I'm on Delphi 10.3. I use TClientdataset a lot in my applications, as a way to locally store small amount of information, usually less than 10000 rows.
Basically i create the table structure and save it on disk :
myClientDataSet.savetofile('c:\mydata.dat') ;
This is a great way to have a persistence mechanism without having to install any database. It works but i noticed that at some point i can't open the file anymore, because when i try to load :
myClientDataSet.loadfromfile('c:\mydata.dat');
I'll get this error :
Mismatch in datapacket.
When this happens, as far as i know the only fix is to delete the file and create a new one, loosing all data (or restoring a backup, if it exists). In some cases this issue will take years to happen, but always happen at some point.
What can i do to avoid this issue and if it already happened, is there a way to recover the data file with this error ?
Thanks
Related
For my latest app update, I have to move the user's Core Data .sqlite database from the Documents directory to the Application Support directory. I use the migratePersistentStore:toURL:options:withType:error: method. I am dealing with one user who somehow managed to save enough data in her database so that it is now a whopping 9G. On top of that, her device is an iPad Air 1 - which had memory issues around the time it came out five or six years ago, and now it is 2018 and she is running the latest iOS. The migratePersistentStore:toURL:options:withType:error: method keeps crashing the app with low memory. But I have no other way to get the .sqlite to the necessary location. Can anyone advise me on how to best approach this situation? Ideally, something I could do in my code - but I would even appreciate suggestions on other ways I could just help the user to manually move that massive database at this point!
I guess you use migratePersistentStore because its the official way to do it. Its a good way, but sometimes you have to make your hands dirty:
Manually-moving database at app-start before you load it
This post shows that CoreData may use more than one file which you have to move. You can list the files in the directory, use a regex find all the files which have to be moved, move them to the new location and then load CoreData as usual from the new location.
This however requires there to be the necessary infrastructure in place to do this. You‘ll figure it out if you want to go this path.
You can hide this behind a feature-flag, only with a code, only for a certain user or after crashing with low memory so it doesn‘t impact users who don‘t need it.
My question surrounds around one single point - data management in mobile application. I have created a mobile application where data comes from server. The data includes both text and images. Following are the steps I am doing for this :
First launch :
1. Get server data.
2. Save server data in Sqlite database.
3. Show Sqlite data.
Next launches :
1. Show Sqlite data.
2. Get server data in background.
3. Delete previous Sqlite data.
4. Save new server data in Sqlite database.
5. Show Sqlite data.
I have couple of questions on these steps :
1. Is this the right approach ? Other way could be showing data every time from server but that would not display the data on screen immediately (depending on internet speed).
2. I also thought of comparing the Sqlite data with the new server data. But faced a big challenge. The new server data might have new records or deleted records. Also, I could not find an appropriate approach to compare each database field with JSON data.
So what is the best approach to compare local Sqlite data with new server data ?
3. Each time I delete the Sqlite data and insert new data and then refresh the screen (which has a UITableView), it blinks for a second which is obvious. How to avoid this issue if steps 3, 4, 5 are followed ?
4. How should I proceed with data update in case I come back on the screen each time or when the application becomes active ? I am very aware of NSOperationQueues or using GCD for that matter. But what if I am crazy and go back and forth to screen again and again. There will be a number of NSOperations in the queue.
It's a challenge to synchronise server data, I've done that before, and if you can spend time on it I'd say it's the best solution.
You may need creation and modification dates on both server and local objects, to compare them - this will let you decide which objects to add, update and delete.
If the server sends you only the recently updated objects you can save a lot of traffic and improve performance (but deleted objects will be harder to detect).
If the data is only changed in the server it's easier, when the app can change the data too it becomes more complicated (but it seems that it's not your case). It also depends on how complex the database is, of course.
If you don't want to invest some time in doing this, just fetching all data everytime works too, even if it is not ideal! Instead of showing the old data and blinking it, you can just make the user wait 2-3 seconds when entering, while you get the new data. Or instead you can fetch the data only when starting the app, and so when you get to that view controller it will be ready already.
It's a complex problem that everyone faces at some point, so I'm curious to see what other people will suggest :)
This is a good question.
I personally think downloading data, store locally and later try to sync is a dangerous scenario. Easy to introduce bugs, master <-> slave issues (what data should be master, if multiple devices would be used etc.)
I think something like this could be a working approach:
1. I would try to look at possibilities to lazy load the data from the server on-demand. That is when a user have a View that should display data, load that specific data with the creation of that specific View. This ensures the data is allways in sync.
2. Tackling the need to reload data from server from every view, could be done by simply storing the downloaded data as objects in memory (not using SqlLite). The view will try to load the needed data trough your cache manager, and it would serve it from memory, if available. If not in memory simply get the data from your server and add it to your memory cache.
The memory cache could be a home made data manager wrapping a Dictionary stored on you AppDelegate, or some global "Singelton" to wrap the cache management/storing and data loading.
3. With lazy loaded data and memory cache you would need to make sure any updates (changes, new records, deleted records) updates your memory data model, as well as pushing these changes to the server as soon as possible. Depending on data size etc. you could force the user to wait, or do it directly as background process.
4. To ensure the data is in sync, you should make sure that you periodically invalidate (delete) the local memory records in the cache and thereby force data updates from the server. Best approach would probably be to have a last updated timestamp for each record in the memory cache. So the periodical invalidator would only delete "old records" from the memory cache (once again not from the server).
To save server from unnecessary data load, the data should still load on demand when the user needs it in a view, and not as part of "cache invalidation".
5. Depending on the data size you might need to look at "cache invalidation". Could be as simple as when xx records are stored, start deleting old objects from memory cache (not server, only locally on device).
6. If data sync is absolutely critical you might want to look at refreshing your memory cache for a record, just before you allow the user to change data. E.g. when user taps "Edit" or similar, you grab the latest data from server for that record. This is just to make sure the user is not going to update a record using outdated data and thereby accidentally overriding any changes made remote, or on another device etc.
--
My take on it. I do not believe there is a "perfect right way" to do this. But this would be what I would try to do.
Hope this will help with some ideas and inspiration.
How about this:
If data exists in SqlLite, load into "in-memory" copy and show it
In background load new server data
delete old sqlite data if it exists (note that the in-memory copy remains)
save new server data to sqlite
load new sqlite data into "in-memory" copy and show it.
If no data was found in step 1, display a "loading" screen to the user during step 2.
I'm making the assumption that the data from SqlLite is small enough to keep a copy in memory to show in your UITable view (The UITable view would always show data from in-memory).
It may be possible to combine steps 4 and 5 if the data is small enough to hold two copies in memory at the same time (you would create a new in-memory copy and swap with the visible copy when complete).
Note:
I don't talk about error handling here, but I would suggest that you don't delete the sqlite data until you have new data to replace it with.
This approach also eliminates the need to determine if this is the first launch or not. The logic always remains the same which should make it a little easier to implement.
Hope this is useful.
You can do same things more efficiently by MultiVersion Concurrency Control (MVCC), which uses a counter (sort of a very simple "time stamp") for every data record, which is updated whenever the record is changed means you need to get those data which is Updated after last sync call that reduces lots of redundant data and bandwidth.
Source: MultiVersion Concurrency Control
I'm importing data into Core Data and find that the save operation is slow. Using the iOS simulator, I watch the sqlite-wal file grow and grow until its over 7GB in size.
I'm importing approx 5000 records with about 10 fields. This isn't a lot of data.
Each object I'm inserting has a to-one relation to various other objects (6 relations total). All of those records combined equal less than 20 fields. There are no images or any binary data or anything that I can see that would justify why the resulting size of the WAL file is so huge.
I read the sqlite docs describing the wal file and I don't see how this can happen. The source data isn't more than 50 MB.
My app is multi-threaded.
I create a managed object context in the background thread that performs the import (creates and saves the core data objects).
Without writing the code out here, has anyone encountered this? Anyone have a thought on what I should be checking. The code isn't super simple and all the parts would take time to input here so lets start with general ideas.
I'll credit anyone who gets me going in the right direction.
Extra info:
I've disabled the undo manager for the context as I don't need that (I think it's nil by default on iOS but I explicitly set it to nil).
I only call save after the entire loop is complete and all managed objects are in ram (ram goes up to 100 MB btw).
The loop and creation of the core data objects takes only 5 seconds or so. The save takes almost 3 minutes as it writes the the awl file.
It seems my comment to try using the old rollback(DELETE) journal mode rather than WAL journal mode fixed the problem. NOTE that there seem to be a range of problems when using WAL journal mode including the following:
this problem
problems with database migrations when using the migratePersistentStore API
problems with lightweight migrations
Perhaps we should start a Core Data WAL problems page and get a comprehensive list and ask Apple to fix the bugs.
Note that the default mode under OS X 10.9 and iOS 7 now uses WAL mode. To change this back add the following option
#{ NSSQLitePragmaOptions : #{ #"journal_mode" : #"DELETE" } }
All changed pages of a transaction get appended to the -wal file.
If you are importing multiple records, you should, if possible, use a single transaction for the entire import.
SQLite cannot do a full WAL checkpoint while some other connection is reading the database (which might just be some statement that you forgot to close).
We are having a strange situation while trying to dbexport/dbimport an Informix database.
while importing the DB we got the error:
1213 - Character to numeric conversion error
I checked at which does does the import stops.
I edited the corresponding file (sed -n '1745813,1745815p' table.unl) and have seen data that look to be corrupt.
3.0]26.0]018102]0.0]20111001.0]0.0]77.38]20111012.0]978]04]0.0072]6.59]6.59]29.93]29.93]77.38]
3.0]26.0]018102]0.0]20111001.0]0.0]143.69]20111012.0]978]04]0.0144]6.59]6.59]48.79]48.79]143.69]
]0.000/]]-0.000000000000000000000000000000000000000000000000000044]8\00\00\07Ú\00\00Õ²\00\00\07P27\00\00\07Ú\00\00i]-0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000999995+']-49999992%(000000000000000000.0]-989074999997704800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0]-999992%(0000000000000000000000.0]]]Ú\00\00]*00000015056480000000000000000000000000000000000000000000000000000000000.0]-92%'9999)).'000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0]-;24944999992%(000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0]-81%-999994;2475200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0]]-97704751999992%(00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0]
The first two lines are OK. The rest seems to be corrupt data.
I do not know how the data appears here since it does not appear in select statement.
I exported only the affected table and figured out that the same data is there.
I looked for a filter that matches all the rows, I used it in another export. This time the corrupt data is not there.
Any idea about what might be the reason behind this?
Best Regards
Arthur
Arthur,
Trying answering the question, why the database is generating corrupted data.
You will need to investigate.
The common causes is :
Occur some crash at your OS/Hardware
Occur some lack of energy
Occur some crash at your database or they process are killed by some admin.
After any of problems above, your FS become corrupt and probably at recovery (fsck) mess the database data.
Probably you are working with a Journaling FS , which ext3,ext4,ntfs is...
If you don't know anything about any events like described before, you need to investigate into the online.log of our Informix database , looking for any start of the engine without a regular shutdown before. Look at your OS logs will help too to found for any involuntary restart of the OS (lack of energy or crash).
Now about the solutions.
Recover a backup
Then you can export just the table corrupted and replace it at your dbexport.
You can do this with archecker. (must be Informix version grater of 10.FC4)
This article maybe will help you if need : Table Level Restore - Pretty Useful Stuff
Export your table just like your describe at the comments.
But this will not recover the corrupt data, they just will "save" the "good" data and discard the "bad" data.
created a new table copy of the first one.
Insert into table 2 select * from table1 where (my filter which matched all rows)
recreated table indexes
renamed tables
Depending how is bad is the corrupted data sometimes you not able to export all "good" data at just one select, you need workaround the "bad" data , check this IBM article:
Unloading around table corruption
Ways to prevent this kind of problem or make easily any recover
First, of course, there is no way to prevent any crash...
What you can do is try minimize the damage after any crash.
Do not use journal file systems!
(at linux, use ext2 FS or RAW devices)
Enable KAIO (for RAW) or DIRECT_IO (of any FS) at Informix configuration.
This will prevent the database to use the OS cache, making more secure the process of writing data at your disk. At some situations this can slower down or speed up your database, depends a lot of your hardware/storage.
Configure your backup to work and test/check it with some frequency.
I recommend configure the backup of full database + logical logs backup.
Depending the version of the Informix and which license you have, you maybe have the rights to configure a cold RSS server ("cluster" secondary node) which will work as active-passive mode at different server and will reduce dramatically you chances to loose any data after any crash at the mainly server.
After any crash, run oncheck to detect the if occur some corruption :
How to use oncheck to detect corruption
I store some data in my iOS app directly in a local .sqlite file. I chose to do this instead of CoreData because the data will need to be compatible with non-Apple platforms.
Now, I'm trying to come up with the best way to sync this file over iCloud. I know you can't sync it directly, for many reasons. I know CoreData is able to sync its DBs, but even ignoring that using CD would essentially lock this file into Apple platforms (I think? I've only looked into CD a bit), I need the iCloud syncing of this file to work across ALL of iCloud's supported platforms - which is supposed to include Windows. I have to assume that there won't be any compatibility for the CoreData files in the Windows API. Planning out the best way to accomplish this would be a lot easier if Apple would tell us any more than "There will be a Windows API [eventually?]"
In addition, I'll eventually need to implement at least one more sync service to support platforms that iCloud does not. It would be helpful, though not required, if the method I use for iCloud can be mostly reused for future services.
For these reasons, I don't think CoreData can help me with this. Am I correct in thinking this?
Moving on from there, I need to devise an algorithm for this, or find an existing one or an existing 3rd party solution. I haven't stumbled across anything yet. However, I have been mulling over a couple possible methods I could implement:
Method 1:
Do something similar to how CoreData syncs sqlite DBs: send "transaction logs" to iCloud instead and build each local sqlite file off of those.
I'm thinking each device would send a (uniquely named) text file listing all the sql commands that that device executed, with timestamps. The device would store how far along in each list of commands it has executed, and continue from that point each time the file is updated. If it received updates to multiple log files at once, it would execute each command in timestamp order.
Things could get 'interesting' efficiency-wise once these files get large, but it seems like a solvable problem.
Method 2:
Periodically sync a copy of the working database to iCloud. Have a modification timestamp field in every record. When an updated copy of the DB comes through, query all the records with newer timestamps than some reference time and update the record in the local DB from the new data.
I see many potential problems with this method:
-Have to implement something further to recognize record deletion.
-The DB file could get conflicts. It might be possible to deal with them by handling each conflict version in timestamp order.
-Determining the date to check each update from could be tricky, as it depends on which device the update is coming from.
There are a lot of potential problems with method 2, but method 1 seems doable to me...
Does anyone have any suggestions as to what might be the best course of action? Any better ideas than my "Method 1" (or reasons why it wouldn't work)?
Try those two solutions from Ray Wenderlich:
Exporting/Importing data through mail:
http://www.raywenderlich.com/1980/how-to-import-and-export-app-data-via-email-in-your-ios-app
File Sharing with iTunes:
http://www.raywenderlich.com/1948/how-integrate-itunes-file-sharing-with-your-ios-app
I found it quite complex but helped me a lot.
Both method 1 and method 2 seem doable. Perhaps a combination of the two in fact - use iCloud to send a separate database file that is a subset of data - i.e. just changed items. Or maybe another file format instead of sqlite db - XML/JSON/CSV etc.
Another alternative is to do it outside of iCloud - i.e. a simple custom web service for syncing. So each change gets submitted to a central server via JSON/XML over HTTP, and then other devices pull updates from that.
Obviously it depends how much data and how many devices you want to sync across, and whether you have access to an appropriate server and/or budget to cover running such a server. iCloud will do that for "free" but all it really does is transfer files. A custom solution allows you to define your syncing model as you wish, but you have to develop and manage it and pay for it.
I've considered the possibility of transferring a database file through iCloud but I think that I would run into classic problems of timing - slow start for the user - and corrupted databases if the app is run on multiple devices simultaneously. (iPad/iPhone for example).
Sooo. I've had to use the transaction logs method. It really is difficult to implement, but once in place, seems ok.
I am using Apple's SharedCoreData sample as the base for this work. This link requires an Apple Developer Account.
I did find a much much better solution from Tim Roadley however this only works for IOS and I needed both IOS and MacOS.
rant> iCloud development really has to get easier and more stable! /rant