A long-running application uses the MS C OBDC API to create and use SQL connections to an Oracle DB. The application was originally designed to establish an ODBC connection at startup and keep that connection indefinitely as the application runs, potentially for weeks or months.
We're seeing very infrequent cases where a connection suddenly dies and I wondered if that's because we're using them wrong, or if it's considered OK to hold a connection like this. Can anyone point me to some definitive information on the subject?
I'm not sure there's definitive information regarding this, but with long-running programs you always have to be prepared for this kind of incidents, they just happen (and not only with db connections but also with sockets that remain open for extended periods). I have no experience with Oracle, but I have a very similar setup with Informix, and this is (in pseudocode) what we do
while (programissupposedtorun) {
opendb();
do {
youractivities();
} while(dbisok);
closedbandcleanup();
}
As long as you are able to correctly detect that the connection died and are able to resume processing without losing data you should be OK.
I'm not familiar with ODBC but such use cases are best handled with a connection pool. Your application can simply request a connection from the pool when it has some work and release it as soon as its done -- the pool will take care of actually (re)connecting to the database.
A quick search for ODBC connection pooling brought this up: Driver Manager Connection Pooling
Related
I'm coding in Java EE and I have some class that manage all the actions with my DB.
I was asking myself when should I open/close the connection with the DB.
Is it better to open and close it in each method ?
Or is it better to open it in the constructor and close it when I finished using my class ?
Thx
there is no generic solution. all decisions depends of concrete task. you should remember next things:
every connect to the database is the application time. if your method will be called too often, your application will waste lot of time to connect and disconnect tasks and much more with the slow network. in case of rarely calls, it will not so important;
if your method connects to database in the constructor for the long time without any operations the connection may be dropped. this is not neccesary, but may cause of the network issues or database connections policy. so before every query should be checked with fast and simple operation like select 'some random text' from dual;
database resources is not infinite and total number of connections is limited. this limit can be very large, but it still exist. so, if your application can be used in parallel several (hungred, thousands) times, it may reach the limits with permanent connections.
if you have no information of future usage of method, I advise use time limited permament connection. it should be opened with first query and closed with timer if method did no queries through this connection for some time like 3-5 seconds. sure, any querier should check connection status before query. open it, if it closed, with touching the closing timer. and don't forget explicitly close connection at destructor.
I would argue that you shouldn't be managing it in code. Every EE server I've worked on has connection pooling to remove the need for you to care about this. Basically you "open" the connection when you need and "close" it when you need. Those words are in quotes because it is up to the pool to manage when a connection is truly opened and closed.
From a design perspective then use the connection only when you need it. Object construction doesn't make sense - what if a method in the class doesn't get called for an hour? What is the purpose of having it open when you don't need it? So if a method needs a connection open and close it in the method.
I am a DBA, not a developer, and could use some insight. The development staff is using VB.NET to create web based applications with connections to a DB2 database. Please assume that the connection string in the web.config file is coded correctly.
I can see the number of orphaned connections from the web servers grow over time. By orphaned I mean that there is no activity associated with the connection for hours, yet I can see other connections being created and destroyed every couple of seconds.
I suspect that the connections are not being properly closed, but there are two different groups looking at the problem and so far haven't turned up anything. By the end of the day I can have hundreds of these connections - all of which are cleared when the application pool is reset every night. (This means it is NOT a database problem)
Is there a coding technique to ensure a connection is closed using vb.net on IIS v7+?
Am I seeing the symptom of another problem on IIS?
You need to have the developers implement the Dispose pattern, which is facilitated by the Using statement in VB.NET, like this (this is pertinent to SQL Server, but whatever connection object you are using for DB2 should work as well):
Using theConnection As New SqlConnection()
'' Command, parameter logic goes here
End Using
By wrapping the connection object inside of a Using block, it will guarantee that the connection is closed and properly disposed of memory-wise, even if there is an exception in the code within the Using block.
Sounds like a code-review is in order for whoever is in charge of the developers.
I am considering using Fluent NHibernate for a new application with SQL Server 2008 and I am having trouble understanding the connection handling behavior I am seeing.
I am monitoring connections using sp_who2 and here is what I see:
When the SessionFactory is created, a single connection is opened. This connection
seems to stay open until the app is killed.
No connection is opened when a new session is opened. (That's ok, I understand NHibernate waits until the last moment to create database connections).
No new connection is opened even when I run a query through NHibernate. I must assume it is using the connection created when the SessionFactory was created, which is still open. I set a breakpoint after the query (before the session was closed), and no new sessions had appeared in sp_who.
Running an entire app through a single connection is not acceptable (obviously). How can I ensure each ISession gets its own connection? I'm sure there's something obvious I'm missing here...
Thanks in advance.
The behaviour that you see is nothing NHibernate specific - Connection pooling is default behaviour in SQL Server.
Even if it may sound awkward at first glance, it actually is a good thing because creating new connections and maintaining them is expensive.
(for more information, see the Wikipedia article about connection pooling)
So there is no need to try to get NH to open a new connection for each session, because reusing existing connections actually improves SQL Server performance.
I have to work with a Project made by another developer. A project Win-Form with Visual-Basic code, with MS-Access as db and some OleDbConnections. There is a bug: sometimes the application can't open the OleDbConnection because the max number of connections has been reached on the db. I know the best way to use the connections is this:
Using cn As New OleDbConnction(s)
...
cn.Close()
End Using
But in the project there are many classes to work with the db, and in many of these classes there are OleDbConnections with "Friend" visibility, that are opened and closed in different times. For this reason it's impossible to put all the OleDbConnections in a Using construct, and it's very very hard to find what operation "forgets" to close one of these OleDbConnection.
A possible solution could be to use only one unique public OleDbConnection, and to check, before opening it, if it isn't already opened.
But someone have told me it's a very bad practice. I suppose he told me this about the performance, but I don't know it exactly.
Can you tell me why one unique public OleDbConnection is so deprecated?
Have you got, for me, an "easy" solution for my problem?
Thank you,
Pileggi
From your description, I see a couple of possible issues that could result in your problem:
nested connections:
You open multiple connections within each-other
open/release connections too fast:
As David-W-Fenton mentionned, with access, every time you open/close a single connection, the lock file will be created/removed. This operation is quite slow and if you quickly open/close the database within you application (execute lots of atomic queries), you may get this issue.
A few possible ways to investigate and solve the issue:
Trace all open/close calls
Add some debug traces that show every time you open and close a connection.
It will allow you to detect nested connections and where your connection pool is being wasted.
Force connection polling
An easy 'fix' may be to explicitely set connection pooling in your connection string. It should be the default behaviour, so maybe it won't do anything to solve your problem, but it's so simple that there is no reason not to try it:
OLE DB Services=-1
Use a connection manager class to create/release connections for you.
Replace all the explicit creations of new OleDbConnection and close operations by your own code.
This would allow you to always re-use a single existing connection throughout your application and allow you to quickly make tweaks for the whole of your app by centralising the behaviour in a single place.
So why holding a single connection is generally deprecated?
Generally, you should not keep connections open throughout your application as they force the database server to keep resources available for you, and it decreases the number of client that can connect (there is always a limited number of connections available).
For Access though -a file-based database without server part- keeping a single connection open is actually preferable because of the delay associated with opening new connections (creation of the lock file). Since Access is not meant to be used with a large number of concurrent users, the resource cost of keeping the connection open is not significant enough to be an issue.
From simple tests, it can be shown that keeping a connection always open allows subsequent connections to open about 10x faster!
The OleDb driver does connection pooling for you, so it is able to re-use connections when they are freed.
By keeping your connections and database operations small and contained, you would be less likely to run into concurrency issues when using threads. Keeping a global connection may become an issue if you are executing multiple operations using the same pipeline to the database.
Just adding some information that works for years successfully for me (it is somewhat similar to what David-W-Fenton suggests)
First, an OleDbConnection to Microsoft Access (MDB, JET) is not using connection pooling. As Microsoft states in KB191572:
Connections that use the Jet OLE DB providers and ODBC drivers are not
pooled because those providers and drivers do not support pooling.
Regarding connection pooling, there is also this blog post from Ivan Mitev that states:
So what does this mean? It is apparent that that the presence of an
actively opened connection made the test with multiple connection
closing and opening finish a lot faster (2-3 times). The only possible
explanation for me is that the connection pool is released each time
there are no active connections. I have to make further investigations
and read something like Pooling in the Microsoft Data Access
Components. Or maybe hold a single opened connection just for the sake
of keeping the pool alive. This would be ugly, but still it is a good
enough workaround! If anyone has a better idea, please share it.
And Microsoft notes in MSDN:
The ADO Connection object implicitly uses IDataInitialize. However,
this means your application needs to keep at least one instance of a
Connection object instantiated for each unique user—at all times.
Otherwise, the pool will be destroyed when the last Connection object
for that string is closed.
Based on all this and my own tests, my solution to "simulate" connection pooling even with Microsoft Access databases roughly follows these steps:
Open one OleDbConnection to the Access database as early as possible in application lifecycle.
Do your normal SQL queries, disposing OleDbConnections as early as possible, just like recommended.
Dispose that one always-open OleDbConnection as late as possible in application lifecycle.
This sped up my applications (mostly WinForms) tremendously.
Please note that this also works for Sqlite which seems to not support connection pooling, too.
this question is related with this one.
The problem I'm having is that our application has a bug and it leaves an unclosed transaction at SQL. I know for sure that the connection has been released with SQLFreeHandle but looks like somehow ODBC maintains the connection open to reuse it latter. As the connection remains open the transaction is not rolled back and I'm having a lock.
Is there a way to force ODBC to shutdown the connection and to close the underlying socket?
Thanks in advance.
It sounds like connection pooling is turned on. This article describes how it can be turned off at the driver level.
As Mark said, connection pooling is turned on.
Typically the wrappers for the sql drivers make a rollback call prior to releasing the connection back to the pool.
If you guys have written your own ODBC driver (or wrapper for it), you might consider just modifying the connection dispose/close/release code to do that rollback if there was an open transaction.