Throughout the program which I am currently working on, I realized that whenever I need to access to SQL Server, I just type a queryString and execute my query using SqlCommand and SqlConnection in C#. At a certain point during the program, I even opened a connection and ran a query in a "for loop".
Is it unhealthy for the program to constantly open-close connections?
***I am not very familiar with the terminology, therefore you might be having some problems understanding what I am talking about:
Is doing this very frequently may cause any problem?:
string queryString = "Some SQL Query";
public void(){
SqlConnection con = new Connection(conString);
SqlCommand cmd = new SqlCommand(queryString,con);
cmd.Parameters.AddWithValue("#SomeParam",someValue);
con.Open();
cmd.ExecuteNonQuery();
con.Close();
}
I use this template almost every class I create,(usually to get,update,insert data from/into a datatable).
Well, is it harmful?
The short answer is yes - it is inefficient to constantly open and close connections. Opening an actual connection is a very expensive process and managing a connection for the lifetime of its need (which usually is the lifetime of the application or process using it) is fraught with errors.
That is why connection pooling was introduced a long time ago. There is a layer beneath your application that will manage the physical opening/closing of connections in a more efficient way. This also helps prevent the chances that an open connection is lost and continues to stay open (which causes lots of problems). By default pooling is enabled so you don't need to do anything to use it.
With pooling - you write code to open a connection and use it for the duration of a particular section of code and then close it. If the connection pool has an open but unused connection, it will reuse it rather than open a new one. When you close the connection, that simply returns the connection to the pool and makes it available to the next open attempt. You should also get familiar with the c# using statement.
Related
I have an application that connect to a SQL Server database with high frequency. Inside this service, there are many scheduled tasks that run every second, and each time I'm executing some query.
I don't understand which solution is better in this condition.
Opening a single SqlConnection and keeping it open while application is running and execute all query with that connection
Each time I want to execute query, opening a new connection and after query execution, close the connection (does this solution suitable for so many scheduled task that runs every 1 second?)
I tried second solution, but is there any better choice?
How do ORMs like EF manage connections?
As you see i have many service. I cant change interval and the interval is important for me. but the code makes so many calls and im following a better way manage connection over database. Also I'm making connection with Using Statement.
Is there any better solution?
you should use SQL Connection Pool feature for that.
It automatically manages in the background if a connection needs to be open or can be reused.
Documentation: https://learn.microsoft.com/en-us/dotnet/framework/data/adonet/sql-server-connection-pooling?source=recommendations
Example copied from that page
using (SqlConnection connection = new SqlConnection(
"Integrated Security=SSPI;Initial Catalog=Northwind"))
{
connection.Open();
// Pool A is created.
}
using (SqlConnection connection = new SqlConnection(
"Integrated Security=SSPI;Initial Catalog=pubs"))
{
connection.Open();
// Pool B is created because the connection strings differ.
}
using (SqlConnection connection = new SqlConnection(
"Integrated Security=SSPI;Initial Catalog=Northwind"))
{
connection.Open();
// The connection string matches pool A.
}
By using the "using" statement, application checks if a connection in this pool can be reused before opening a new connection. So the overhead of opening and closing the connections disappears.
But after your last edit you seem to have other problems in your current architecture. Like the other poster recommends you can try to use the "with (nolock)" parameter in your sql statements. It creates dirty reads, but maybe that's ok for your application.
Alternatively if all your services use the same select statement maybe a stored procedure or a caching mechanism could help.
I assume that you are already opening/closing your SQL connections in either a "using" statement or explicitly in your code ( try/catch/finally ). If so you are already making use of connection pooling as it is enabled in ADO.Net by default ("By default, connection pooling is enabled in ADO.NET").
Therefore I don't think that your problem is so much a connection/resource problem as it is a database concurrency issue. I assume it to be either 1 of 2 issues :
Your code is making so many calls to the SQL server that it is exhausting all the available connections and nobody else can get one
Your code is locking tables in SQL that is causing other code/applications to timeout
If it is case 1, try and redesign your code to be "less chatty" to the database. Instead of making several inserts/updates per second, perhaps buffer the changes and make a single insert/update every 3-5 seconds in batch mode ( obvs if possible ). Or maybe your SQL statements are taking longer than 1 second to execute and you are calling them every second causing in a backlog scenario?
If it is case 2, try and redesign the SQL tables in such a way that the "reading" applications are not influenced by the "writing" application. Normally this involves a service that periodically writes aggregated data to a read-only table for viewing or at very least adding a "WITH(NOLOCK)" hint to the select clauses to allow dirty reads ( i.e. it wont lock the table to read, but may result in slightly out of date dataset i.e. eventual consistency )
Good luck
I have my business-logic implemented in simple static classes with static methods. Each of these methods opens/closes SQL connection when called:
public static void DoSomething()
{
using (SqlConnection connection = new SqlConnection("..."))
{
connection.Open();
// ...
connection.Close();
}
}
But I think passing the connection object around and avoiding opening and closing a connection saves performance. I made some tests long time ago with OleDbConnection class (not sure about SqlConnection), and it definitely helped to work like this (as far as I remember):
//pass around the connection object into the method
public static void DoSomething(SqlConnection connection)
{
bool openConn = (connection.State == ConnectionState.Open);
if (!openConn)
{
connection.Open();
}
// ....
if (openConn)
{
connection.Close();
}
}
So the question is - should I choose the method (a) or method (b) ? I read in another stackoverflow question that connection pooling saved performance for me, I don't have to bother at all...
PS. It's an ASP.NET app - connections exist only during a web-request. Not a win-app or service.
Stick to option a.
The connection pooling is your friend.
Use Method (a), every time. When you start scaling your application, the logic that deals with the state will become a real pain if you do not.
Connection pooling does what it says on the tin. Just think of what happens when the application scales, and how hard would it be to manually manage the connection open/close state. The connection pool does a fine job of automatically handling this. If you're worried about performance think about some sort of memory cache mechanism so that nothing gets blocked.
Always close connections as soon as you are done with them, so they underlying database connection can go back into the pool and be available for other callers. Connection pooling is pretty well optimised, so there's no noticeable penalty for doing so. The advice is basically the same as for transactions - keep them short and close when you're done.
It gets more complicated if you're running into MSDTC issues by using a single transaction around code that uses multiple connections, in which case you actually do have to share the connection object and only close it once the transaction is done with.
However you're doing things by hand here, so you might want to investigate tools that manage connections for you, like DataSets, Linq to SQL, Entity Framework or NHibernate.
Disclaimer: I know this is old, but I found an easy way to demonstrate this fact, so I'm putting in my two cents worth.
If you're having trouble believing that the pooling is really going to be faster, then give this a try:
Add the following somewhere:
using System.Diagnostics;
public static class TestExtensions
{
public static void TimedOpen(this SqlConnection conn)
{
Stopwatch sw = Stopwatch.StartNew();
conn.Open();
Console.WriteLine(sw.Elapsed);
}
}
Now replace all calls to Open() with TimedOpen() and run your program. Now, for each distinct connection string you have, the console (output) window will have a single long running open, and a bunch of very fast opens.
If you want to label them you can add new StackTrace(true).GetFrame(1) + to the call to WriteLine.
There are distinctions between physical and logical connections. DbConnection is a kind of logical connection and it uses underlying physical connection to Oracle. Closing/opening DbConnection doesn't affect your performance, but makes your code clean and stable - connection leaks are impossible in this case.
Also you should remember about cases when there are limitations for parallel connections on db server - taking that into account it is necessary to make your connections very short.
Connection pool frees you from connection state checking - just open, use and immediately close them.
Normally you should keep one connect for each transaction(no parallel computes)
e.g when user execute charge action, your application need find user's balance first and update it, they should use same connection.
Even if ado.net has its connection pool, dispatching connection cost is very low, but reuse connection is more better choice.
Why not keep only one connection in application
Because the connection is blocking when you execute some query or command,
so that means your application is only doing one db operation at sametime,
how poor performance it is.
One more issue is that your application will always have a connection even though your user is just open it but no operations.If there are many user open your application, db server will cost all of its connection source in soon while your users have not did anything.
I read that .NET uses connection pooling.
For example, if I instantiate a bunch of SqlConnection objects with the same connection string, then internally .NET will know to use the same connection.
Is this correct?
Also, in a big web-based application, any tips on the best way to harness this "power" ?
Setting up the TCP connection between your Web application and SQL Server can be an expensive operation. Connection pooling allows connections to the database to be reused for subsequent data requests. Rather than setting up a new TCP connection on each request, a new connection is set up only when one is not available in the connection pool. When the connection is closed, it is returned to the pool where it remains connected to the database, as opposed to completely tearing down that TCP connection.
Always close your connections when you're finished with them. No matter what anyone says about garbage collection within the Microsoft .NET Framework, always call Close or Dispose explicitly on your connection when you are finished with it. Do not trust the common language runtime (CLR) to clean up and close your connection for you. The CLR will eventually destroy the class and force the connection closed, but you have no guarantee when the garbage collection on the object will actually happen.
To use connection pooling optimally, there are a couple of rules to live by. First, open the connection, do the work, and then close the connection. It's okay to open and close the connection multiple times on each request if you have to, rather than keeping the connection open and passing it around through different methods. Second, use the same connection string (and the same thread identity if you're using integrated authentication). If you don't use the same connection string, for example customizing the connection string based on the logged-in user, you won't get the same optimization value provided by connection pooling. And if you use integrated authentication while impersonating a large set of users, your pooling will also be much less effective.
The .NET CLR data performance counters can be very useful when attempting to track down any performance issues that are related to connection pooling.
http://msdn.microsoft.com/en-us/magazine/cc163854.aspx
If you use the following syntax, when ever the using block is left the dispose method will be called, even if an exception occurs.
using(SqlConnection connection = new SqlConnection())
{
// Work with connection object here.
}
//connection object gets disposed here.
not sure if this is entirely related, but I just took over a project and noticed the original programming team failed to do something very important.
when you have a SQLConnection, let's call it conn and you do this:
conn.Open();
and then you perform some SQL statement, be it a select, insert or update. it is entirely possible that it will fail. So of course, you should do this:
try { conn.Open() }
catch (SqlException ex)
{
//do your logging/exception handling
}
however, people forget to add the Finally block.
finally {
if (conn.State == System.Data.ConnectionState.Open)
conn.Close();
}
you want to make sure if you have an exception that the connection does not stay open, so make sure you close it.
I am using multiple queries to pull data from the same server in my application. The issue is that I have to open a new connection every time I have a new query.
Is it even possible to:
Open the connection
Run query
Pull results
Run another query
Pull another result
Run final query
Pull another result
Close connection.
Although you may not yet know it, you are doing it correctly.
Open the connection, do your query, close it. Preferably using a using block or try/finally.
This may sound like a lot of overhead, but the connection pool in the .NET Framework Data Provider for SQL Server will actually optimize this for you.
In fact closing the connection is recommended.
Here is a quote from the documentation:
It is recommended that you always
close the Connection when you are
finished using it in order for the
connection to be returned to the pool.
This can be done using either the
Close or Dispose methods of the
Connection object. Connections that
are not explicitly closed might not be
added or returned to the pool. For
example, a connection that has gone
out of scope but that has not been
explicitly closed will only be
returned to the connection pool if the
maximum pool size has been reached and
the connection is still valid.
Here is an example of some code that does this:
try {
conn.Open();
// Perform query here
} finally {
conn.Close();
}
For reference:
http://msdn.microsoft.com/en-us/library/8xx3tyca(VS.71).aspx
If you are using ASP.NET with the same connection string you will be using a pooled connection that may never get physically closed, so you will pretty much always use an available open connection.
It's very possible. Assuming that you are talking about Connection and a DataReader. If you have to create a different connection every time, it sound like something is going wrong.
Without seeing any code, I am guessing that you are leaving the DataReader open. This is a BIG mistake. By default DataReaders completely consume the connection and leaving it unclosed can lead leaks. Close the DataReader, then execute another. I'd recommend wrapping the DataReader in a using block.
Rob
Short answer: Yes. This should be possible with most data providers.
Long answer: It depends on what you are using for your data access. However, you probably do not need to worry about it. Many data provider frameworks have connection pooling built in, so the subsequent connection creation/opening shouldn't "really" open a connection.
Sure, if you're using a SqlConnection object you can just do something like this:
connection.Open();
cmd.ExecuteReader(); // or any other form of getting the data
cmd2.ExecuteReader();
.
.
.
.
connection.Close();
I'd also like to add, if you're using a few SqlDataAdapters for your queries, although you normally don't need to open the connection by yourself, if you DO explicitly call connection.Open() it then won't close the connection for you automatically, allowing you to execute multiple queries with only one connection.
If you are using C# to open a connection. use using statement will help you clean up the resource/connection even if there is some excepion throwing out.
using (SqlConnection connection =
new SqlConnection(connectionString)
{
connection.Open();
//issue command
}
And read this:
http://msdn.microsoft.com/en-us/library/8xx3tyca(VS.71).aspx, you can "Controlling Connection Pooling with Connection String Keywords", and the system will handle pooling for you.
The following code executes a simple insert command. If it is called 2,000 times consecutively (to insert 2,000 rows) an OleDbException with message = "System Resources Exceeded" is thrown. Is there something else I should be doing to free up resources?
using (OleDbConnection conn = new OleDbConnection(connectionString))
using (OleDbCommand cmd = new OleDbCommand(commandText, conn))
{
conn.Open();
cmd.ExecuteNonQuery();
}
The system resources exceeded error is not coming from the managed code, its coming from you killing your database (JET?)
You are opening way too many connections, way too fast...
Some tips:
Avoid round trips by not opening a new connection for every single command, and perform the inserts using a single connection.
Ensure that database connection pooling is working. (Not sure if that works with OLEDB connections.)
Consider using a more optimized way to insert the data.
Have you tried this?
using (OleDBConnection conn = new OleDBConnection(connstr))
{
while (IHaveData)
{
using (OldDBCommand cmd = new OldDBCommand())
{
cmd.Connection = conn;
cmd.ExecuteScalar();
}
}
}
I tested this code out with an Access 2007 database with no exceptions (I went as high as 13000 inserts).
However, what I noticed is that it is terribly slow as you are creating a connection every time. If you put the "using(connection)" outside the loop, it goes much faster.
In addition to the above (connecting to the database only once), I would also like to make sure you're closing and disposing of your connections. As most objects in c# are managed wrt memory, connections and streams don't have this luxury always, so if objects like this aren't disposed of, they are not guaranteed to be cleaned up. This has the added effect of leaving that connection open for the life of your program.
Also, if possible, I'd look into using Transactions. I can't tell what you're using this code for, but OleDbTransactions are useful when inserting and updating many rows in a database.
I am not sure about the specifics but I have ran across a similar problem. We utilize an Access database with IIS to serve our clients. We do not have very many clients but there are alot of connections being opened and closed during a single session. After about a week of work, we recieve the same error and all connection attempts fail. To correct the problem, all we had to do was restart the worker processes.
After some research, I found (of course) that Access does not perform well in this environment. Resources do not get released correctly and over time the executable will run out. To solve this problem, we are going to move to an Oracle database. If this does not fix the problem, I will keep you updated on my findings.
This could be occurring because you are not disposing the Connection and Command object created. Always Dispose the object at the end.
OledbCommand.Dispose();