutions
(630) 205-1593
http://www.theminimumyouneedtoknow.com
http://www.infiniteexposure.net
http://www.johnsmith-book.com
http://www.logikalblog.com
http://www.interestingauthors.com/blog
http://lesedi.us
_______________________________________________
Interest mailing list
Interest@qt-project.org
http://lists.qt-project.org/mailman/listinfo/interest
--
Roland Hughes, President
Logikal Solutions
(630) 205-1593
http://www.theminimumyouneedtoknow.com
http://www.infiniteexposure.net
http://www.johnsmith-book.com
http://www.logikalblog.com
http://www.interestingauthors.com/blog
http://lesedi.us
--
Roland Hughes, President
Logikal Solutions
(630) 205-1593
http://www.theminimumyouneedtoknow.com
http://www.infiniteexposure.net
http://www.johnsmith-book.com
http://www.logikalblog.com
http://www.interestingauthors.com/blog
http://lesedi.us
|
