Exception of multi thread reading QSqlTableModel
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@tovax said in Exception of multi thread reading QSqlTableModel:
If there are more threads that modify the database, then it is necessary to establish signal slot connections from these threads to all other threads.
As @Christian-Ehrlicher already written,
QSqlDatabaseis not thread-safe, so you have 2 options:- do all DB stuff in one thread, by using signals/slots it is quit easy.
- use as many
QSqlDatabaseconnections as you have threads ==> take a look here for example: https://forum.qt.io/topic/103626/problem-with-sqlite-database-and-threads-database-is-locked/14
@KroMignon said in Exception of multi thread reading QSqlTableModel:
do all DB stuff in one thread, by using signals/slots it is quit easy.
It's easy to write data by using signals/slots, but is it necessary to copy a memory copy for reading data? Because DB data cannot be read directly from other threads...
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@KroMignon said in Exception of multi thread reading QSqlTableModel:
do all DB stuff in one thread, by using signals/slots it is quit easy.
It's easy to write data by using signals/slots, but is it necessary to copy a memory copy for reading data? Because DB data cannot be read directly from other threads...
@tovax
Yes, this is the issue. As you say, emitting a signal to ask the main thread to do a write (setData()) for your thread is not so difficult. Your problem comes on trying to synchronise read access (data()).It is easier to maintain per-thread connections for that purpose. But that comes with other overheads, as discussed. Depends on how much of what you need to do when.
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Again: why do you think you need threads for simple reading data from the database? What heaviy computation do you do with those values?
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Again: why do you think you need threads for simple reading data from the database? What heaviy computation do you do with those values?
@Christian-Ehrlicher
The main purpose of thread is to process time-consuming algorithms and real-time control. Reading data from the database is only to obtain the global parameters needed in the algorithm and control process. -
Then read your data in one thread, start/inform the workers that there is data and let them do the work. When you a mutex to protect read/write access to your data you don't even need to copy it. But that's plain threading stuff which you should read about before trying to actually writing something. Threading is not easy and should be avoided when not really needed for basic programmers.
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@Christian-Ehrlicher
The main purpose of thread is to process time-consuming algorithms and real-time control. Reading data from the database is only to obtain the global parameters needed in the algorithm and control process.@tovax
I was going to say as @Christian-Ehrlicher has just said. Can you not take the reading from the database outside the threads and then access some shared data only for the computations.@Christian-Ehrlicher
Isn't the following a problem: is it not the case that for a reader accessingdata()that callsrowCount()and that can cause physical call to database forfetchMore()?? Or doesdata()/rowCount()never do that, I can't recall? -
Then read your data in one thread, start/inform the workers that there is data and let them do the work. When you a mutex to protect read/write access to your data you don't even need to copy it. But that's plain threading stuff which you should read about before trying to actually writing something. Threading is not easy and should be avoided when not really needed for basic programmers.
Some results of time-consuming algorithm and motion control need to be written to database, such as time, current position and so on. In other words, there will be frequent data interaction between some child threads and the database.
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@Christian-Ehrlicher
The main purpose of thread is to process time-consuming algorithms and real-time control. Reading data from the database is only to obtain the global parameters needed in the algorithm and control process.@tovax said in Exception of multi thread reading QSqlTableModel:
The main purpose of thread is to process time-consuming algorithms and real-time control. Reading data from the database is only to obtain the global parameters needed in the algorithm and control process.
I think you are "over-engineering" your programm.
For me, the steps your need are:- getting computation parameters from db
- starting multiple computation operations
So I would do it like @Christian-Ehrlicher said:
- in main thread preparing data for computation ==> reading required data from DB
- start thread for computation ==> I would do it with
QtConcurrent::run()and dedicatedQThreadPoolto avoid thread creation/destruction and to be able to wait for computation end withQFutureWatcher.
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@tovax said in Exception of multi thread reading QSqlTableModel:
The main purpose of thread is to process time-consuming algorithms and real-time control. Reading data from the database is only to obtain the global parameters needed in the algorithm and control process.
I think you are "over-engineering" your programm.
For me, the steps your need are:- getting computation parameters from db
- starting multiple computation operations
So I would do it like @Christian-Ehrlicher said:
- in main thread preparing data for computation ==> reading required data from DB
- start thread for computation ==> I would do it with
QtConcurrent::run()and dedicatedQThreadPoolto avoid thread creation/destruction and to be able to wait for computation end withQFutureWatcher.
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For example, the results generated by child thread1 need to be written to the database, and used by child thread2. -
For example, the results generated by child thread1 need to be written to the database, and used by child thread2.@tovax from the looks of it you are missing one component: the controller.
Your use case is typically one that should rather use MVC rather than just MV.
One thing that is not clear is how are you child threads triggered ?
Are you creating them on demand on are they long-lived ? In both cases, the suggestions from @KroMignon are good in terms of encapsulation of the threaded components and separation of the storage side.
@JonB The one connection per thread rule is rather a constraints of the class design: you shall not use the exact same database connection from different threads in the same spirit as you should not modify GUI object outside the GUI thread.
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For example, the results generated by child thread1 need to be written to the database, and used by child thread2.@tovax said in Exception of multi thread reading QSqlTableModel:
For example, the results generated by child thread1 need to be written to the database, and used by child thread2.
You can solve this with signals/slots:
- Client1: you need a signal (or more) for writing result, which can be connected to Main to store in DB and to Client2 to update his parameters.
- Client2: you need a signal for writing new position
- Main: you need a signal to setup Client1 and another to setup Client2
Maybe Client1 and Client2 needs an additional signals to request settings update.
But that don't seems to be a very difficult architecture to implement. -
@tovax from the looks of it you are missing one component: the controller.
Your use case is typically one that should rather use MVC rather than just MV.
One thing that is not clear is how are you child threads triggered ?
Are you creating them on demand on are they long-lived ? In both cases, the suggestions from @KroMignon are good in terms of encapsulation of the threaded components and separation of the storage side.
@JonB The one connection per thread rule is rather a constraints of the class design: you shall not use the exact same database connection from different threads in the same spirit as you should not modify GUI object outside the GUI thread.
@SGaist said in Exception of multi thread reading QSqlTableModel:
@JonB The one connection per thread rule is rather a constraints of the class design: you shall not use the exact same database connection from different threads
Absolutely, as I said from the start! The question at hand is whether for this user's case he should indeed go for separate connections per thread, or whether he should work on "shared" (somehow) data.
Since @Christian-Ehrlicher has not replied to it, could you kindly answer my earlier question, at least for my information:
Isn't the following a problem: is it not the case that for a reader accessing
data()that callsrowCount()and that can cause physical call to database forfetchMore()?? Or doesdata()/rowCount()never do that, I can't recall? -
@tovax
Most important decision: do you really mean that thread1 must physically write to the database and thread2 must physically (re-)read the data from the database, or do you mean that thread2 can/should read the (changed) data in-memory.@JonB
First of all, the data generated by each child thread must be written to the database in order to prevent sudden power failure. Then, the data generated by each child thread can be shared with each other threads.
I just think the unified use of the database is logically simpler, thread2 can read the changed data in memory. -
@tovax said in Exception of multi thread reading QSqlTableModel:
For example, the results generated by child thread1 need to be written to the database, and used by child thread2.
You can solve this with signals/slots:
- Client1: you need a signal (or more) for writing result, which can be connected to Main to store in DB and to Client2 to update his parameters.
- Client2: you need a signal for writing new position
- Main: you need a signal to setup Client1 and another to setup Client2
Maybe Client1 and Client2 needs an additional signals to request settings update.
But that don't seems to be a very difficult architecture to implement.@KroMignon said in Exception of multi thread reading QSqlTableModel:
Maybe Client1 and Client2 needs an additional signals to request settings update.
Thank you for your detailed steps. The actual application is much more complicated than the block diagram, so I'm worried that this method will need to create too many signal slots.
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