System.Data.SQLite

This will come up frequently, so I thought I'd sticky a post about it.

The SQLite engine itself works fine in a multithreaded environment as long as you follow the rules.  The chief rule is: you may not have multiple threads calling sqlite functions on the same connection pointer, nor may multiple threads call sqlite functions on pointers that hold a reference to the same connection pointer.  Sure you could implement your own synchronization objects over the top of these pointers to prevent simultaneous access, but the ADO.NET wrapper does not do this -- therefore the same rules apply to ADO.NET.  I'll expand a bit:

It is perfectly legal for two threads to each have their own instance of a SQLiteConnection object, and for each of those connections to reference the same database file.  This is the "right" way to do things.  You can also call the Clone() method on an open connection at any time and pass the cloned connection to a secondary thread.  It is NOT safe to pass a connection to another thread and THEN clone it -- clone it FIRST and pass the clone to the secondary thread.

Just about every other object in the wrapper holds a reference to the underlying SQLiteConnection object too, and therefore should never be shared between threads.  SQLiteCommand, SQLiteDataReader, SQLiteCommandBuilder, SQLiteDataAdapter, and SQLiteTransaction are just some of the objects that either directly or indirectly hold a reference to a SQLiteConnection and should never be shared across threads.  SQLiteCommand also implements ICloneable, so if you must share a command across threads, make a clone first.

So to summarize:  If multiple threads must talk to the same database file, then make multiple connections to it and make sure you don't share them.  Also remember that SQLite's locking mechanism is not row or table level, it is file level locking.  Therefore you cannot be actively iterating through a datareader while a write operation is pending, and all write operations are suspended while a datareader is reading.  The ADO.NET wrapper has built-in retry and timeout mechanisms to handle these scenarios, but if you are using multiple threads and attempt to mix a read with a write, or a write with a read, you're going to hit a deadlock.

Robert

 

rsimpson wrote:

Sure you could implement your own synchronization objects over the top of these pointers to prevent simultaneous access, but the ADO.NET wrapper does not do this -- therefore the same rules apply to ADO.NET.

To clarify, is this acceptable to do?  I'm using a SQLite database as a log file and have threads coming from everywhere which want to write to the log file.  Some come from async operation thread pools, so there's no way of assigning every thread its own connection.  To get around this problem I implemented my own locking mechanism on a single SQLiteConnection so that only one thread will be executing a statement at a time.

It seems to work well but am I going to run into a problem in the underlying code due to some thread-specific resource?  I'd like to avoid creating a specific thread to do my logging.

It works as long as you coded it properly :)  The SQLite engine itself doesn't have any threading issues, so as long as you've guarded your connection (and anything that executes on that connection) with a mutex, you should be fine.

Robert

 

rsimpson wrote:

It works as long as you coded it properly :)

Well that's always up for debate.  Thanks for alleviating my fears, I was just hoping that something wasn't allocated in thread-local storage or something crazy like that which would cause an invisible problem.  Awesome!

Did the 3.5.0 update relax the restrictions in regards to this?  I know the SQLite added some pretty heavy use of mutexes in that release.

Hi,

 I'm wondering: how would such a locking mechanism work actually ? I'm working on a project in a client server enviroment. Sqlite is used at the client site to cache data from the server.

I'm hitting a "database is locked" situation cause of the fact that incoming data from the server (over a socket) needs to be inserted or updated in the database. On the other side, you have the client app which you have no control over the actions of the user. In this situation, we hit the database locking A LOT.. and i honestly have no clue how to solve this

Noctris: I'm not sure about this, but check if you dispose all the commands and datareaders after usage.

Robert: We have a locking problem in our project and I could write a test to reproduce it. It just opens two connections and transactions (using NHibernate) in two threads. The first transaction is waiting for the second to finish, the second transaction runs into a timeout with a database lock error. Both don't do anything in the database, just open a transaction. Is this due to Sqlite restrictions? I can't find it out in the documentation and forums of Sqlite.

Pseudo Code of my test:

cn1 = new Connection
cn2.BeginTransaction
queue new Thread
{
  cn2 = new Connection
  cn2.BeginTransaction  <<== locking exception after 30sec
  event.Set
}
event.Wait

SQLite knows three kinds of transaction: deferred, immediate, or exclusive. If my understanding is correct, immediate and exclusive will immediatly lock the database. So my guess would be you're starting one of those on the first connection, which causes the locking exception on the second connection...

Thanks. This means, I should find out how NHibernate opens the connection to the database. I just wasn't sure if SqlLite is able to handle multiple connections at all (except of locking the whole database when opening a connection) and if it is worth to find out more about it. Currently, our application runs fine with SqlServer, but only partially on SqlLite.

I am running into threading issues myself, where each thread does create its own connection, but i still run into problems and fails with:

System.Data.SQLite.SQLiteException: Unable to open the database file
unable to open database file

I've posted a repro case here: http://sqlite.phxsoftware.com/forums/p/1770/7470.aspx

Any suggestions as to what I'm doing wrong would be greatly appreciated. Right now I've built a wrapper that invokes a lock on a global syncroot, but  i'd really prefer to do it that way. I even tried using a connection pool of 1 connection, hoping that would serialize the access to the file, but no luck.

Have you tried using a ReaderWriterLock on your wrapper?  That way you won't block reads, and all reads will wait till the writer finishes then are granted access to do whatever they want. 

Can anyone provide a working example how to work with sqlite in a multithreaded environment?

What you will want to do is organize your code so that whenever you are hitting a certain database, you either lock for read or for write.  I would advise using a ReaderWriterLock.  You'll want the lock to be a singleton.

Say you have a class called TestObjectData (organize it however you want), which holds static methods for inserting, selecting etc. 

Insert(TestObject o)
{

   try

    {

    rwLock.AcquireWriterLock(300);

    //do your inserts

     }

     finally

     {

        if(rwLock.IsWriterLockHeld)
        {

              rwLock.ReleaseWriterLock();

        }

     }

}

 

For reads it is the same, however you will want to acquire a read lock instead of a write lock. Rule of thumb is whenever you are hitting a database, it locks all tables in that database, so you will need to handle this condition within code.

 I am also suffering from the same database locking Problem. My requirement is when one thread is running (read/write) the second thread also have to work for read and write  safely on the same database file. but now its not happening whether i created seperate connections for each threads. 

In above code you have used  "rwLock" from where you get this object?

My application is in VB.net  and my both threads working Inserts and updates.

Please help me 

 Hi thank you for your post about the SQLite. This is a very nice post and it contains many important information here. So I liked it. And keep it up. Thanks.