Just wondering if there is a way that I can close all SQL connections and commands in the cleanest possible way when a connection is lost in VB.NET (can also be in C#.NET).
What I'm trying to achieve:
Using the System.Net.NetworkInformation.NetworkAvailabilityChanged event, I'm monitoring whether the connection has been lost. If it has been lost, I'm disabling all user input - works great. Network comes back on, UI is enabled. (thumbs up)
Now, however, comes my predicament. If an SQL query is executing before the connection drops and then the network is lost, then the query returns a null value as expected, however if that was mid population of datatables/fields, then I get NullReferenceExceptions.
My question is:
Is there anyway to cleanly exit a sub after the connection has dropped? I've tried Application.ExitThread, but that doesn't seem to quite cut it. Do I need to put dropped connection handlers within my objects, so that when the connection is dropped, the respective object won't return or try to assign null data?
Any help is greatly appreciated. Not asking for plain code, need explanations if at all possible. Cheers.
If you follow the "using" best practice, like:
Using cn As New SqlConnection(connectionString)
...
End Using
Then the compiler will generate code that cleans up the connection when an exception is thrown.
Related
We are developing a SQL Database, and client connects to server with rpc.
Think about a case: there're Transaction_A(TxnA) & Transaction_B(TxnB). TxnA may be like UPDATE tbl SET a=a+1 WHERE id=1 .
When client gets CONN_TIMEOUT exception, the reason might be server crashed or network issues. Therefore, TxnA may or maynot complete write. And meanwhile TxnB may write to the same table and update table meta (like advancing the sequence number or revision).
So it seems no way to check TxnA completed or not by the client. And the query may not be idempotent, such as the 'UPDATE a=a+1' example above. So it cannot be retried recklessly.
My question: Is there any solution to solve this issue? I'm not sure how other SQL system or storage system work with this issue. Try google some keywords, but fail to get the answers.
Thank you for any inspiration.
I'm actually programming a quite big application and I was wondering when I should open/close a database connection.
My first thought was to open it at the beginning and keep the same DB connection the whole time. But this approach seems to have risks of failing (not sure why tho).
My second idea was to open/close as soon as possible like mentioned here: Best Practice on initializing a Database Connection
but as mentionned by someone in comment: "For the database, opening a connection is a very expensive operation." So the question remains:
When do I open / close a DB connection?
For context:
My application has multiple workspaces, for example, one workspace to see information about users, another one to process them, and so on. I had the idea to open the DB connection when I open a new workspace but when using the application it is possible to stay a very long time on the same workspace, which would be as bad as my first thought. But opening / closing the DB connection after seeing the info of one client isn't the smartest idea either.
Should I use a time-based refresh of the DB connection? Every 5 minutes I close and reopen a connection?
I'm not sure how to do this properly.
I'm wondering is there a way to recognize the OfflineComamd is being executed or internal flag or something to represent this command has been passed or mark it has been executed successfully. I have issue in recognizing the command is passed or not with unstable internet. I keep retrieve the records from database and comparing each and every time to see this has been passed or not. But due to the flow of my application, I'm finding it very difficult to avoid duplicates.IS there any automatic process to make sure commands executed automatically or something else?
2nd question, I can use UITimer to check isOffline() to make sure internet is connected or not on the forms. Is there something equivalent on server page or where queries is written to see internet is disconnected or not. When the control moved to queries and internet is disconnected I see the dialog open from form page being frozen for unlimited time and will not end. I have to close and re-open the app to continue the synchronization process.At the same time I cannot set a timeout for dialog because I'm not sure how long it will take the complete the Synchronization process. Please advise.
Extending on the same topic but I have created a new issue just to give more clarity on my questions.
executeOfflineCommand skips a command while executing from storage on Android
There is no way to know if a connection will stay stable as it requires knowledge of the future. You can work like transaction services do where the server side processes an offline command as a transaction using the approach of 2-phase commit.
In this approach you have an algorithm similar to this:
Client sends command to server
Server returns a special unique ID for the command
Client asks server to perform the unique id
Server acknowledges that the command was performed
If the first 2 stages didn't complete you just do that again. The worst thing that could happen is some orphan commands on the server.
If the 3rd option didn't complete you just do it again. The server knows whether it processed the command and will just acknowledge it if it was already processed.
I am using c3p0 for my connection pooling. The ComboPooledDataSource I use is configured as below.
#Bean
public DataSource dataSource() {
ComboPooledDataSource dataSource = new ComboPooledDataSource();
dataSource.setUser("user");
dataSource.setDriverClass("oracle.jdbc.OracleDriver");
dataSource.setJdbcUrl("test");
dataSource.setPassword("test");
dataSource.setMinPoolSize("10");
dataSource.setMaxPoolSize("20");
dataSource.setMaxStatements("100");
return dataSource;
}
I am facing some issues with this. I get warnings that this might leak connections. Also the below error from time to time,
as all the connections are being used up.
java.sql.SQLException: Io exception: Got minus one from a read call
at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:112)
at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:146)
at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:255)
at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:387)
at oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:439)
at oracle.jdbc.driver.T4CConnection.<init>(T4CConnection.java:165)
at oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:35)
at oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:801)
at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:135)
at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection
(WrapperConnectionPoolDataSource.java:182)
at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection
(WrapperConnectionPoolDataSource.java:171)
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource
(C3P0PooledConnectionPool.java:137)
at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
And from the DB stat, could see almost 290 inactive connections. I am having around 8 applications deployed in two servers,
connecting to the same DB.
My queries are
How do I make sure th connections are closed and not to have these many inactive connections?
Would configuring idle time and timeout resolve this issue?
What would happen if the server is brought down/tomcat is shutdown, will the connections remain open?
Connections are mainly used during startup to load cache, so is there a way of not using these connections afterwards?
What should I do for the existing inactive connections?
Given maxPoolSize of 20 and eight deployments, you should expect to see up to 180 Connections, which may be inactive if the application has seen periods of traffic which has now subsided. You have configured nothing to encourage a fast scaling down of your pools -- set maxIdleTime and/or maxIdleTimeExcessConnections and/or maxConnectionAge.
You should probably tell Spring how to close the DataSource you've defined. Use #Bean(destroyMethodName="close") instead of #Bean alone above your dataSource() method.
You have not configured any sort of Connection testing, so even broken Connections might remain in the pool. Please see Simple Advice On Connection Testing.
If the issue were a Connection leak, clients would eventually hang indefinitely, as the pool would be out of Connections to check out, but would already have reached maxPoolSize, and so wouldn't be able to acquire more from the DBMS. Are you seeing clients hang like that?
The way you avoid Connection leaks is, post-Java7, to always acquire Connections from your DataSource via try-with-resources. That is, use...
try ( Connection conn = myDataSource.getConnection() ) {
...
}
rather than just calling getConnection() in a method that might throw an Exception or in a try block. If you are using an older version of Java, you need to use the robust resource cleanup idiom, that is, acquire the Connection in a try block and be sure that conn.close() is always closed in the finally block, regardless of any other failures in the finally block. If you are not working with the DataSource directly, but letting Spring utilities work with it, hopefully those utilities are doing the right thing. But you should post whatever warning you are receiving that warns you of potential Connection leaks!
If your application has little use for Connections after it has "warmed up", and you want to minimize the resource footprint, set minPoolSize to a very low number, and use maxIdleTime and/or maxIdleTimeExcessConnections and/or maxConnectionAge as above to ensure that the pool promptly scales down when Connections are no longer in demand. Alternatively you might close() the DataSource when you are done with its work, but you are probably leaving that to Spring.
We are using ASP.NET MVC with LINQ to SQL. We added some features and tested them all to perfection on our QA box. We are using Windows Server 2003 and SQL Server 2005. So when we pushed out changes to the Live web server we also used Red Gate SQL Compare to push new database changes to the LIVE database. We tested again between the few of us, no problems. Time for bed.
The morning comes and users are starting to hit the app, and BOOM. We have no idea why this would happen as we have not been doing any new types of code things that we were not doing before. However we did notice that during the SQL Compare sync the names of all the foreign keys were different between the two databases, not the IDs in the tables, FK_AssetAsset_A0EB67 to FK_AssetAsset_B67EF8 (for example, don't remember the exact number of trailing mixed characters during the SQL Compare), we are not sure why but that is another variable in this problem.
Strangely once this was all pushed out we could then replicate the errors on QA, but not before everything was pushed to LIVE.
QA and LIVE databases are on the same SQL Server, but the apps are on different instances of Windows Server 2003.
Errors generated:
Index was outside the bounds of the array.
Invalid attempt to call FieldCount when reader is closed.
Server failed to resume the transaction.
There is already an open DataReader associated with this Command which must be closed first.
A transport-level error has occurred when sending the request to the server.
A transport-level error has occurred when receiving results from the server.
Invalid attempt to call Read when reader is closed.
Invalid attempt to call MetaData when reader is closed.
Count must be positive and count must refer to a location within the string/array/collection. Parameter name: count
ExecuteReader requires an open and available Connection. The connection's current state is connecting.
Any one have any idea what the heck could have happened?
EDIT: Since we were able to replicate the errors all of a sudden on QA, it might not be a user load issue... Needless to say we all feel really screwed here.
Concurrency always brings bugs out of the woodwork. I'd recommend you check for objects that could be shared among requests (such as static members and singletons) and refactor your code so that as little as possible is shared.
As far as specifics go, for the error "There is already an open DataReader associated with this Command which must be closed first," you may want to try adding MultipleActiveResultSets=True to your connection strings.
It sounds like you're crossing the streams a bit and trying to share DataContexts across requests. My suggestion would be to wire in a dependancy injection framework that creates a new instance of the dependancy for each request.
I use Castle's IoC and wire it into the controller factory so that when it sees a dependancy on a repository it creates a new instance of that repository for each request. If you go this route let me know and I can shoot you a few more resources.