I have a SQLite database that is saved on a network share and can be accessed by multiple users.
Now the schema of this database needs to be changed (by a self written program) and I need to make sure that during the update operation no other user can open the database (neither for writing nor for reading).
To my knowledge the PRAGMA locking_mode=EXCLUSIVE could be used to lock the database. Unfortunately the exclusive lock is only obtained when the first write operation is executed.
That means that in the time between opening the database, setting the locking mode and the first write operation a different user would be able to open the database.
Is there any way to obtain the exclusive lock on opening the database with System.Data.SQLite from C#?
EDIT
As you requested some code, here you go:
void UpdateDatabaseSchema(Boolean UpdateNeeded)
{
// make sure that all SQLite* objects are disposed correctly by using-statement, otherwise database will not be closed correctly!
using (var Connection = new SQLiteConnection("./Database.db"))
using (var Command = Connection.CreateCommand())
{
Connection.Open();
Command.CommandText = "PRAGMA locking_mode=EXCLUSIVE;";
Command.ExecuteNonQuery();
Command.CommandText = "PRAGMA locking_mode;";
using (var DataReader = Command.ExecuteReader())
{
while (DataReader.Read())
{
var Test = DataReader.GetString(0);
}
}
if (UpdateNeeded)
{
if (System.Windows.Forms.MessageBox.Show("Do you want to update the database schema?", "Update needed", System.Windows.Forms.MessageBoxButtons.YesNo) == System.Windows.Forms.DialogResult.Yes)
{
Command.CommandText = "CREATE TABLE Users (Test TEXT NOT NULL DEFAULT 0);";
Command.ExecuteNonQuery();
}
}
}
}
Obviously the reading of the locking mode is only there for debugging (and will be removed for productive code).
Now what happens if another user opens the same database - a tiny bit - later than the first user but is faster in clicking "Yes"? The first user will get an error because the schema has already been changed.
Yes, I could write the statement differently but this update is just an example, there might - and will - be more difficult queries in the future and I don't want to care about these race conditions in every command (at least if possible).
Hence I need to lock the database on opening.
#C Perkins:
- Handling the problem on file system basis is something I had not thought about, I'll have a look into that possibility, thanks for the input!
- Online docs also mentions "The first time the database is written, an exclusive lock is obtained and held." I know it is only released when the connection is closed, the problem is it is only obtained on the first write operation.
My testing indicates that an exclusive lock is obtained by executing any kind of database-changing command whether or not it actually makes a change. In other words, the following two commands will end up obtaining an exclusive lock, but the WHERE false makes the command a null op.
//* The following only changes the mode, but does not lock the file
Command.CommandText = "PRAGMA locking_mode=EXCLUSIVE;";
Command.ExecuteNonQuery();
try {
using (var cmdLock = Connection.CreateCommand())
{
//* The following command will force an exclusive file lock to be obtained.
//* Although 'WHERE false' will cause the actual UPDATE to fail,
//* the actual statement is valid SQL and will not cause an error.
cmdLock .CommandText = "UPDATE Users SET Test = 'bogus' WHERE false;";
cmdLock .ExecuteNonQuery();
}
//* Exclusive lock obtained
//... free to do updates
}
catch {
MessageBox.Show("Failed to obtain exclusive lock, try again later.", "Lock failed")
}
I want to lock a table for a given amount of time in SQL Server . I am using C# at code level. How can I achieve it, I also need to verify that the table is locked or not. I do not have much knowledge of locking tables in SQL Server. I want to achieve it using entity framework. Any help is much appreciated.
You can try as shown below.
using (var context = new YourContext())
{
using (var dbContextTransaction = context.Database.BeginTransaction())
{
//Lock the table during this transaction
context.Database.ExecuteSqlCommand("SELECT 1 FROM YourTable WITH (TABLOCKX)
WAITFOR DELAY '00:03:00'");
//your work here
dbContextTransaction.Commit();
}
}
Note : The table lock will be released after it exits the BeginTransaction block
How to use SqlTransaction in .net 2.0 so that when I start reading data from a table, that table is blocked for others (other programs) to read/write to that table?
If SqlTransaction is not a good option, than what is?
This should be allowed by using Serializable transaction together with TABLOCKX hint in initial select statement. TABLOCKX should take exclusive lock on the table so nobody else can use that table and Serializable transaction should demand HOLDLOCK which means that all locks are kept until end of the transaction (you can use HOLDLOCK directly).
Update: I just tested different scenarios in Management studio and it
looks like you do not need to
explicitly use Serializable
transaction. Using TABLOCKX within any
transaction is enough.
Be aware that such approach can be big bottleneck because only one transaction can operate on such table = no concurrency. Even if you read and work with single record from million nobody else will be able to work with the table until your transaction ends.
So the command should look like:
SELECT * FROM Table WITH (TABLOCKX) WHERE ...
To use serializable transaction you can use SqlTransaction:
using (SqlConnection connection = new SqlConnection(connectionString))
{
connection.Open();
SqlTransaction transaction = connection.BeginTransaction(IsolationLevel.Serializable);
try
{
...
transaction.Commit();
}
catch (Exception)
{
transaction.Rollback();
...
}
}
Or System.Transactions.TransactionScope (default isolation level should be Serializable).
using (TransactionScope scope = new TransactionScope())
{
using (SqlConnection connection = new SqlConnection(connectionString))
{
...
}
scope.Complete();
}
I have these two exceptions generated when I try to get data from SQL database in C#:
System.Data.SqlClient.SqlException: Transaction (Process ID 97) was deadlocked on lock resources with another process and has been chosen as the deadlock victim.
OR
System.Data.SqlClient.SqlException: Transaction (Process ID 62) was deadlocked on lock resources with another process and has been chosen as the deadlock victim.
OR
System.Data.SqlClient.SqlException: Transaction (Process ID 54) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction.
this is the code:
using (SqlConnection con = new SqlConnection(datasource))
{
SqlCommand cmd = new SqlCommand("Select * from MyTable Where ID='1' ", con);
cmd.CommandTimeout = 300;
con.Open();
SqlDataAdapter adapter = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
adapter.Fill(ds);
con.Close();
return ds.Tables[0];
}
These happened every time.
Any ideas on how these can be resolved?
There are a couple of things you can do to lessen the number of deadlocks you receive, and some things you can do to completely eliminate them.
First off, launch SQL Server Profiler and tell it to give you a deadlock graph. Running this trace will tell you the other query which is conflicting with yours. Your query is quite simple, though I seriously doubt you have a SELECT * query off a table called MyTable in your system...
Anyway, armed with the deadlock graph and the other query, you should be able to tell what resources are deadlocking. The classic solution is to change the order of both queries such that the resources are accessed in the same order -- this avoids cycles.
Other things you can do:
Speed up your queries by, among other things, applying the correct indexes to them.
Enable snapshot isolation on the database and use SET TRANSACTION ISOLATION LEVEL SNAPSHOT in your transactions where appropriate. Also enable read committed with row-versioning. In many cases, this is enough to eliminate most deadlocks completely. Read about transaction isolation levels. Understand what you're doing.
Not that this is going to help the deadlock issue, but you should be disposing your other IDisposable objects much like you're disposing your SqlConnection as such:
using (SqlConnection con = new SqlConnection(datasource))
using (SqlCommand cmd = new SqlCommand("Select * from MyTable Where ID='1' ", con))
{
cmd.CommandTimeout = 300;
con.Open();
using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
using (DataSet ds = new DataSet())
{
adapter.Fill(ds);
return ds.Tables[0];
}
}
You might be able to avoid the lock with a locking hint in your query thusly:
Select * from MyTable with (nolock) Where ID='1'
I want to be clear though, you're allowing for reads of uncommitted data with this solution. It's a risk in a transactional system. Read this answer. Hope this helps.
Basically, the SQL server concurrency model makes it so you can never avoid this exception (eg. completely unrelated transaction might block eachother if they happen to lock the same index page or something). The best you can do is keep your transactions short to reduce the likelyhood, and if you get the exception, do what it says and retry the transaction.
I'm performing a large number of INSERTS to a SQLite database. I'm using just one thread. I batch the writes to improve performance and have a bit of security in case of a crash. Basically I cache up a bunch of data in memory and then when I deem appropriate, I loop over all of that data and perform the INSERTS. The code for this is shown below:
public void Commit()
{
using (SQLiteConnection conn = new SQLiteConnection(this.connString))
{
conn.Open();
using (SQLiteTransaction trans = conn.BeginTransaction())
{
using (SQLiteCommand command = conn.CreateCommand())
{
command.CommandText = "INSERT OR IGNORE INTO [MY_TABLE] (col1, col2) VALUES (?,?)";
command.Parameters.Add(this.col1Param);
command.Parameters.Add(this.col2Param);
foreach (Data o in this.dataTemp)
{
this.col1Param.Value = o.Col1Prop;
this. col2Param.Value = o.Col2Prop;
command.ExecuteNonQuery();
}
}
this.TryHandleCommit(trans);
}
conn.Close();
}
}
I now employ the following gimmick to get the thing to eventually work:
private void TryHandleCommit(SQLiteTransaction trans)
{
try
{
trans.Commit();
}
catch (Exception e)
{
Console.WriteLine("Trying again...");
this.TryHandleCommit(trans);
}
}
I create my DB like so:
public DataBase(String path)
{
//build connection string
SQLiteConnectionStringBuilder connString = new SQLiteConnectionStringBuilder();
connString.DataSource = path;
connString.Version = 3;
connString.DefaultTimeout = 5;
connString.JournalMode = SQLiteJournalModeEnum.Persist;
connString.UseUTF16Encoding = true;
using (connection = new SQLiteConnection(connString.ToString()))
{
//check for existence of db
FileInfo f = new FileInfo(path);
if (!f.Exists) //build new blank db
{
SQLiteConnection.CreateFile(path);
connection.Open();
using (SQLiteTransaction trans = connection.BeginTransaction())
{
using (SQLiteCommand command = connection.CreateCommand())
{
command.CommandText = DataBase.CREATE_MATCHES;
command.ExecuteNonQuery();
command.CommandText = DataBase.CREATE_STRING_DATA;
command.ExecuteNonQuery();
//TODO add logging
}
trans.Commit();
}
connection.Close();
}
}
}
I then export the connection string and use it to obtain new connections in different parts of the program.
At seemingly random intervals, though at far too great a rate to ignore or otherwise workaround this problem, I get unhandled SQLiteException: Database file is locked. This occurs when I attempt to commit the transaction. No errors seem to occur prior to then. This does not always happen. Sometimes the whole thing runs without a hitch.
No reads are being performed on these files before the commits finish.
I have the very latest SQLite binary.
I'm compiling for .NET 2.0.
I'm using VS 2008.
The db is a local file.
All of this activity is encapsulated within one thread / process.
Virus protection is off (though I think that was only relevant if you were connecting over a network?).
As per Scotsman's post I have implemented the following changes:
Journal Mode set to Persist
DB files stored in C:\Docs + Settings\ApplicationData via System.Windows.Forms.Application.AppData windows call
No inner exception
Witnessed on two distinct machines (albeit very similar hardware and software)
Have been running Process Monitor - no extraneous processes are attaching themselves to the DB files - the problem is definitely in my code...
Does anyone have any idea whats going on here?
I know I just dropped a whole mess of code, but I've been trying to figure this out for way too long. My thanks to anyone who makes it to the end of this question!
brian
UPDATES:
Thanks for the suggestions so far! I've implemented many of the suggested changes. I feel that we are getting closer to the answer...however...
The code above technically works however it is non-deterministic! It is not guaranteed to do anything aside from spin in neutral forever. In practice it seems to work somewhere between the 1st and 10th iteration. If i batch my commits at a reasonable interval damage will be mitigated but I really do not want to leave things in this state...
More suggestions welcome!
It looks like you failed to link the command with the transaction you've created.
Instead of:
using (SQLiteCommand command = conn.CreateCommand())
You should use:
using (SQLiteCommand command = new SQLiteCommand("<INSERT statement here>", conn, trans))
Or you can set its Transaction property after its construction.
While we are at it - your handling of failures is incorrect:
The command's ExecuteNonQuery method can also fail and you are not really protected. You should change the code to something like:
public void Commit()
{
using (SQLiteConnection conn = new SQLiteConnection(this.connString))
{
conn.Open();
SQLiteTransaction trans = conn.BeginTransaction();
try
{
using (SQLiteCommand command = conn.CreateCommand())
{
command.Transaction = trans; // Now the command is linked to the transaction and don't try to create a new one (which is probably why your database gets locked)
command.CommandText = "INSERT OR IGNORE INTO [MY_TABLE] (col1, col2) VALUES (?,?)";
command.Parameters.Add(this.col1Param);
command.Parameters.Add(this.col2Param);
foreach (Data o in this.dataTemp)
{
this.col1Param.Value = o.Col1Prop;
this. col2Param.Value = o.Col2Prop;
command.ExecuteNonQuery();
}
}
trans.Commit();
}
catch (SQLiteException ex)
{
// You need to rollback in case something wrong happened in command.ExecuteNonQuery() ...
trans.Rollback();
throw;
}
}
}
Another thing is that you don't need to cache anything in memory. You can depend on SQLite journaling mechanism for storing incomplete transaction state.
Run Sysinternals Process Monitor and filter on filename while running your program to rule out if any other process does anything to it and to see what exacly your program is doing to the file. Long shot, but might give a clue.
We had a very similar problem using nested Transactions with the TransactionScope class. We thought all database actions occurred on the same thread...however we were caught out by the Transaction mechanism...more specifically the Ambient transaction.
Basically there was a transaction higher up the chain which, by the magic of ado, the connection automatically enlisted in. The result was that, even though we thought we were writing to the database on a single thread, the write didn't really happen until the topmost transaction was committed. At this 'indeterminate' point the database was written to causing it to be locked outside of our control.
The solution was to ensure that the sqlite database did not directly take part in the ambient transaction by ensuring we used something like:
using(TransactionScope scope = new TransactionScope(TransactionScopeOptions.RequiresNew))
{
...
scope.Complete()
}
Things to watch for:
don't use connections across multiple threads/processes.
I've seen it happen when a virus scanner would detect changes to the file and try to scan it. It would lock the file for a short interval and cause havoc.
I started facing this same problem today: I'm studying asp.net mvc, building my first application completely from scratch. Sometimes, when I'd write to the database, I'd get the same exception, saying the database file was locked.
I found it really strange, since I was completely sure that there was just one connection open at that time (based on process explorer's listing of active file handles).
I've also built the whole data access layer from scratch, using System.Data.SQLite .Net provider, and, when I planned it, I took special care with connections and transactions, in order to ensure no connection or transaction was left hanging around.
The tricky part was that setting a breakpoint on ExecuteNonQuery() command and running the application in debug mode would make the error disappear!
Googling, I found something interesting on this site: http://www.softperfect.com/board/read.php?8,5775. There, someone replied the thread suggesting the author to put the database path on the anti-virus ignore list.
I added the database file to the ignore list of my anti-virus (Microsoft Security Essentials) and it solved my problem. No more database locked errors!
Is your database file on the same machine as the app or is it stored on a server?
You should create a new connection in every thread. I would simplefy the creation of a connection, use everywhere: connection = new SQLiteConnection(connString.ToString());
and use a database file on the same machine as the app and test again.
Why the two different ways of creating a connection?
These guys were having similiar problems (mostly, it appears, with the journaling file being locked, maybe TortoiseSVN interactions ... check the referenced articles).
They came up with a set of recommendations (correct directories, changing journaling types from delete to persist, etc). http://sqlite.phxsoftware.com/forums/p/689/5445.aspx#5445
The journal mode options are discussed here: http://www.sqlite.org/pragma.html . You could try TRUNCATE.
Is there a stack trace during the exception into SQL Lite?
You indicate you "batch my commits at a reasonable interval". What is the interval?
I would always use a Connection, Transaction and Command in a using clause. In your first code listing you did, but your third (creating the tables) you didn't. I suggest you do that too, because (who knows?) maybe the commands that create the table somehow continue to lock the file. Long shot... but worth a shot?
Do you have Google Desktop Search (or another file indexer) running? As previously mentioned, Sysinternals Process Monitor can help you track it down.
Also, what is the filename of the database? From PerformanceTuningWindows:
Be VERY, VERY careful what you name your database, especially the extension
For example, if you give all your databases the extension .sdb (SQLite Database, nice name hey? I thought so when I choose it anyway...) you discover that the SDB extension is already associated with APPFIX PACKAGES.
Now, here is the cute part, APPFIX is an executable/package that Windows XP recognizes, and it will, (emphasis mine) ADD THE DATABASE TO THE SYSTEM RESTORE FUNCTIONALITY
This means, stay with me here, every time you write ANYTHING to the database, the Windows XP system thinks a bloody executable has changed and copies your ENTIRE 800 meg database to the system restore directory....
I recommend something like DB or DAT.
While the lock is reported on the COMMIT, the lock is on the INSERT/UPDATE command. Check for record locks not being released earlier in your code.