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No ManagedConnections available within configured blocking timeout ( 30000 [ms] ))

You might have seen this error when working with jdbc datasource deployed on JBoss. This error is raised when free connections are not available in the connection pool for a new request. This happens due to a combination of factors.

Jboss pool size configuration

Low MaxSize setting in ManagedConnectionPool configuration

Database connection size

When not enough connections are accepted by the database.

Peak in requests

Typically, connection pool is tuned to handle various patterns of load. However, any unexpected surge in load/requests can deplete connections in the pool.

Long running transactions

They can hold a connection for a long time not making it available for waiting requests, and those waiting requests may eventually timeout.

Connections Leak

Connections are not closed/returned properly by the application, and those connections linger on eating up the whole pool. This can also happen if the application closes the connection directly without returning it to the pool. In fact, the connection given to the application by the pool is a wrapper of actual JDBC connection. Attempting to close the wrapper connection does not close the underlying JDBC connection but rather returns the connection to the pool. However, this is pool vendor dependent. See documentation.

Short blocking-timeout-millis

This pool configuration in JBOSS specifies the maximum time in milliseconds a worker thread block, while waiting for a connection before throwing an exception. Note that this blocks only while waiting for a permit for a connection, and will never throw an exception if creating a new connection takes an inordinately long time.
There are many ways to trouble shoot this error. First step is to understand if this is caused by a leak. Connection leak will eventually toast your server. But if this exception is caused by other means like pool size setting or surge in requests, the server would continue to serve few requests, which can be met by the available connections.
Your can troubleshoot this error through JMX Console- http://host:port/jmx-console. JMX console reports various metrics of jboss mbeans. For connection pool, look for ManagedConnectionPool mbean under JBoss.JCA category.
Logs are the most crucial help. Turn on jboss connection pool and hibernate jdbc logging, In jboss-log4j.xml, add-
<appender name="JDBC">
<errorHandler />
<param name="File" value="${jboss.server.log.dir}/jdbc.log" />
<param name="Append" value="true" />
<param name="MaxFileSize" value="5000KB" />
<param name="Threshold" value="DEBUG" />
<layout>
<param name="ConversionPattern" value="%d %-5p [%c] %m%n" />
</layout>
</appender>
<category additivity="true" name="org.hibernate.jdbc">
<priority value="DEBUG" />
<appender-ref ref="JDBC" />
</category>
<category additivity="true" name="org.jboss.resource.adapter.jdbc">
<priority value="DEBUG" />
<appender-ref ref="JDBC" />
</category>
<category additivity="true"
name="org.jboss.resource.connectionmanager">
<priority value="DEBUG" />
<appender-ref ref="JDBC" />
</category>

References

http://docs.jboss.org/jbossas/jboss4guide/r4/html/ch7.chapt.html

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