I have a MVC application that stores some information in memory and writes that data asynchronously to a database.
This mechanism works fine. However, I want to prevent an IISreset or a reboot to flush the data before it is written.
Is there a way to receive an event to tell IIS and/or the OS to run some handling code prior to shutting down?
(Update): I am fully aware that I will never be able to detect a crash, hard reset etc. Losing some data on a hard reset is not nice but I will deal with it (and make sure the application is usable after the server comes back).
Use the "office metaphor" to describe my approach: If you receive a BSOD, your work is lost if you haven't saved. But if you shutdown the way you should, you will be asked to save before.
Related
I wrote myself a server monitoring utility using Vaadin 18.0.2 for the UI. This works very nicely execpt for one major issue:
The monitoring is meant to become part of our "team-screen", i.e. a big screen displaying team news, build results, server status, etc.). As such the application has be able to run for longer periods without any user interaction.
The application once per minute queries a bunch of servers re. livelihood and response times and then updates a corresponding result screen. For some reason it always crashes after pretty reproducibly 80 minutes with a com.vaadin.flow.component.UIDetachedException:
com.vaadin.flow.component.UIDetachedException: null
at com.vaadin.flow.component.UI.handleAccessDetach(UI.java:412)
at com.vaadin.flow.component.UI.access(UI.java:512)
at com.vaadin.flow.component.UI.access(UI.java:499)
...
Once this happens all subsequent UI updates have the same issue, i.e. the display "freezes".
The above code is called by a listener which always first requests a Vaadin UI handle and then updates the screen. While it keeps getting such a UI there obviously something's wrong with it after these 80 minutes and then - while the application's monitoring core continues to run - all UI updates are impossible. Neither reloading the page in the browser or even terminating and restarting the browser helps. The application's web-interface is "dead" after that and only an application restart can revive it. This is of course a complete killer or "no-go" for this kind of application. :-(
I suspect that there is some session timeout or UI object duration limit being reached but I haven't encountered any setting to control that so far. I also found no docs mentioning that some token or handle or similar needs to be regularly refreshed or so.
What I specifically don't understand is, why a browser page reload or even a browser restart doesn't help here. One doesn't get a login-in screen as one might expect if some token or session cookie would have expired or so, but the application's web interface is completely "dead" after that time. One doesnt even get any response anymore from the server but the browser requests just time out...
Any hint or guidance what could be the cause for these exceptions or what mechanism is behind this behavior?
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 loosing my patience with "delay write failed" errors. It silently disconnects the database from the application so nothing gets saved in the database while using it. Is there a way to detect the occurrence itself so I can flash a warning ? Or perhaps monitoring the connection itself for a disconnection ? Everyone seems to miss the balloon tip from the Windows XP so I figured to flash a more visible warning that the application must be restarted. It seems Microsoft has found a way to force people to upgrade....
I suppose this could be done with a timer and constantly check connected users:
cxlabel1.Caption := IntToStr(DataModule2.ABSDatabase1.GetDBFileConnectionsCount);
But I was thinking more of checking/detecting for the occurence itself. Is there something in Delphi that can detect this?
I would like to hear your ideas on this...
Putting this as an answer because the comment length is limited.
I have seen this before. IIRC, the problem you have is that the Delayed Write Error is an OS error, it has nothing to do with your application. Windows has already told you that the write has been committed correctly to disk. You would have to try and hook into the OS errors to see when this is happening.
You need to get the network issues resolved because that's where the problem is. In our situation it was a faulty router that was causing the problem.
It's unfair to expect users to check for the error message and then handle it. They could be out at lunch when it occurs as it's not immediate. They also have no way of know what has been saved and what hasn't. It's only a matter of time before your database is corrupted.
The problem with a timer is that it might tell you everything is fine because it triggers after the network resolves the problems.
A far better approach would be to switch to a Client/Server database. You can do this by setting up your own server that listens for web service or another remote call or switch to a database that supports client/server instead of using a file based database. This will tell you immediately when there is a problem with the save of data.
Since I've started developing my Blackberry app, the biggest problems I've encountered all had to do with SQLite Databases.
Right now I'm putting my app through a stress test, and when problems pop up I address them by printing out statuses to the console and taking care of things line by line. Right now (after mashing buttons on my app) I received a "Database is locked" error and I'm not sure what to do.
It seems that once the database is locked it's locked for good until it is unlocked........ my question is how can I unlock it?? First of all, how can I check to see if it's locked??
I'm sure our users won't be mashing buttons like I did, but you never know. I want to account for every possible scenario.
Thanks
EDIT: This is what happens in my application..... When I launch it starts a thread, this thread performs a cleanup on one of my tables based on how old certain pieces of data are (uses DELETE). The thread then continues to get a USER object from my DB (read only), it then uses this USER object as a parameter to call a web service. The data retrieved from the web service is INSERTED into my database. (It's a little more complex than that as a few read/write operations are performed at this time. After that, the thread fires a callback method to update my UI.
This all works fine. I can exit the app WHILE the thread is running and relaunch and a flag will prevent it from starting a new instance of the same thread (unless the other one is done of course).
Now my problem: My app's home screen is a list of buttons, when the user clicks one of these buttons another, more detailed list is loaded (this requires a READ ONLY call to the database). When I launch the app (firing the web service calling thread) and then click a button on the main screen right away, the table gets locked. (Not always, sometimes it takes 4 or 5 tries, sometimes more, sometimes less). But if I keep doing this it WILL eventually lock making it impossible to make any calls to my DB, hence no more UI (which depends on the DB).
The DB call that populates the UI on the second screen is READ ONLY, can't I have as many of these as I need?? What causes the DB to lock?? What's the difference between a DB lock and File System error (12)??
I seemed to have fixed the problem. I was under the impression that if a read/write connection was open then a read-only connection could be created safely.
This doesn't seem to be the case. If I have a read/write connection open then no other connections can open until that one is finished.
I basically created one read/write connection, set a flag to identify it as open, and during my read connection use the same Database object if the flag is open, or create a read only if it's closed.
So far so good.
Sqlite does not support concurrent modification. In practice on BlackBerry, this means you can only open the database from one part of the code at a time. To maintain this one-at-a-time access, you need to close the database when you are done with it, as #AnkitRox points out.
However you also need to guard against concurrent access. Even if your code properly closes the database, it is possible for two different threads to access the database. In that case, you will need one to wait. In Java-ME this is typically accomplished through the 'synchronized' keyword, and using the same lock object for all database access.
Check properly that, you are opening and closing database before and after execution of query respectively.
Because if Database is going to open without closing it properly, then it gives errors.
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.