I want to get the path or even the connection string of a sql.DB instance. The package I'm writing doesn't know what the database path is and the application may have multiple database files. However I need to be able to map a specific database to a specific buffered channel, and ignore any additional calls for a database the package has already seen.
The application uses the github.com/mutecomm/go-sqlcipher driver to instantiate the database, if that makes any difference.
Is it possible to discriminate between instances of sql.DB based on the file path of the source database? If so, how do I do it?
Related
I'm quite new in c# and Xamarin android and I want to backup and restore my offline SQLite database created with sqliteOpenHelper Class, send and use that on another device. thanks in advance.
In Android API sets, the providers for SQLite library are available under, android.database.sqlite package. The most prominent types in the package are:
1. SQLiteOpenHelper: This is the main class that you need to inherit your classes from, in order to handle the database creation, opening or closing events. Even the events such as create new tables, or deleting the old tables and upgrading your databases to a latest version (such as upgrading the schema), are all handled here in this class-derived classes of yours.
2. SQLiteDatabase: This is the object that you get and use to either push the data to the database, or to read the data from the database.
3. SQLiteCursor: This is the cursor implementation for working with the data records that are returned after Query commands.
The way they all communicate is that, the main class for the data manipulation first of all inherits from SQLiteOpenHelperto get the functions to handle, then later has a field of type SQLiteDatabasein it to execute the functions for writing or reading the data.
For more information, you can check:
https://basicsofwebdevelopment.wordpress.com/2017/02/19/learning-sqlite-databases-in-xamarin-for-android/
https://www.c-sharpcorner.com/article/xamarin-android-develop-sqlite-database-using-sqliteopenhelper/
I'm referring to SQLite-PCL tutorial here: https://code.tutsplus.com/tutorials/an-introduction-to-xamarinforms-and-sqlite--cms-23020
I'm very new to SQLite, so I'm lacking in knowledge in lots of basic things - I have tried Googling, can't understand most of it.
Is the call to new SQLiteConnection actually opens up the database or just saying that "the road to the database has been established, whether you access it or not is up to you"?
How do I check if there's already existing database in the devices? And if there is, how do I access it? I have Googled this, but it all seems to be a bit extreme - can't I just call simple OPEN the database?
Is it okay to have multiple SQLiteConnection instances to the same database, if I can be sure that I'm not going to do multiple transaction at the same time?
After I have INSERT into the database, close the app, open up the app back - how do I make sure that there is database created in previous session? Any way to debug this? Because I have no idea if the database is there or not, and I don't know how to access it either..
SQLiteConnection returns a connection object that is used to make subsequent queries
use File.Exists to see if the db file is already present
Yes
Again, use File exists to see if the db file is physically present
Xamarin's ToDo sample provides a good overview of using SQLite with Forms.
How to create data base link in oracle 11 g to Access Tables.
You seem to have copied the example in the documentation without really understanding it.
The USING 'local' part of the statement is creating a link to 'the local database', where local is the service name of a database. (The example is a bit confusing, to be fair).
When the link is used it tries to interpret local as a service name, appending the current database's domain, as the docs say:
USING 'connect string'
Specify the service name of a remote database. If you specify only the
database name, then Oracle Database implicitly appends the database
domain to the connect string to create a complete service name.
Therefore, if the database domain of the remote database is different
from that of the current database, then you must specify the complete
service name.
If you're trying to create a link back into the same database - which would be a bit odd but I've seen it done in place of grant access across schemas, and that seems to be what the example is hinting at - then you can replace 'local' in the USING clause with the service name of your current database (e.g. USING 'orcl', or whatever).
You can also use a TNS alias; if your tnsnames.ora has an entry for SOME_DB which points to the SID or service name of another database, you can have USING'some_db'`. You should be able to use any connect string I think; certainly Easy Connect is allowed. There's more in the net services admin guide.
I have a simple access database that resides on a network drive. All of the people that need to make any modifications to this database have access on their machine, but they would like to have a read-only website just displaying the data contained in it. I set up a website and tried using the accessdatasource, and while I could connect, it had issues whenever a user had the database open in access. I swapped to using a sqldatasource with a connection string set up according to www.connectionstrings.com, but I seem to be having either the same or similar issues, depending on how I set up the string/database. Basically, the entire issue is, if the database is opened by any user, the webpage is prevented from opening the database. Is it possible to open the database read only from the webpage?
It is an access 2000 database, but everyone is using copies of access 2007, and in all of these instances, the ASP.NET user has read/write access to the network directory containing the database, and read access to the database itself.
When I use the connection string:
ConnectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=\\<source>\<database>.mdb;User Id=<USER>;Password=<PASSWORD>;Mode=Share Deny Write;"
Where is not the default "admin" user, and is configured to have open/run (not open/exclusive) permissions on the database, I get the error:
Cannot start your application. The workgroup information file is missing or opened exclusively by another user.
As far as I can tell, there is no workgroup information file, but I am not really sure what I am doing there.
When is the default "admin" user, I get the error:
Could not use ''; file already in use.
If someone has the database currently open, otherwise, it works fine.
And finally, if I use the connection string:
ConnectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=\\<source>\<database>.mdb;User Id=<USER>;Password=<PASSWORD>;Mode=Read;"
I get the error:
Could not lock file.
I have been googling this for the past several days now and I feel like I've just been going around in circles. Any insight at all would be appreciated.
As it's Access 2000 format there will definately have to be a workgroup file (.mdw) even if that has no security set as such. I would try the connection string whereby you also specify the workgroup location e.g:
Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\mydatabase.mdb;Jet OLEDB:System Database=system.mdw;User ID=myUsername;Password=myPassword;
Furthermore, I wouldn't recommend setting the read/write properties in the connection string, instead I think you would be better to create an account in the workgroup file and only assign it read only priviledges.
Update:
This is the ODBC connection method:
Driver={Microsoft Access Driver (*.mdb)};Dbq=C:\mydatabase.mdb;Uid=Admin;Pwd=;
I am completely new to SQLite and I intend to use it in a M2M / client-server environment where a database is generated on the server, sent to the client as a file and used on the client for data lookup.
The question is: can I replace the whole database file while the client is using it at the same time?
The question may sound silly but the client is a Linux thin client and to replace the database file a temporary file would be renamed to the final file name. In Linux, a program which has still open the older version of the file will still access the older data since the old file is preserved by the OS until all file handles have been closed. Only new open()s will access the new version of the file.
So, in short:
client randomly accesses the SQLite database
a new version of the database is received from the server and written to a temporary file
the temporary file is renamed to the SQLite database file
I know it is a very specific question, but maybe someone can tell me if this would be a problem for SQLite or if there are similar methods to replace a database while the client is running. I do not want to send a bunch of SQL statements from the server to the client to update the database.
No, you cannot just replace an open SQLite3 DB file. SQLite will keep using the same file descriptor (or handle in Windows-speak), unless you close and re-open your database. More specifically:
Deleting and replacing an open file is either useless (Linux) or impossible (Windows). SQLite will never get to see the contents of the new file at all.
Overwriting an SQLite3 DB file is a recipe for data corruption. From the SQLite3 documentation:
Likewise, if a rogue process opens a
database file or journal and writes
malformed data into the middle of it,
then the database will become corrupt.
Arbitrarily overwriting the contents of the DB file can cause a whole pile of issues:
If you are very lucky it will just cause DB errors, forcing you to reopen the database anyway.
Depending on how you use the data, your application might just crash and burn.
Your application may try to apply an existing journal on the new file. Sounds painful? It is!
If you are really unlucky, the user will just get back invalid results from any queries.
The best way to deal with this would be a proper client-server implementation where the client DB file is updated from data coming from the server. In the long run that would allow for far more flexibility, while also reducing the bandwidth requirements by sending updates, rather than the whole file.
If that is not possible, you should update the client DB file in three discrete steps:
Send a message to the client application to close the DB. This allows the application to commit any changes, remove any journal files and clean-up its internal state.
Replace/Overwrite the file.
Send a message to the client application to re-open the DB. You would have to setup all prepared statements again, though.
If you do not want to close the DB file for some reason, then you should have your application - or even a separate process - update the original DB file using the new file as input. The SQLite3 backup API might be of interest to you in that case.