I've written some code to write some data to SQL Server, using Dapper. I don't need to wait for this write to complete before continuing other work, so want to use Task.Run() to make this asynchronous.
I have (using) statements for calling this in the rest of my system:
using (IDataAccess ida = new DAL())
{
ida.WriteMessageToDB(id, routingKey, msgBody);
}
My DAL will automatically check the dbConnection.State when the using statement is ran, and attempt a simple fix if it's closed. This works just fine for any non-async/TPL select calls.
However, when I throw a load of writes at the same time, the Task.Run() code was falling over as the connection was closed for some of them - essentially I think the parallel nature of the code meant the state was being closed by other tasks.
I 'fixed' this by doing a check to open the Connection.State within the Task.Run() code, and this appears to have 'solved' the problem. Like so:
Task.Run(() =>
{
if (dbConnection.State == ConnectionState.Closed)
{
dbConnection.Open();
}
if (dbConnection.State == ConnectionState.Open)
{
*Dapper SQL String and Execute Commands*
}
});
When I run SELECT * FROM sys.dm_exec_connections from SSMS after this, I see a lot more connections. To be expected?
Now as I understand it:
Dapper doesn't deal with connection pooling
SQL Server should automatically deal with connection pooling?
Is there anything wrong with this solution? Or a better way of doing it? I'd like to use connection pooling for obvious reasons, and as painlessly as possible.
Thanks in advance.
Thanks Juharr - I've upvoted your reply.
For reference to others, I changed write function to await and Dapper async:
private async Task WriteMessageToDB(Guid id, string tableName, string jsonString)
{
string sql = *Redacted*
await dbConnection.ExecuteScalarAsync<int>(sql, new { ID = id, Body = jsonString });
}
And then created a new task in the caller that monitors the outcome.
This is working consistently under load, and not seeing excessive new connections being created either.
Related
I'm trying to figure out the best way to batch insert about 37k rows into my Sql Server using DAPPER.
My problem is that when I use Parallel.ForEach - the number of connections to the database increases over a short period of time - finally hitting nearly or about 100 ... which gives connection pool errors. If I force the max degree of parall then it's hit that max number and stays there.
Setting the maxdegree feels wrong.
It currently is doing about 10-20 inserts a second. This is also in a simple Console App - so there's no other database activity besides what's happening in my Parallel.ForEach loop.
Is using Parallel.ForEach the incorrect thing in this case because this is not-CPU bound?
Should I be using async/await ? If so, what stopping this from doing hundreds of db calls in one go?
Sample code which is basically what I'm doing.
var items = GetItemsFromSomewhere(); // Returns 37K items.
Parallel.ForEach(items => item)
{
using (var sqlConnection = new SqlConnection(_connectionString))
{
var result = sqlConnection.Execute(myQuery, new { ... } );
}
}
My (incorrect) understanding of this was that there should on be about 8 or so connections at any time to the db. The Connection Pool will release the connection (which remains instantiated in the Connection Pool, waiting to be used). And if the Execute takes .. i donno .. lets say even a 1 second (the longest running time for an insert was about 500ms .. and that's 1 in every 100 or so) ... that's ok .. that thread is blocked and chills until the Execute completes. Then the scope completes (and Dispose is auto called) and the connection closed. With the connection closed, the Parallel.ForEach then grabs the next item in the collection, goes to the connection pool and then grabs a spare connection (remember - we just closed one, a split second ago) ... rinse.repeat.
Is this wrong?
Notes:
.NET 4.5
Sql 2012
Console app.
Using Dapper.NET for sql code.
First of all: If it is about performance, use SqlBulkCopy. This works with SQL-Server. If you are using other database servers, they might have their own SqlBulkCopy-solution (Oracle has one).
SqlBulkCopy works like a bulk-select: One state opens one connection and streams all the data from the server to the client. With an insert, it works the other way arround: It streams all the new records from the client to the server.
See: https://msdn.microsoft.com/en-us/library/ex21zs8x(v=vs.110).aspx
If you insist of using parallellism, you might want to consider the follow code:
void BulkInsert<T>(object p)
{
IEnumerator<T> e = (IEnumerator<T>)p;
using (var sqlConnection = new SqlConnection(_connectionString))
{
while(true)
{
T item;
lock(e)
{
if (!e.MoveNext())
return;
item = e.Current;
}
var result = sqlConnection.Execute(myQuery, new { ... } );
}
}
}
Now create your own threads and invoke this method on these threads with one and the same parameter: The iterator which runs through your collection. Each threat opens its own connection once, starts inserting, and after all items are inserted, the connection is closed. This solutions uses as many connections as your created threads.
PS: Multiple variants of above code are possible . You could call it from background threads, from Tasks, etc. I hope you get the point.
You should use SqlBulkCopy instead of inserting one by one. Faster and more efficient.
https://msdn.microsoft.com/en-us/library/ex21zs8x(v=vs.110).aspx
credits to the answer owner
Sql Bulk Copy/Insert in C#
I am trying to write a background process that checks to see if a database is broken and I am a little bit confused on what exactly would constitute as "broken".
Looking at the official documentation found here on the Microsoft Developers Network for ConnectionState there is a member entitled "broken". At what point would this member result to true, or how exactly would it be used?
This is currently how I am checking if the DB is broken:
public bool DatabaseConnection()
{
bool statusUp = true;
using (var databaseConnection = new SqlConnection(ConfigData.ConnectionStrings.DatabaseConnectionString))
{
try
{
databaseConnection.Open()
}
catch (SqlException ex)
{
const string message = "Could not establish a connection with Database.";
Log.DatabaseStatusDown(message, ex);
statusUp = false;
}
finally { databaseConnection.Close(); }
}
return statusUp
}
I know Using statement leverages the IDisposable class and the connection will be disposed of but I am extra paranoid. Is this efficient? If not, would a more efficient way to determine if my connection is broken would be to do something like this?
public bool DatabaseConnection()
{
using (var databaseConnection = new SqlConnection(ConfigData.ConnectionStrings.DatabaseConnectionString))
{
return databaseConnection.State == ConnectionState.Broken;
}
}
I will be running this process every two minutes, and something tells me the first method I outlined will not be efficient. Would the second method work to determine if my DB is broken? What exactly does Microsoft define as broken for this particular enum?
I wouldn't use ConnectionState.Broken. It is reserved for future use.
The first technique is actually pretty lightweight. All it does is get a connection from the connection pool, which is held locally. Disposing the connection will return the connection to the pool for use by other processes.
I would perhaps consider actually sending a command to the SQL Server, e.g. "SELECT 'ping'" or something lightweight. If you don't get a resultset back it indicates that your SQL Server couldn't service the request for whatever reason.
Not an expert, but I believe broken would indicate that an already connected database connection had an unrecoverable connection issue (Server closed it, etc). It wouldn't be very reliable, and then possibly only detectable after a failed attempt to do something.
It doesn't make sense to check for Broken on a connection you just made. It's only useful for a long-living connection - and it basically tells you to reopen the connection.
It doesn't tell you anything about the state of the database, or the database server. The only thing it tells you is whether the concrete connection is working or not.
If you're always creating new connections, the only thing you care about is whether connection.Open throws an exception or not. And of course, the ExecuteXXX methods etc. - the connection can drop at any point.
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 have a ASP.net based application.
The CPU on the SQL Server box is constantly ~90 - 100%
There are a lot of inneficient queries, which I am currently working on, however, looking at the code from a previous coder, he never seemed to close (or dispose) the SqlConnection
When I run the folloing query, I get around 450 connections that are "Awaiting Command"
SELECT Count(*) FROM
MASTER.DBO.SYSPROCESSES WHERE
DB_NAME(DBID) = 'CroCMS' AND DBID != 0
AND cmd = 'AWAITING COMMAND'
Is this likely to be causing a problem?
I read this and it seems to relate:
http://www.pythian.com/news/1270/sql-server-understanding-and-controlling-connection-pooling-fragmentation/
We are also getting a lot of timeouts, specifically when replication is enabled..
I'm not sure if this is related.. Have disabled replication (transactional) for now and it seems ok..
(This server is a subscriber to our in office Database server)
Would disposing of the SQL connection object help?
Yes, dispose them. Otherwise ignore them for now. Possibly the pool is as large because the statements are slow. I would more suggest:
Fixing the statements.
Check the applicaion that it only uses one connection PER REQUEST (i.e. not open multiple at the same time).
If the problem does not get better after optiomizing SQL - you can revisit the pool.
You should always dispose the command object when your done with it. that way the connection pooling can be used better.
easist is to use the using statment.
using (
var sqlCommand = new SqlCommand(
"storedprocname",
new SqlConnection("connectionstring"))
{ CommandType = CommandType.StoredProcedure })
{
// do what you should.. setting params executing etc etc.
}
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.