Prevent a connection deadlock. Deadlock
can occur if the application requires more than one concurrent connection
per thread, and the database connection pool is not large enough for
the number of threads. Suppose each of the application threads requires
two concurrent database connections and the number of threads is equal
to the maximum connection pool size. Deadlock can occur when both
of the following are true:
- Each thread has its first database connection, and all are in
use.
- Each thread is waiting for a second database connection, and none
would become available since all threads are blocked.
To prevent the deadlock in this case, increase the maximum
connections value for the database connection pool by at least one.
This will allow at least one of the waiting threads to obtain a second
database connection and avoid a deadlock scenario.
For general
prevention of connection deadlock, code your applications to use only
one connection per thread. If you code the application to require
C concurrent
database connections per thread, the connection pool must support
at least the following number of connections, where
T is the
maximum number of threads:
T * (C - 1) + 1
Enable deferred enlistment. In the application
server environment, deferred enlistment refers to the technique in
which the application server waits until the connection is used before
the connection is enlisted in the application's unit of work (UOW)
scope.Consider the following illustration of deferred enlistment:
- An application component that uses deferred enlistment calls the getConnection method
from within a global transaction.
- The application component does not immediately use the connection.
- When the application issues the call for initial use of the connection,
the transaction manager intercepts the call.
- The transaction manager enlists the XA resource for the connection
and calls the XAResource.start method.
- The connection manager associated with the XA resource sends the
call to the database.
Given the same scenario, but the application component does not
use deferred enlistment, the component container immediately enlists
the connection in the transaction. Thus the application server incurs,
for no purpose, an additional load of all of the overhead associated
with that transaction. For XA connections, this overhead includes
the two phase commit (2PC) protocol to the resource manager.
Deferred
enlistment offers better performance in the case where a connection
is obtained, but not used, within the UOW scope. The technique saves
the cost of transaction participation until the UOW in which participation
must occur.
Check with your resource adapter provider if you
need to know if the resource adapter provides this functionality.
The application server relational resource adapter automatically supports
deferred enlistment.
Incorporating a deferred enlistment
in your code:
The J2EE Connector Architecture (JCA) Version
1.5 specification calls the deferred enlistment technique
lazy
transaction enlistment optimization. This support is initiated
using the LazyEnlistableManagedConnectionmarker interface, and the
LazyEnlistableConnectionManager() method on the connection manager:
package javax.resource.spi; import javax.resource.ResourceException; import javax.transaction.xa.Xid; interface LazyEnlistableConnectionManager { // application server void lazyEnlist(ManagedConnection) throws ResourceException; } interface LazyEnlistableManagedConnection { // resource adapter }
If
the lazy enlistment for the connection is not attempted by the relational
resource adapter, you might receive an ORA-01002 exception. This exception
occurs when the ResultSet object associated with an unshared connection
is cached across the transaction boundary. To solve this problem,
add the beginTranForResultSetScrollingAPIs custom property to the
configuration settings for your data source, and set this property
to true. Setting this custom property to true
allows DataSource to add the lazy enlist option for result operations.
The default value is false.
To specify
a value for the beginTranForResultSetScrollingAPIs custom property,
in the administrative console, click .
Click the name of the data source for which you want to set this property,
and then under Additional Properties, click .
Control connection pool
sharing. You can use the defaultConnectionTypeOverride,
or globalConnectionTypeOverride connection pool custom property for
a particular connection factory or datasource to control connection
sharing , or to globally set a datasource connection type.
The
defaultConnectionTypeOverride property changes the default sharing
value for a connection pool. This property enables you to control
connection sharing for direct look-ups. If resource references are
configured for this datasource/connection factory they will take
precedence over this property and the resource ref settings will
be used. For example: if an application is doing direct look-ups,
and you do not want the connections pool shared. set this property
to unshared.
The value specified for the globalConnectionTypeOverride
custom property takes precedence over all of the other connection
sharing settings. For example if you set this property to unshared,
all connection requests are unshared for both direct look-ups, and
resource reference lookups. This property provides you with a quick
way to test the consequences of moving all connections for a particular
datasoure/connection factory to unshared or shared without changing
the resource reference setting in all the deployment descriptors for
an application. The globalConnectionTypeOverride property also enables
you to move between shared, and unshared connections for a particular
datasource or connection factory without having to change any resource
references.
If you specify values for both the defaultConnectionTypeOverride
and the globalConnectionTypeOverride properties, only the values
specified for the globalConnectionTypeOverride property is used to
determine connection sharing type.
To add these new custom
properties to the settings for a datasource or connection factory
connection pool, a new custom property on the connection pool, in
the administrative console click . Then specify defaultConnectionTypeOverride or globalConnectionTypeOverride in
the Name field, and shared or unshared in
the Value field.