BEA WebLogic Interview Questions and Answers 7

How does a file store compare with a JDBC store?
The following are some similarities and differences between file stores and JDBC stores:
* Both have the same transaction semantics, including rolling back transactions (e.g., received messages are put back on the queue).
* Both have the same application interface (no difference in application code).
* The file store should be much faster.
* JDBC may make it easier to handle failure recovery since the JDBC interface can access the database from any client machine; with the file store, the disk must be shared or migrated.
* File store reliability is limited to reliability of disk and O/S; run it on Veritas or a RAID 5 system. Database reliability may be higher.
* File stores will require more memory, but perhaps not significantly more; it depends on how fragmented the file store gets, if the application works roughly
* FIFO it shouldn't get very fragmented at all.
* File stores generate no additional network traffic, database stores do if the database server is on a different JVM or machine.

How do the WLS JMS 6.1 server/destination message maximum and threshold values work?
The byte and message maximum values are quotas - not flow control. Message quotas prevent a WebLogic JMS server from filling up with messages and possibly running out of memory, causing unexpected results. When you reach your quota, JMS prevents further sends with a ResourceAllocationException (rather than blocking). You can set quotas on individual destinations or on a server as a whole.
The thresholds are also not flow control - though they would be better suited to that application than the quotas. The thresholds are simply settings that when exceeded cause a message to be logged to the console to let you know that you are falling behind.
Note that the messages maximum setting on a connection factory is not a quota. This specifies the maximum numbers of outstanding messages that can exist after they have been pushed from the server but before an asynchronous consumer has seen them; it defaults to a value of 10.

How do I configure JDBC so that the JMS JDBC Store recovers automatically?
Several customers have reported a problem where they are using a JDBC store, the DBMS goes down and back up, but JMS can no longer use the store until WLS is shutdown and restarted. You can get around this problem by configuring the following attributes on the JDBC Connection Pool associated with the JMSJDBCStore:

TestConnectionsOnReserve="true"\
TestTableName="[[[catalog.]schema.]prefix]JMSState"

If they are not set, then if the JDBC resource goes down and comes back up, JMS cannot re-use the connection pool until WLS is shutdown and restarted. This has been tested against WLS 6.0 SP02 and WLS 6.1.

Does WebLogic JMS support clustering?
WebLogic JMS supports cluster-wide, transparent access to destinations from any server in the cluster. A system administrator can establish cluster-wide, transparent access to destinations from any server in the cluster by configuring multiple connection factories and using targets to assign them to WebLogic Servers. Each connection factory can be deployed on multiple WebLogic Servers.

The application uses the Java Naming and Directory Interface (JNDI) to look up a connection factory and create a connection to establish communication with a JMS server. Each JMS server handles requests for a set of destinations. Requests for destinations not handled by a JMS server are forwarded to the appropriate server.

You can configure multiple JMS servers on the various nodes in the cluster as long as you give them different names. You can assign destinations to the various JMS servers.

One problem to be aware of is the propagation delay in replicating entries in JNDI. If you have an MDB deployed on one node but reference a destination on another node, the deployment may fail with a javax.naming.NamingException exception. The problem occurs because the server is not synced up to the JNDI from the remote server (JMS server) yet, so the JNDI lookup of destination as part of MDB deployment will fail. One workaround is for each MDB to reference a local destination. Another approach is deploy the MDBs after the server boots (plus a delay for JNDI propagation). To get around losing messages before the MDB is deployed, use durable subscribers. This problem is fixed for MDBs in WLS 6.1, where the MDB will be deployed and reconnection will be retried until the destination is available. Note that this is still a problem for EJBs in general that try to reference a non-local JMS destination.

How do I do HTTP tunneling?
If you want to use HTTP tunneling (wrap every message in HTTP to get through a firewall), you need to add TunnelingEnabled="true" into your &lr;ver> definition in the config.xml file or check the appropriate box on the console. Then use a URL like http://localhost:7001 instead of t3://localhost:7001 for Context.PROVIDER_URL when getting your InitialContext. If you want HTTP tunneling with SSL, use https://localhost:7002 (where https uses HTTP tunneling with SSL and 7002 is the secure port that you configured). You will pay a performance penalty for doing this, so only use tunneling it if you really need to (i.e., need to go through a firewall).

Which of the following statements are true regarding the identity of two EJBs?
a. Two stateful session beans are identical if their data attributes are identical.
b. Two stateful session beans are identical if their session contexts are equal.
c. Two stateless session beans are identical if they are of the same type.
d. Two stateless session beans are identical if their session contexts are equal.
e. Two entity beans are identical if they have same primary key but different home interface.
f. Two entity beans are identical if they have different primary key but same home interface.

B and C are correct. Since the stateful session beans maintain the conversational state of the clients, they are identical when their session contexts are equal. Two stateful session beans may have identical data attributes, but if the session contexts are different they are not identical. Thus choice A is incorrect and B is correct. Since stateless beans do not retain the conversational state, they are considered identical if they are of the same type. Thus choice C is correct.
If two entity objects have the same home interface and primary key, they are considered identical. The EJB specification does not mention object equality based on the = = operator. Also, if you compare two object references using the Java API, Object.equals(Object obj), the result is unspecified. The only way to compare object equality is through the isIdentical (EJBObject) API. Thus choice E and F are incorrect.

Why is my JMS work not part of a user transaction (i.e., called within a transaction but not rolled back appropriately)? How do I track down transaction problems?
Usually this problem is caused by explicitly using a transacted session which ignores the external, global transaction by design (JMS spec requirement). A transacted JMS session always has its own inner transaction. It is not affected by any transaction context that the caller may have.

It may also be caused by using a connection factory that is configured with "UserTransactionsEnabled" set to false.
1. You can check if the current thread is in a transaction by adding these two import lines:
import javax.transaction.*
import weblogic.transaction.*;

and adding the following lines (i.e., just after the begin and just before every operation).

Transaction tran = TxHelper.getTransaction();
System.out.println(tran);
System.out.println(TxHelper.status2String(tran.getStatus()));

This should give a clear idea of when new transactions are starting and when infection is occurring.

2. Ensure that the thread sending the JMS message is infected with a transaction. Check that the code is not using a transacted session by setting the first parameter of createQueueSession or createTopicSession to false. Note that creating the connection and/or session is orthogonal to the transaction. You can begin your transaction before or after. You need only start the transaction before you send or receive messages.

3. Check that the UserTransactionsEnabled flag is explicitly set to true for the connection factory in the config.xml file since the default for user-configured connection factories for this value is false. If you are using one of the pre-configured connection factories they are set as follows:

weblogic.jms.ConnectionFactory disables user transactions so
don't use this one for the case where user transactions are
desired;

javax.jms.QueueConnectionFactory and
javax.jms.TopicConnectionFactory enable user transactions.

4. You can trace JTA operations by starting the server with this additional property:
-Dweblogic.Debug.DebugJMSXA=true
You should see trace statements like these in the log:
XA ! XA(3163720,487900)
This can be used to ensure that JMS is infected with the transaction.