CODEWITHSUNDEEP
c#sqlite
Brian Sweeney
Brian Sweeney

Reputation: 6803

Database file is inexplicably locked during SQLite commit

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.

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!

Upvotes: 11

Views: 15672

Answers (10)

Snake
Snake

Reputation: 1

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.

Upvotes: 0

Liron Levi
Liron Levi

Reputation: 1169

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.

Upvotes: 11

Lu&#237;s Fernando Quitaiski
Lu&#237;s Fernando Quitaiski

Reputation: 61

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!

Upvotes: 3

Kev
Kev

Reputation: 121

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()
}

Upvotes: 5

arolson101
arolson101

Reputation: 1502

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.

Upvotes: 2

Aviad Ben Dov
Aviad Ben Dov

Reputation: 6409

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?

Upvotes: 2

Svante Svenson
Svante Svenson

Reputation: 12478

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.

Upvotes: 5

BlueShepherd
BlueShepherd

Reputation: 593

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?

Upvotes: 2

Jay
Jay

Reputation: 14471

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.

Upvotes: 3

tuinstoel
tuinstoel

Reputation: 7306

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?

Upvotes: 2

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