Develop Reference

what does 'testConnectionOnCheckin' in c3p0 mean?

I have been trying to go through the c3p0 documentation but not able to understand 'testConnectionOnCheckin' property.
Docs says - "Connections are tested before they are included in pool".
Does this property apply to only new connections that c3p0 creates are tested before they are included in pool? What is point of checking new connections? Wouldn't they generally be valid?
Also couple of days my application logs were showing following:
[managed:2 unused:2 excluded:1]
And my application was throwing exception for one particular connection which I assume is 'excluded' one. Is 'excluded' connection counted in pool and can c3p0 hand it over to application without checking validity? If not, then would setting 'testConnectionOnCheckin' test this excluded connection for validity before it is used by my application?
I apologize for too many questions but it's just that I am confused.
Thanks
Jitendra

testConnectionOnCheckin tests Connections after they are checked-in by clients [ie via Connection.close()], but before they are reintegrated into the Connection pool. I'm not sure what documentation you are looking at, but see
http://www.mchange.com/projects/c3p0/#testConnectionOnCheckin
http://www.mchange.com/projects/c3p0/#configuring_connection_testing
I generally recommend testing Connections with a combination of idleConnectionTestPeriod and testConnectionsOnCheckIn (and a fast preferredTestQuery).
An "excluded" Connection is a Connection currently in use by a client, but which c3p0 has noticed is faulty. c3p0 marks these Connections to be destroyed rather than reintegrated into the pool when they are checked-in by the client.
I hope this helps!

How to reconnect when the LDAP server is restarted?

I have a situation where through a Java program, I create a javax.naming.ldap.LdapContext and do a search() operation on it - which makes an underlying connection. Then I put the Java app thread to sleep, during which I restart the LDAP server (OpenLDAP, just to note). When the App thread wakes up and tries to do any operation on the LdapContext created earlier, it throws "CommunicationException: Connection is closed".
What I want is to be able to re-establish the connection.
I see that LdapContext has a reconnect() method - where I pass controls as null. However, this does not have any effect. What I saw in the Sun LDAP implementation that during the time when the LDAP server was restarted, the ConnectionPool maintained by the Sun implementation marked the underlying com.sun.jndi.ldap.LdapClient instance with a "usable=false". Upon reconnect() call - it simply calls ensureOpen(), which again checks if the usable flag is false or not - if it's false; then it throws CommunicationException - so back to square one.
My question is: how does a Java app survive an external LDAP server restart? Is creation of new LdapContext again is the only way out?
Appreciate any insights.
Here is the stacktrace of the exception:
javax.naming.CommunicationException: connection closed [Root exception is java.io.IOException: connection closed]; remaining name 'uid=foo,ou=People,dc=example,dc=com'
at com.sun.jndi.ldap.LdapCtx.doSearch(LdapCtx.java:1979)
at com.sun.jndi.ldap.LdapCtx.searchAux(LdapCtx.java:1824)
at com.sun.jndi.ldap.LdapCtx.c_search(LdapCtx.java:1749)
at com.sun.jndi.toolkit.ctx.ComponentDirContext.p_search(ComponentDirContext.java:368)
at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.search(PartialCompositeDirContext.java:338)
at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.search(PartialCompositeDirContext.java:321)
at javax.naming.directory.InitialDirContext.search(InitialDirContext.java:248)
Caused by: java.io.IOException: connection closed
at com.sun.jndi.ldap.LdapClient.ensureOpen(LdapClient.java:1558)
at com.sun.jndi.ldap.LdapClient.search(LdapClient.java:504)
at com.sun.jndi.ldap.LdapCtx.doSearch(LdapCtx.java:1962)
... 26 more

Just enable JNDI connection pooling and it will all be taken care of for you behind the scenes. See the JNDI Guide to Features and the LDAP Provider documentation. It's controlled by just a couple of properties.

The UnboundID LDAP SDK provides a means to auto-connect wherein that auto-reconnect operation is invisible to the client.

We had this problem at work. The solution we came up with (may not be the best answer). Was to create a watchdog thread that would check the connection at some fixed rate. If the connection did not work, it would re-initialize the connection with LDAP.

You should note that this is related essentially to LDAP connection pooling. As defined here:
A connection is retrieved from the pool, used, returned to the pool, and then, retrieved again from the pool for another Context instance.
Thus, the reuse of a previous connection may cause such problem:
You may test the behavior without using LDAP connection pooling by setting
com.sun.jndi.ldap.connect.pool=false
Also, another possible cause may be the timeout of reading the LDAP operations. In fact, the reading operation is not notified about the closure of the LDAP server after a specific timeout. For more information, you may take a look at this link

How to reestablish a JDBC connection after a timeout?

I have a long-running method which executes a large number of native SQL queries through the EntityManager (TopLink Essentials). Each query takes only milliseconds to run, but there are many thousands of them. This happens within a single EJB transaction. After 15 minutes, the database closes the connection which results in following error:
Exception [TOPLINK-4002] (Oracle TopLink Essentials - 2.1 (Build b02-p04 (04/12/2010))): oracle.toplink.essentials.exceptions.DatabaseException
Internal Exception: java.sql.SQLException: Closed Connection
Error Code: 17008
Call: select ...
Query: DataReadQuery()
at oracle.toplink.essentials.exceptions.DatabaseException.sqlException(DatabaseException.java:319)
.
.
.
RAR5031:System Exception.
javax.resource.ResourceException: This Managed Connection is not valid as the phyiscal connection is not usable
at com.sun.gjc.spi.ManagedConnection.checkIfValid(ManagedConnection.java:612)
In the JDBC connection pool I set is-connection-validation-required="true" and connection-validation-method="table" but this did not help .
I assumed that JDBC connection validation is there to deal with precisely this kind of errors. I also looked at TopLink extensions (http://www.oracle.com/technetwork/middleware/ias/toplink-jpa-extensions-094393.html) for some kind of timeout settings but found nothing. There is also the TopLink session configuration file (http://download.oracle.com/docs/cd/B14099_19/web.1012/b15901/sessions003.htm) but I don't think there is anything useful there either.
I don't have access to the Oracle DBA tables, but I think that Oracle closes connections after 15 minutes according to the setting in CONNECT_TIME profile variable.
Is there any other way to make TopLink or the JDBC pool to reestablish a closed connection?
The database is Oracle 10g, application server is Sun Glassfish 2.1.1.

All JPA implementations (running on a Java EE container) use a datasource with an associated connection pool to manage connectivity with the database.
The persistence context itself is associated with the datasource via an appropriate entry in persistence.xml. If you wish to change the connection timeout settings on the client-side, then the associated connection pool must be re-configured.
In Glassfish, the timeout settings associated with the connection pool can be reconfigured by editing the pool settings, as listed in the following links:
Changing timeout settings in GlassFish 3.1
Changing timeout settings in GlassFish 2.1
On the server-side (whose settings if lower than the client settings, would be more important), the Oracle database can be configured to have database profiles associated with user accounts. The session idle_time and connect_time parameters of a profile would constitute the timeout settings of importance in this aspect of the client-server interaction. If no profile has been set, then by default, the timeout is unlimited.

Unless you've got some sort of RAC failover, when the connection is terminated, it will end the session and transaction.
The admins may have set into some limits to prevent runaway transactions or a single job 'hogging' a connection in a pool. You generally don't want to lock a connection in a pool for an extended period.
If these queries aren't necessarily part of the same transaction, then you could try terminating and restarting a new connection.
Are you able to restructure your code so that it completes in under 15 minutes. A stored procedure in the background may be able to do the job a lot quicker than dragging the results of thousands of operations over the network.

I see you set your connection-validation-method="table" and is-connection-validation-required="true", but you do not mention that you specified the table you were validating on; did you set validation-table-name="any_table_you_know_exists" and provide any existing table-name? validation-table-name="existing_table_name" is required.
See this article for more details on connection validation.
Related StackOverflow article with similar problem - he wants to flush the entire invalid connection pool.

How to remove invalid database connection from pool

I am using connection pooling of tomcat with oracle database. It is working fine, but when i use my application after a long time it is giving error that "connection reset". I am getting this error because of physical connection at oracle server closed before logical connection closed at tomcat datasource. So before getting the connection from datasource i am checking the connection validity with isValid(0) method of connection object which gives false if the physical connection was closed. But i don't know how to remove that invalid connection object from the pool.

This could be because on the db server, there is a timeout to not allow connections to live beyond a set time, or to die if it does not receive something saying it is still valid. One way to fix this is to turn on keepalives. These basically ping the db server saying that they are still valid connections.
This is a pretty good link on Tomcats DBCP configurations. Take a look at the section titled "Preventing dB connection pool leaks". That looks like it may be a good place to start.

I used validatationquery while configuring the datasource in server.xml file. It is going to check the validity of the connection by executing the query at database before giving to the application.
for Oracle
validationQuery="/* select 1 from dual */"
for MySql
validationQuery="/* ping */"

Try closing it and opening it if it's invalid. I mean u would reinitialize it in this way so u won't need to remove it from the pool and reuse it.

If we want to dispose an ill java.sql.connection from Tomcat jdbc connection pool,
we may do this explicitly in the program.
Unwrap it into an org.apache.tomcat.jdbc.pool.PooledConnection,
setDiscarded(true) and close the JDBC connection finally.
The ConnectionPool will remove the underlying connection once it has been returned.
(ConnectionPool.returnConnection(....))
e.g.
PooledConnection pconn = conn.unwrap(PooledConnection.class); pconn.setDiscarded(true);
conn.close();

Blocking on DBCP connection pool (open and close connection). Is database connection pooling in OpenEJB pluggable?

We use OpenEJB on Tomcat (used to run on JBoss, Weblogic, etc.). While running load tests we experience significant performance problems with handling JMS messages (queues). Problem was localized to blocking on database connection pool getting or releasing connection to the pool. Blocking prevented concurrent MDB instances (threads) from running hence performance suffered 10-fold and worse. The same code used to run on application servers (with their respective connection pool implementations) with no blocking at all.
Example of thread blocked:
Name: JMS Resource Adapter-worker-23
State: BLOCKED on org.apache.commons.pool.impl.GenericObjectPool#1ea6b4a owned by: JMS Resource Adapter-worker-19
Total blocked: 18,426 Total waited: 0
Stack trace:
org.apache.commons.pool.impl.GenericObjectPool.returnObject(GenericObjectPool.java:916)
org.apache.commons.dbcp.PoolableConnection.close(PoolableConnection.java:91)
- locked org.apache.commons.dbcp.PoolableConnection#1bcba8
org.apache.commons.dbcp.managed.ManagedConnection.close(ManagedConnection.java:147)
com.xxxxx.persistence.DbHelper.closeConnection(DbHelper.java:290)
....
Couple of questions.
I am almost certain that some transactional attributes and properties contribute to this blocking, but MDBs are defined as non-transactional (we use both annotations and ejb-jar.xml). Some EJBs do use container-managed transactions though (and we can observe blocking there as well). Are there any DBCP configurations that may fix blocking?
Is DBCP connection pool implementation replaceable in OpenEJB? How easy (difficult) to replace it with another library?
Just in case this is how we define data source in OpenEJB (openejb.xml):
<Resource id="MyDataSource" type="DataSource">
JdbcDriver oracle.jdbc.driver.OracleDriver
JdbcUrl ${oracle.jdbc}
UserName ${oracle.user}
Password ${oracle.password}
JtaManaged true
InitialSize 5
MaxActive 30
ValidationQuery SELECT 1 FROM DUAL
TestOnBorrow true
</Resource>

My 2 cts...
1 - Are there any DBCP configurations that
may fix blocking?
Although I cannot see it in the doc, I think there should also be a setting attribute named 'WhenExaustedAction' in the Resource node that could take a value "GROW" (value 2) as opposed to "BLOCK" (value 1) or "FAIL" (value 0). This comes straight from the Pools common.
Both Hibernate and Cayenne do use this DBCP setting. Don't know about OpenEJB though.
No need to say that this would work only if all connections are dutifully closed of course (which is sometimes hard to guarantee).
Then you could probably see through JMX how many connections you need at peak activity time and you could then set the maxActive to a higher value evolved from these measures.
2 - Is DBCP connection pool implementation
replaceable in OpenEJB? How easy
(difficult) to replace it with another
library?
Sorry no idea. Would imagine yes. Or possibly DBCP allows another connection pool manager.
UPDATE: Just had a look in the code and it seems DBCP is the only option for connection pooling.
Incidentally I've seen that the whenExhaustedAction settings. is not supported by openejb.xml.
There would however, still be one option left, since you are using an Oracle Database.
One thing you could try is to use Oracle implicit connection caching (assuming version 10g) and leave DBCP with an arbitrary "sufficient" amount of connections.
To do so, you would need to configure in the openejb.xml resource block, ConnectionProperties properties and use Oracle JDBC connection properties. That is connectionCachingEnabled=true and at least connectionCacheName and connectionCacheProperties. In this way I would lure DBCP into believing it's doing the real job and actually using Oracle's pooling mechanism. That would also mean taking little risks with DBCP and thereby a more liberal sizing of the maxActive setting.

Resolved issue with dbcp blocking by changing pool configuration (openejb.xml):
TestOnBorrow false
Thank you, Andy, from OpenEJB team!