I have a TClientDataset that is managed in Thread 1.
In a different thread I have a cloned Image of the TClientDataset.
Will I run into threading problems?
Edit
The cloned image is used in a read only mode.
The short answer is no, a ClientDataSet is not thread safe. The more involved answer is that it depends on how you use it. No matter how many clones of the ClientDataSet you have, they are safe to use from multiple threads so long as you are only reading the data. This is true even if you are setting different ranges, current records, filters, and so forth.
And, though you did not ask about this, you can free any of the cloned ClientDataSets, even the original ClientDataSet that was cloned, without problems.
On the other hand, if there is any need to ever post a record to any of the clones, or reload the data, you must use a synchronization object. If the changes are rare, the TMultiReadExclusiveWriteSynchronizer class is great one to use for this purpose.
In a word: No.
All of the VCL should be considered "thread unsafe". Any calls to visual components in a TThread should be made in a Synchronize event.
Any VCL/RTL class should be created and destroyed entirely within a TThread.
The clone process should be handled with a TCriticalSection, but each TClientDataSet should be fine in its own thread.
Related
I'd like to be able to open a TDataSet asynchronously in its own thread so that the main VCL thread can continue until that's done, and then have the main VCL thread read from that TDataSet afterwards. I've done some experimenting and have gotten into some very weird situations, so I'm wondering if anyone has done this before.
I've seen some sample apps where a TDataSet is created in a separate thread, it's opened and then data is read from it, but that's all done in the separate thread. I'm wondering if it's safe to read from the TDataSet from the main VCL thread after the other thread opens the data source.
I'm doing Win32 programming in Delphi 7, using TmySQLQuery from DAC for MySQL as my TDataSet descendant.
Provided you only want to use the dataset in its own thread, you can just use synchronize to communicate with the main thread for any VCL/UI update, like with any other component.
Or, better, you can implement communication between the mainthread and worker threads with your own messaging system.
check Hallvard's solution for threading here:
http://hallvards.blogspot.com/2008/03/tdm6-knitting-your-own-threads.html
or this other one:
http://dn.codegear.com/article/22411
for some explanation on synchronize and its inefficiencies:
http://www.eonclash.com/Tutorials/Multithreading/MartinHarvey1.1/Ch3.html
I have seen it done with other implementations of TDataSet, namely in the Asta components. These would contact the server, return immediately, and then fire an event once the data had been loaded.
However, I believe it depends very much on the component. For example, those same Asta components could not be opened in a synchronous manner from anything other than the main VCL thread.
So in short, I don't believe it is a limitation of TDataSet per se, but rather something that is implementation specific, and I don't have access to the components you've mentioned.
One thing to keep in mind about using the same TDataSet between multiple threads is you can only read the current record at any given time. So if you are reading the record in one thread and then the other thread calls Next then you are in trouble.
Also remember the thread will most likely need its own database connection. I believe what is needed here is a multi-threaded "holding" object to load the data from the thread into (write only) which is then read only from the main VCL thread. Before reading use some sort of syncronization method to insure that your not reading the same moment your writing, or writing the same moment your reading, or load everything into a memory file and write a sync method to tell the main app where in the file to stop reading.
I have taken the last approach a few times, depdending on the number of expected records (and the size of the dataset) I have even taken this to a physical disk file on the local system. It works quite well.
I've done multithreaded data access, and it's not straightforward:
1) You need to create a session per thread.
2) Everything done to that TDataSet instance must be done in context of the thread where it was created. That's not easy if you wanted to place e.g. a db grid on top of it.
3) If you want to let e.g. main thread play with your data, the straight-forward solution is to move it into a separate container of some kind,e.g. a Memory dataset.
4) You need some kind of signaling mechanism to notify main thread once your data retrieval is complete.
...and exception handling isn't straightforward, either...
But: Once you've succeeded, the application will be really elegant !
Most TDatasets are not thread safe. One that I know is thread safe is kbmMemtable. It also has the ability to clone a dataset so that the problem of moving the record pointer (as explained by Jim McKeeth) does occur. They're one of the best datasets you can get (bought or free).
If many asynchronous threads draw in a global TBitmap, it will rise me an error? Should I create my code using a critical section? (From my surf on the internet I found that the TBitmap.Draw is not thread safe)
Another question: If many synchronous threads draw in a global TBitmap and a VCL Timer read asynchronously the content from the TBitmap will this rise me an error?
Thanks!
Yes, you do need to protect the TBitmap from concurrent access across multiple threads. A critical section is fine for serializing your drawing code, HOWEVER that is not enough by itself! The main thread caches GDI resources and performs cleanup on them periodically, which will affect your TBitmap. As such, you will ALSO need to Lock/Unlock() the TBitmap.Canvas whenever drawing/rendering to ensure the VCL does not rip out its resources behind your back.
Since your threads are all modifying the same bitmap, you need to serialize all access to that bitmap. That means reading its contents as well as writing to it.
Of course, this assumes that multiple threads drawing to a shared bitmap is the right solution to your problem. Without knowing what your actual problem is, I could not comment on that.
UPDATE
You must also use Lock/Unlock when drawing to the bitmap because of the issue described in Remy's answer. Which should be the accepted answer to this question.
Use monitors or semaphores to control your threads when they do changes in your TBitmap Pixels !
Can you use the TThread.Synchronize method instead?
http://docs.embarcadero.com/products/rad_studio/delphiAndcpp2009/HelpUpdate2/EN/html/delphivclwin32/Classes_TThread_Synchronize#TThread#TThreadMethod.html
According to the doco for TThread class
Following are issues and recommendations to be aware of when using threads:
Keeping track of too many threads consumes CPU time; the recommended limit is 16 active threads per process on single processor systems.
When multiple threads update the same resources, they must be synchronized to avoid conflicts.
Most methods that access an object and update a form must only be called from within the main thread or use a synchronization object such as TMultiReadExclusiveWriteSynchronizer.
I have a native Visual C++ COM object and I need to make it completely thread-safe to be able to legally mark it as "free-threaded" in th system registry. Specifically I need to make sure that no more than one thread ever accesses any member variable of the object simultaneously.
The catch is I'm almost sure that no sane consumer of my COM object will ever try to simultaneously use the object from more than one thread. So I want the solution as simple as possible as long as it meets the requirement above.
Here's what I came up with. I add a mutex or critical section as a member variable of the object. Every COM-exposed method will acquire the mutex/section at the beginning and release before returning control.
I understand that this solution doesn't provide fine-grained access and this might slow execution down, but since I suppose simultaneous access will not really occur I don't care of this.
Will this solution suffice? Is there a simpler solution?
This solution should work, but I'd recommend mutexes over critical sections as they handle time-outs, which provide some level of fall back in case of deadlock. You also want to be very careful that a function locking a mutex does not call another function that has already locked the same mutex in the same thread. This shouldn't be a problem for your COM interface, so long as you don't add extra functionality on top of your mutex to the interface. You could hit issues if the COM includes call backs.
If you are certain that actual concurrent access is not going to happen in practice, then mutexing the entire execution is not an unreasonable approach.
I don't want multiple windows, each with its own UI thread, nor events raised on a single UI thread, not background workers and notifications, none of that Invoke, BeginInvoke stuff either.
I'm interested in a platform that allows multiple threads to update the same window in a safe manner. Something like first thread creates three buttons, the second thread another five, and they both can access them,change their properties and delete them without any unwanted consequences.
I want safe multi-threaded access to the UI without Invoking, a platform where the UI objects can be accessed directly from any thread without raising errors like "The object can only be accessed from the thread that created it". To let me do the synchronizing if I have to, not prevent me from cross-tread accessing the UI in a direct manner.
I'm gonna get down voted but ... Go Go Gadget Soapbox.
Multi threaded GUI are not possible in the general case. It has been attempted time and time again and it never comes out well. It is not a coincidence that all of the major windowing frameworks follow the single threaded ui model. They weren't copying each other, it's just that the constraints of the problem lead them to the same answer. Many people smarter than you or i have tried to solve this.
It might be possible to implement a multi-thread ui for a particular project. I'm only saying that it can't be done in the general case. That means it's unlikely you'll find a framework to do what you want.
The gist of the problem is this. Envision the gui components as a chain (in reality it's more like a tree, but a chain is simple to describe). The button connects to the frame, connects to the box, connects to the window. There are two source of events for a gui the system/OS and the user. The system/OS event originate at the bottom of the chain (the windowing system), the user event originate at the top of the chain (the button). Both of these events must move through the gui chain. If two threads are pushing these events simultaneously they must be mutex protected. However, there is no known algorithm for concurrently traversing a double linked list in both directions. It is prone to dead lock. GUI experts tried and tried to figure out ways to get around the deadlocking problem, and eventually arrived at the solution we use today called Model/View/Controller, aka one thread runs the UI.
You could make a thread-safe Producer/Consumer queue of delegates.
Any thread that wants to update a UI component would create a delegate encapsulating the operations to be performed, and add it to the queue.
The UI thread (assuming all components were created on the same thread) would then periodically pull an item from the queue, and execute the delegate.
I don't believe a platform like that exists per se
There is nothing stopping you from saying taking .Net and creating all new controls which are thread safe and can work like that(or maybe just the subset of what you need) which shouldn't be an extremely large job(though definitely no small job) because you can just derive from the base controls and override any thread-unsafe methods or properties.
The real question though is why? It would definitely be slower because of all the locking. Say your in one thread that is doing something with the UI, well it has to lock the window it's working on else it could be changed without it knowing by the other thread. So with all the locking, you will spend most of your drawing time and such waiting on locks and (expensive) context switches from threads. You could maybe make it async, but that just doesn't seem safe(and probably isn't) because controls that you supposedly just created may or may not exist and would be about like
Panel p=new Panel();
Button b=new Button();
WaitForControlsCreated(); //waits until the current control queue is cleared
p.Controls.Add(b);
which is probably just as slow..
So the real question here is why? The only "good" way of doing it is just having an invoke abstracted away so that it appears you can add controls from a non-UI thread.
I think you are misunderstanding how threads really work and what it takes to actually make an object thread safe
Accept that any code updating the GUI has to be on the GUI thread.
Learn to use BeginInvoke().
On Windows, Window handles have thread affinity. This is a limitation of the Window manager. It's a bad idea to have multiple threads accessing the same window on Windows.
I'm surprised to see these answers.
Only the higher level language frameworks like C# have thread restrictions on GUI elements.
Windows, at the SDK layer, is 100% application controlled and there are no restrictions on threads except at insignificant nitty gritty level. For example if multiple threads want to write to a window, you need to lock on a mutex, get the device context, draw, then release the context, then unlock the mutex. Getting and releasing a device context for a moment of drawing needs to be on the same thread... but those are typically within 10 lines of code from each other.
There isn't even a dedicated thread that windows messages come down on, whatever thread calls "DispatchMessage()" is the thread the WINPROC will be called on.
Another minor thread restriction is that you can only "PeekMessage" or "GetMessage" a window that was created on the current thread. But really this is very minor, and how many message pumps do you need anyway.
Drawing is completely disconnected from threads in Windows, just mutex your DC's for drawing. You can draw anytime, from anywhere, not just on a WM_PAINT message.
BeOS / Haiku OS
Based on my guessing of your requirement, you want a single Windows Form and having ways to execute certain routines asynchronously (like multi-threading), yes?
Typically (for the case of .NET WinForms) Control.Invoke / Control.BeginInvoke is used to a certain effect what I think you want.
Here's an interesting article which might help: http://www.yoda.arachsys.com/csharp/threads/winforms.shtml
I'd like to be able to open a TDataSet asynchronously in its own thread so that the main VCL thread can continue until that's done, and then have the main VCL thread read from that TDataSet afterwards. I've done some experimenting and have gotten into some very weird situations, so I'm wondering if anyone has done this before.
I've seen some sample apps where a TDataSet is created in a separate thread, it's opened and then data is read from it, but that's all done in the separate thread. I'm wondering if it's safe to read from the TDataSet from the main VCL thread after the other thread opens the data source.
I'm doing Win32 programming in Delphi 7, using TmySQLQuery from DAC for MySQL as my TDataSet descendant.
Provided you only want to use the dataset in its own thread, you can just use synchronize to communicate with the main thread for any VCL/UI update, like with any other component.
Or, better, you can implement communication between the mainthread and worker threads with your own messaging system.
check Hallvard's solution for threading here:
http://hallvards.blogspot.com/2008/03/tdm6-knitting-your-own-threads.html
or this other one:
http://dn.codegear.com/article/22411
for some explanation on synchronize and its inefficiencies:
http://www.eonclash.com/Tutorials/Multithreading/MartinHarvey1.1/Ch3.html
I have seen it done with other implementations of TDataSet, namely in the Asta components. These would contact the server, return immediately, and then fire an event once the data had been loaded.
However, I believe it depends very much on the component. For example, those same Asta components could not be opened in a synchronous manner from anything other than the main VCL thread.
So in short, I don't believe it is a limitation of TDataSet per se, but rather something that is implementation specific, and I don't have access to the components you've mentioned.
One thing to keep in mind about using the same TDataSet between multiple threads is you can only read the current record at any given time. So if you are reading the record in one thread and then the other thread calls Next then you are in trouble.
Also remember the thread will most likely need its own database connection. I believe what is needed here is a multi-threaded "holding" object to load the data from the thread into (write only) which is then read only from the main VCL thread. Before reading use some sort of syncronization method to insure that your not reading the same moment your writing, or writing the same moment your reading, or load everything into a memory file and write a sync method to tell the main app where in the file to stop reading.
I have taken the last approach a few times, depdending on the number of expected records (and the size of the dataset) I have even taken this to a physical disk file on the local system. It works quite well.
I've done multithreaded data access, and it's not straightforward:
1) You need to create a session per thread.
2) Everything done to that TDataSet instance must be done in context of the thread where it was created. That's not easy if you wanted to place e.g. a db grid on top of it.
3) If you want to let e.g. main thread play with your data, the straight-forward solution is to move it into a separate container of some kind,e.g. a Memory dataset.
4) You need some kind of signaling mechanism to notify main thread once your data retrieval is complete.
...and exception handling isn't straightforward, either...
But: Once you've succeeded, the application will be really elegant !
Most TDatasets are not thread safe. One that I know is thread safe is kbmMemtable. It also has the ability to clone a dataset so that the problem of moving the record pointer (as explained by Jim McKeeth) does occur. They're one of the best datasets you can get (bought or free).