I'd like to know the correct approach for running two simultaneous queries using NHibernate. Right now, I have a single ISession object that I use for all my queries:
session = sessionFactory.OpenSession();
In one thread, I'm loading some data which takes 10-15 seconds, but I don't need it right away so I don't want to block the entire program while it's loading:
IDbCommand cmd = session.Connection.CreateCommand();
cmd.CommandType = CommandType.TableDirect;
cmd.CommandText = "RecipesForModelingGraph";
IDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
// Do stuff
}
reader.Close();
This works fine, however in another thread I might be running a query such as:
var newBlah = new Blah();
session.Save(newBlah);
When the above transaction commits, I occasionally get an exception:
Additional information: There is already an open DataReader associated
with this Command which must be closed first.
Now, I thought maybe this was because I was running everything in the same transaction. So, I surrounded all my loading code with:
using (ITransaction transaction = session.BeginTransaction(IsolationLevel.Serializable))
{
// Same DataReader code as above
}
However, the problem has not gone away. I'm thinking maybe I need each thread to have its own ISession object. Is this the correct approach, or am I doing something wrong. Note, I only want a single open connection to the database. Also, keep in mind the background thread is only loading data and nothing else, so I'm not worried about isolation levels and data changing as its being read.
The session is tied to the thread and the Commands created are linked to the sessions connection object. So yes, if a commit or close is executed while an open reader exists you will get an exception.
You could Join() your threads and wait until all are complete before closing/committing.
Related
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")
}
Some first things that people learned in their early use of MySQL that closing connection right after its usage is important, but why is this so important? Well, if we do it on a website it can save some server resource (as described here) But why we should do that on a .NET desktop application? Does it share the same issues with web application? Or are there others?
If you use connection pooling you won't close the physical connection by calling con.Close, you just tell the pool that this connection can be used. If you call database stuff in a loop you'll get exceptions like "too many open connections" quickly if you don't close them.
Check this:
for (int i = 0; i < 1000; i++)
{
var con = new SqlConnection(Properties.Settings.Default.ConnectionString);
con.Open();
var cmd = new SqlCommand("Select 1", con);
var rd = cmd.ExecuteReader();
while (rd.Read())
Console.WriteLine("{0}) {1}", i, rd.GetInt32(0));
}
One of the possible exceptions:
Timeout expired. The timeout period elapsed prior to obtaining a
connection from the pool. This may have occurred because all pooled
connections were in use and max pool size was reached.
By the way, the same is true for a MySqlConnection.
This is the correct way, use the using statement on all types implementing IDsiposable:
using (var con = new SqlConnection(Properties.Settings.Default.ConnectionString))
{
con.Open();
for (int i = 0; i < 1000; i++)
{
using(var cmd = new SqlCommand("Select 1", con))
using (var rd = cmd.ExecuteReader())
while (rd.Read())
Console.WriteLine("{0}) {1}", i, rd.GetInt32(0));
}
}// no need to close it with the using statement, will be done in connection.Dispose
Yes I think it is important to close out your connection rather than leaving it open or allowing the garbage collector to eventually handle it. There are a couple of reason why you should do this and below that I'll describe the best method for how
WHY:
So you've opened a connection to the database and sent some data back and forth along this pipeline and now have the results you were looking for. Ideally at this point you do something else with the data and the end results of your application is achieved.
Once you have the data from the database you don't need it anymore, its part in this is done so leaving the connection open does nothing but hold up memory and increase the number of connections the database and your application has to keep track of and possibly pushing you closer to your maximum number of connections limit.
"But wait! I have to make a lot of database calls in rapid
succession!"
Okay no problem, open the connection run your calls and then close it out again. Opening a connection to a database in a "modern" application isn't going to cost you a significant amount of computing power/time, while explicitly closing out a connection does nothing but help (frees up memory, lowers your number of current connections).
So that is the why, here is the how
HOW:
So depending on how you are connecting to your MySQL database you a probably using an IDisposible object to help manage the connection. Here is what MSDN has to say on using an IDisposable:
As a rule, when you use an IDisposable object, you should declare and
instantiate it in a using statement. The using statement calls the
Dispose method on the object in the correct way, and (when you use it
as shown earlier) it also causes the object itself to go out of scope
as soon as Dispose is called. Within the using block, the object is
read-only and cannot be modified or reassigned.
Here is my personal take on the subject:
Using a using block helps to keep your code cleaner (readability)
Using a usingblock helps to keep your code clear (memory wise), it will "automagically" clean up unused items
With a usingblock it helps to prevent using a previous connection from being used accidentally as it will automatically close out the connection when you are done with it.
In short, I think it is important to close connections properly, preferably with a con.close() type statement method in combination with a using block
As pointed out in the comments this is also a very good question/answer similar to yours: Why always close Database connection?
I'm working on an ASP.NET application where, as part of some logic, I want to lock some tables and do work on them. The method runs in a separate thread running as a kind of background task, spawned via a Task. The problem comes in with the error handling...
The code looks more or less like this:
MySqlConnection connection = new MySqlConnection(ConfigurationManager.AppSettings["prDatabase"]);
try
{
connection.Open();
MySqlCommand lock_tables = new MySqlCommand(Queries.lockTables(), connection);
lock_tables.ExecuteNonQuery();
// do a bunch of work here
MySqlCommand unlock_tables = new MySqlCommand(Queries.unlockTables(), connection);
unlock_tables.ExecuteNonQuery();
}
catch (MySqlException mex)
{
// Mostly error logging here
}
finally
{
connection.Close();
}
Pretty simple stuff. Everything works fine and dandy assuming nothing goes wrong. That's a terrible assumption to make, though, so I deliberately set up a situation where things would foul up in the middle and move to the finally block.
The result was that my table locks remained until I closed the app, which I learned by trying to access the tables with a different client once the method completed. Needless to say this isn't my intention, especially since there's another app that's supposed to access those tables once I'm done with them.
I could quickly fix the problem by explicitly releasing the locks before closing the connection, but I'm still left curious about some things. Everything I've read before has sworn that closing a connection should implicitly release the table locks. Obviously in this case it isn't. Why is that? Does connection.Close() not actually completely close the connection? Is there a better way I should be closing my connections?
Try wrapping your Connection and MySqlCommand instance in a using statement. That will release the objects as soon as it leaves the brackets.
using(MySqlConnection conn = new MySqlConnection(connStr))
{
conn.Open();
using(MySqlCommand command = new MySqlCommand("command to execute",conn))
{
//Code here..
}
}
I would like to give a user the ability to cancel a running query. The query is really slow. (Query optimization is besides the point.) This is mainly out of my curiosity.
MSDN says:
If there is nothing to cancel, nothing occurs. However, if there is a
command in process, and the attempt to cancel fails, no exception is
generated.
Cmd - SqlCommand
DA - DataAdapter
Conn - SqlConnection
CurrentSearch - Thread
LongQuery - Singleton
Here's what I have:
var t = new Thread(AbortThread);
t.Start();
void AbortThread()
{
LongQuery.Current.Cmd.Cancel();
LongQuery.Current.Cmd.Dispose();
LongQuery.Current.DA.Dispose();
LongQuery.Current.Conn.Close();
LongQuery.Current.Conn.Dispose();
LongQuery.Current.Cmd = null;
LongQuery.Current.DA = null;
LongQuery.Current.Conn = null;
CurrentSearch.Abort();
CurrentSearch.Join();
CurrentSearch = null;
}
I noticed that CurrentSearch.Abort() was blocking, that's why I wrapped it in a thread, which probably means that the thread is still working.
Finally, is there anything else than this that I can do to cancel a query? Is it actually possible to cancel such a long query from .NET?
IF you really absolutely want to kill it for good use this approach:
store away the session ID right before starting the long-running query by calling SELECT ##SPID AS 'SESSIONID' on the same connection
When you want to kill it:
Open a new DB connection
issue a KILL command for that session ID
BEWARE as the MSDN documentation states you need the permission ALTER ANY CONNECTION to do this
Yes, you can kill a process from .NET. Here is an example. Please note you will need proper permissions and you have to figure out the process in question. I don't have a quick sample of determining which process your query is running under.
You example aborts the thread, but that does not mean the work on SQL Server was terminated. If you think about it this way: when you go through a bad cell zone and the call drops, if you mom/wife/friend was droning on, do they instantly stop talking? That is an analogy of aborting the thread, at least in the case of working with a database server.
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.