I have a stand-alone Java windows application developed based on Swing. It connects to a MySQL database for data storage. In case the database connection fails, I am getting a link failure exception from the MySQL JDBC driver (MySQLNonTransientConnectionException). I don't want to re-instantiate my database connection object or the whole program in case such a link failure issue happens. I just want to tell the user to try again later without having to restart the entire application. If the user is asked to restart the entire application, that would probably give a negative impression on the quality of the program. What do you think would be the preferred way for a standard java application to fail-over after such a database link failure without having to re-instantiate all the communication objects? Thanks in advance.
Use a Connection Pool (such as C3PO or DBCP). Your application takes the Connections from the pool, executes the statement(s) and puts the Connection back into the pool. The pool can be configured to test the JDBC Connections. For example, if they become stale, they can be automatically reinstantiated by the pool.
If your application takes the Connection from the pool, it will be a valid Connection. Let the pool handle the management of valid/invalid/stale JDBC Connections.
Related
We are using H2 started as database server process and listening on standard TCP/IP port 9092.
Our application is deployed in a Tomcat using connection pooling. We do a purge during idle time which at the end results in closing all connections to H2. From time to time we observe errors when the application tries to open the connection to H2 again:
SCHEDULERSERVICE schedule: Exception: Database may be already in use: "Waited for database closing longer than 1 minute". Possible solutions: close all other connection(s); use the server mode [90020-199]
org.h2.jdbc.JdbcSQLNonTransientConnectionException: Database may be already in use: "Waited for database closing longer than 1 minute". Possible solutions: close all other connection(s); use the server mode [90020-199]
at org.h2.message.DbException.getJdbcSQLException(DbException.java:617)
at org.h2.message.DbException.getJdbcSQLException(DbException.java:427)
at org.h2.message.DbException.get(DbException.java:205)
at org.h2.message.DbException.get(DbException.java:181)
at org.h2.engine.Engine.openSession(Engine.java:209)
at org.h2.engine.Engine.createSessionAndValidate(Engine.java:178)
at org.h2.engine.Engine.createSession(Engine.java:161)
at org.h2.server.TcpServerThread.run(TcpServerThread.java:160)
at java.lang.Thread.run(Thread.java:748)
at org.h2.message.DbException.getJdbcSQLException(DbException.java:617)
at org.h2.engine.SessionRemote.done(SessionRemote.java:607)
at org.h2.engine.SessionRemote.initTransfer(SessionRemote.java:143)
at org.h2.engine.SessionRemote.connectServer(SessionRemote.java:431)
at org.h2.engine.SessionRemote.connectEmbeddedOrServer(SessionRemote.java:317)
at org.h2.jdbc.JdbcConnection.<init>(JdbcConnection.java:169)
at org.h2.jdbc.JdbcConnection.<init>(JdbcConnection.java:148)
at org.h2.Driver.connect(Driver.java:69)
at java.sql.DriverManager.getConnection(DriverManager.java:664)
The problem occurs when the Tomcat connection pool closes all idle connection (unused) and one connection still in use is closed afterwards.
The next attempt to open a new connection fails, a retry is successfully after some wait time.
Under which circumstances does this exception happen?
What does the exception mean?
Are there any recommendations to follow to avoid the problem?
It sounds to me that H2 does a database close after the last connection has been closed.
When does the database close occure?
How can database closures been controlled?
Thx in advance
Thorsten
Embedded database in web applications needs careful handling of its lifecycle.
You can add a javax.servlet.ServletContextListener implementation (marked with #WebListener annotation or included into web.xml) and add explicit database shutdown to its contextDestroyed() methods.
You can force database shutdown here with connection.createStatement().execute("SHUTDOWN"). If your application needs to write something to database during unload, it should do it before that command.
Without the explicit shutdown H2 closes the database when all connections are closed, if some other behavior wasn't configured explicitly (with parameters in JDBC URL, for example). For example, DB_CLOSE_DELAY sets the additional delay, maybe your application uses that setting and therefore H2 doesn't close the database immediately, or application doesn't close all connections immediately.
Anyway, when you're trying to update the web application of the fly, Tomcat tries to initialize the new version before its old version is unloaded. If H2 is in classpath of the web application itself, the new version will be unable to connect to the database during short period of time when the new version is already online but the old version isn't unloaded yet.
If you don't like it, you can run the standalone H2 Server process and use remote connections to it in your web applications.
Another option is to move H2 to the classpath of Tomcat itself and configure the connection pool as resource in the server.xml, in that case it shouldn't be affected by the lifecycle of your applications.
In both these cases you shouldn't use the SHUTDOWN command.
UPDATED
With client-server connections to a remote server such exception means that server decided to close the database because there are no active connection. This operation can't be interrupted and reverted in the middle. On attempt to open a new connection to the same database during this process it waits at most for 1 minute for completion of this process to re-open the database again. This timeout is not configurable.
There are two possible solutions.
DB_CLOSE_DELAY setting can be used with some large value in seconds. When all connections are closed, database will stay online for the specified number of seconds. -1 also can be used to set an infinite timeout.
You can try to speed up the shutdown process, but you have to figure out what takes so much time by yourself. The file compaction procedure is limited to 200 milliseconds by default, it may take a longer time, but I think it shouldn't be that long. Maybe you have a lot of temporary objects or uncommitted data. Maybe you have a very high fragmentation of database file. It's hard to say what's going wrong without further investigation.
I have a question related to the scenario when connecting from a Java application using the Microsoft JDBC Driver 4.0 to a SQL Server 2014 with AlwaysOn Availability Groups set up for high availability.
With this set up, we will be connecting to an availability group listener (specified in the db connecting string instead of any particular instance), so that the DB fail-over etc. is handled gracefully by the listener and it tries to connect to the next available instance behind the scenes if current primary goes down in the AG cluster.
Question(s) I have is,
In the data-source that is configured on the j2ee application server side (we use WebSphere), what happens to those connections already pooled by the data-source?
When a database goes down, though the AG listener would try to reconnect on the db side to the next available DB, will the AG Listener also through the jdbc driver send an event or something to the data-source created on the app server and make sure those connections that are already pooled by the datasource to be discarded and have it create new ones so that transactions on the application side wont fail (though they might for a while till new connections are created and fail over is successful) or the java application has to find out only after requesting it from the datasource?
WebSphere Application Server is able to cope with bad connections and removes them from the pool. Exactly when this happens depends on some configurable options and on how fully the Microsoft JDBC driver takes advantage of the javax.sql.ConnectionEventListener API to send notifications to the application server. In the ideal case where a JDBC driver sends the connectionErrorOccurred event immediately for all connections, WebSphere Application Server responds by removing all of these connections from the pool and by marking any connection that is currently in-use as bad so that it does not get returned to the pool once the application closes the handle. Lacking this, WebSphere Application Server will discover the first bad connection upon next use by the application. It is discovered either by a connectionErrorOcurred event that is sent by the JDBC driver at that time, or lacking that, upon inspecting the SQLState/error code of an exception for known indicators of bad connections. WebSphere Application Server then goes about purging bad connections from the pool according to the configured Purge Policy. There are 3 options:
Purge Policy of Entire Pool - all connections are removed from
the pool and in-use connections marked as bad so that they are not
pooled.
Purge Policy of Failing Connection Only - only the
specific connection upon which the error actually occurred is
removed from the pool or marked as bad and not returned to the pool
Purge Policy of Validate All Connections - all connections are
tested for validity (Connection.isValid API) and connections found
to be bad are removed from the pool or marked as bad and not
returned to the pool. Connections found to be valid remain in the
pool and continue to be used.
I'm not sure from your description if you are using WebSphere Application Server traditional or Liberty. If traditional, there is an additional option for pre-testing connections as they are handed out of the pool, but be aware that turning this on can have performance implications.
That said, the one thing to be aware of is that regardless of any of the above, your application will always need to be capable of handling the possibility of errors due to bad connections (even if the connection pool is cleared, connections can go bad while in use) and respond by requesting a new connection and retrying the operation in a new transaction.
Version 4 of that SQL Server JDBC driver is old and doesn't know anything about the always on feature.
Any data source connection pool can be configured to check the status of the connection from the pool prior to doling it out to the client. If the connection cannot be used the pool will create a new one. That's true of all vendors and versions. I believe that's the best you can do.
If the server running Oracle database reboots, this probably should invalidate all previous JDBC connections in the connection pool (running as part of application on another server).
Some connection pools can handle this, others require manual re-initialization, about Oracle I do not know. Would Oracle connection pool (versions 11g and up) be able to detect such situation and recover, or do I need to check for this myself and reinitialize?
Ideally, I would like to know also that would happen for the connection in use that has been borrowed from the pool. Would such connection proxy be able to use the rebooted server?
I have read http://docs.oracle.com/cd/B10501_01/java.920/a96654/connpoca.htm without particular success.
I have a very short Java application that just opens a connection to a remote MySQL database, reads some data, prints it, and exits. The most time-consuming part of the application is the database connection.
Currently I have only a single thread, and my only concern is to save the time of opening the connection.
I thought of several ways to make it faster, but it turned out they do not help:
Connection Pooling - doesn't help because the pool lives only only during a single run of the application. When the application is terminated, the pool is gone, and when I re-run the application, I have to re-open all the connections in the pool.
mysql-proxy - connects only to the local server: mysql-proxy for a remote MySQL server
TCP/IP server - I thought of holding a local TCP/IP server that will keep a persistent open connection and send it to a TCP/IP client on request. However, Connection objects cannot be serialized, so I have no way to pass the Connection object from client to server.
Any other option?
Generally connection to a DB is a most time-consuming operation. If the application is to be started and stopped then there is little that you can do.
Using connection-pooling in a web-server and call that by running your app which talks to the web server using JSON might be an option.
You said you have a very short application so your 3rd option might work if you put the database logic into you "option 3 TCP/IP server" and just forward the results to your connecting client. This is a typical application server pattern.
Another thing you should consider about network look up https://stackoverflow.com/q/3641155/1055715 which Marc B has mentioned in his comment.
It turns out the best solution is to use mysql-proxy with a script that handles connection pooling (a combination of my first two options). I found one such script here:
http://forge.mysql.com/tools/tool.php?id=151
It was probably written for an older version of mysql-proxy, so I had to fix it (if anyone need the fixed version - write me).
It works like a charm - I run the exact same application as before, the only change is in the connection string: instead of connecting to "qa-srv:3308" (the remote server) I connect to "127.0.0.1:4040" (the proxy server).
I have code that connects to a SQL DB and queries it based on user input. I would like to connect using a Pool instead to speed up the query times. I attempted to write a Pool and Manager class but I am getting an unreported NamingException when I try to get a connection from the pool. I have also already caught NamingExceptions in my getConnection() function.
Does anyone know why I am getting this error?
Or could point me in the right direction to create a valid ConnectionPool?
You should use the latest JDBC driver, which supports connection pooling internal, so you don't have to write your own.
You have 3 options:
If your code runs in an application server, configure a connection pool in the server.
Some JDBC drivers provide a connection pool implementation (but many only provide a poolable data source that is meant for integration with an app server).
Use commons-dbcp or some equivalent library to create a connection pool.