I am currently working on a cross-platform application using Monodroid/Xamarin. It uses a SQLite.net database and at the moment I have only built the Android UI.
In the application there is a long running method on a background thread which does several database reads and writes.
I was testing this long running method on a Nexus 7 running android 4.2.2 and it seems as though after a second screen rotation, a SQLite exception is thrown and the application crashes.
The exception stack trace varies depending on when you rotate the screen but the exception is always object not set to an instance inside Prepare2.
The interesting part is when testing on Samsung Galaxy S2 running 4.1.2 an exception still occurs but it is a different exception.
Furthermore, testing on a Samsung Galaxy Tab 2 and emulators the error is not thrown at all and the method completes just fine.
Any ideas, hints, suggestions would be much appreciated.
As a side question - in all the cross platform applications i have seen using Xamarin - the database layer and or repository is always static - whats the reason for this?
Thanks everyone
It's hard to identify the problem from what you've written - would be easier if you'd included some code about how you are accessing the database from your app.
However, I suspect that the problem here will be due to some interaction between the lifecycle of your UI Activity and the way it interacts with the SQLite database.
By default, when you rotate an Activity, then the existing instance is destroyed and a new one is created - your old Activity will be called to store its state in SaveInstanceState and your new rotated Activity will get called with a bundle in OnCreate.
This is why the database layer is often static/singleton in the samples - it's an easy way to allow the same database instance to be shared by multiple Activities/Components.
I guess that your Activity is doing something with the database during OnCreate - and that this is being called after each rotation/creation? Alternatively maybe your long-running method is trying to do something after the Activity that's hosting it has been finished and disposed?
In the end, this turned out to be a issue with mulithreading and Linq not playing well together. Thanks to Stuart for his answer.
Related
In our application we have been using an encrypted sqlite database in db3 format that is downloaded from server and then again after processing is uploaded. The app is live and is used by several users.
Sometimes, very intermittently in one or two instances, the database gets corrupted. The user has to discard the entire application and reinstall again to work resulting in data loss.
Only once we could detect that one of the tables got missing from the database through no drop table command was written anywhere in the code.
Did anyone face this instance before? Any idea why does this happens?
Please note: The application is iPad application written in objective C.
One of the main reason:
iDevices shut down quite a time before they'd actually run out of power. Before your device shuts down your App will get notified that it's going to background, and then get notified that it's going to quit. If you're handling those two notifications properly (i.e. closing all SQLite connections at one or the other) then you should not be getting database corruption.
I am working on a Delphi application using the TService functionalities.
It is a large project that was started by someone else.
The application uses several separate threads for processing, communicating with clients, database access, etc. The application’s main job is to poll regularly (every 2-300ms) certain devices and, a couple times a day, execute specific actions.
Now somewhere there is an unhandled exception for which I cannot seem to find the cause:
According to the debugger, the faulting method is System.Classes.StdWndProc.
This also seems to be confirmed by analyzing the crash dump file with WinDbg.
After numerous tests and debugging, I noticed that the crash happened on my dev computer every day at almost the same time.
I looked at the windows log and found this event:
This, coupled with the fact that the stack trace from Delphi indicates that a message with ID=26 (WM_WININICHANGE) was processed, made me believe that there might be something wrong with my usage of FormatDateTime() or DateTimeToStr() when regional settings are reloaded.
I checked every call and made sure to be using the thread-safe overload with a local instance of TFormatSettings.
However today the service crashed again.
A few points that I think are worth mentioning:
The application is also installed on a Windows 2008 server and has
been running OK for over a month.
On 2012r2 I tried forcing DEP off, but it didn’t change anything.
The service’s OnExecute() method is not implemented. I create a base thread in TService. ServiceStart() which then in turn creates the main data module and all the other threads.
The service is not marked as interactive and is executed with the Local System account.
All data modules are created with AOwner=nil.
With a special parameter, the application can be started a normal windowed application with a main form (which is created only in this case). The exception does not seem to happen when running in GUI mode.
Almost all threads have a message pump and use PostThreadMessage() to exchange information. There are no window handle allocations anywhere.
I have checked the whole project and there are no timers or message dialogs or other graphical components anywhere.
I have activated range as well as overflow checking and found no issues.
I am a loss for what to do here. I have checked and re-checked the code several times without finding anything that could explain the error.
Looking online I found several reports that seem to be pertinent to my situation, but none that actually explains what is going on:
https://answers.microsoft.com/en-us/windows/forum/windows_10-other_settings/windows-10-group-policy-application-hang/72016ea4-ba89-4770-b1de-6ddf14b0a51f
https://www.experts-exchange.com/questions/20720591/Prevent-regional-settings-from-changing-inside-a-TService-class.html
https://forums.embarcadero.com/thread.jspa?messageID=832265
Before taking everything apart I would like to know if anyone experienced anything similar or has any tips.
Thanks
Edit: looking at the call stack and the first method executed I am thinking of the TApplication instance that is used in TService.
LPARAM is always 648680.
LPARAM is a pointer to a string (you can cast it to PChar).
You could try to catch WM_SETTINGS_CHANGE and log anytime it's processing.
I think you could use TApplicationEvents component. The component has OnSettingsChange event. The event provides a setting area name (section name) and a flag that points to changed parameter.
Have a look at doc-wiki.
There is a full list of possible parameters.
I did not test it, but maybe you can use Abort procedure in OnSettingsChange event handler to stop the message distribution. Of course it's not a solution, but it may work.
Can someone help me out, i am facing a issues since many days.
My Application is lagging(Users feel like the app is almost struck) at the launch screen while handling image data from server and saving it in the local SQLite Database. It would be great if someone could provide me with some solution for the same.
Thank You.
It is hard to figure out the cause without any code but if the user is complaining that the application is lagging, there is a good chance that you are doing work on the main thread and locking up the UI. You should do as much of this work as you can on a background thread as to not freeze the User Interface. If your app is useless unless this data is ready, you should design a flow where the application lets the user know that it is doing some work instead of just freezing the application UI.
We had to use CoFreeUnusedLibrariesEx for fixing a bug with heap not being cleared after using a MSXML library
Refer this link:
http://blogs.msdn.com/b/marcelolr/archive/2008/11/13/msxml-heaps-not-being-released.aspx
But this caused another issue with TTimers which takes while to show up and disappears when Delphi app is bounced and again shows after a while.
This app uses TTimers to schedule it's job like running a XML transform .
Here is the issue:
When TTimer.Enable is called it throws an error not enough Timer available.
I know this is a masked error and I would have to figure out how to get to the actual error.
This is a single threaded application with only one timer.
Here are the links I looked into
Most Common reason seems to be invalid windows handle
https://groups.google.com/forum/#!topic/borland.public.delphi.winapi/UrIskaFZggU
There are other threads that suggested that OS ran out of resources for TIMERS I'm not sure if that is relevant to me.
I'm Just trying to understand what is Interaction between CoFreeUnusedLibrariesEx and TTimers that it gradually some how robs it of resources and makes us bounce the app to get it working.
How do I go about solving this issue , I'm looking for some directions?
CoFreeUnusedLibrariesEx should not affect TTimers. But if loading and unloading a (buggy) dll leaks any user objects (this includes timers, window handles, ...) then I could imagine that you run out of user objects.
Use Windows Task Manager and configure it so it will show the "USER Objects" in the "Processes" tab. Then compare the number of user objects when you call CoFreeUnusedLibrariesEx and when you don't call CoFreeUnusedLibrariesEx.
I've been reading up on SQLite3 included in the iOS firmware which might serve my needs for the app i'm writiung.
What I can't figure out is if it is persistent or goes away like some objects do.
For example if I do sqlite3_open() which appears to be a C function rather than an Objective-C object, if I open this at the start of my application, will it stay persistent until I close it no matter how many views I push/pop all over the place.
Obviously that would depend on where I put it but if I was doing a universal app and had some central functions for loading / saving data which were common to both iPhone/iPad, if, in my didFinishLoading: I put a call to open the SQLite database and then called various exec's of queries, would it remain persistent throughout the lifecycle of the application.
or
Am I better off opening and closing as needed, i'm coming from a PHP background so i'd normally open a database at the start of the script and then run many queries and then finally close it before browser output.
From the 1,000,000th i've learned over the last few months about iOS programming, I think the latter might be the better way as there's possibility of app exit prematurely or it going to background.
I'd just like a second opinion on my thinking please.
I dont know directly, but I think you are right - you only need to open it once at the start of your app.
Looking at sqlitepersistentobjects, an ORM framework for iOS, it only opens the DB when its first used, and never closes it except when there is a problem opening it :)
Single opened sqlite database used throughout the app from different places in your app is fine.
You are using word "persistent" which is confusing. What you mean is "reuse of single connection, for executing different statements in the app, possibly from different threads". Persistence has completely different meaning in context of databases - it means that the requested modification of data has been safely stored to media (disk, flash drive) and the device can even unexpectedly shut down without affecting written data.
It's recommended to keep running sqlite statements from a single, dedicated thread.
It's not recommended to connect to sqlite database from different processes for and executing parallel modifications.
A good alternative solution is to use sqlite async extension which sends all writes to a dedicated, background thread.
You can check out https://github.com/mirek/CoreSQLite3 framework if you want to use custom built (newer version) of sqlite.