Some of the interview Questions

Some of the interview Questions that i faced recently. Interview was form 1 hour 25 minutes -------------

• if admin server is down and if we changed any setting in managed server, will admin get all those setting after getting up?
• Can a stuck thread still do reasonable work?
• How do you deal with the corrupted config.xml?
• how do you deal with the corrupted configuration like JDBC , JMS files?
• What are the available logs Created in Weblogic?
• What is a TTL in weblogic?
• What are the difference between Connection pool and data source?
• What happens if we delete the log files when the server is running & when the server is not running?
• Can we change the name of the log files when the process is running?
• What is HTTP tunneling in weblogic?
• How many types of weblogic installation ?
• How do you clean the Cache in Weblogic?
• What are the recommendations for the number of WebLogic Server Instance (JVM) per CPU?
• Tell about the Weblogic Boot Up sequence?
• Does WebLogic Server dynamically use additional CPUs added to a managed server on the fly?
• Can a WebLogic Server (WLS) admin server running on a 32-bit JDK be configured with a managed server running on a 64-bit JDK?
• What is the difference between -Dweblogic and setting values in weblogic console?
• How to disable admin port in weblogic without weblogic console?
• What are Pinned services in Weblogic?
• Why we need Weblogic Inactive Connection Timeout in Weblogic
• How do I integrate JNI ( Native code ) code into weblogic?
• I want to set Prodcution Mode to true in Admin Domain , would this also run all the managed servers in Production Mode ?
• What is the deployment process in weblogic?
• How does the deployment to a cluster work if one of the member is down
• How do you differentiate between a server hang and server crash issue?
• what is the difference between .jar .war and .ear files?
• What are deployment descriptors?
• What is a shutdownhook?
• What is a File Descriptor?
• What is fast swap in weblogic?
• How do we find the applied patches done in Weblogic
• What are core , thread and heap dump?
• What is a network channel in Weblogic?
• What are Resource adapters?
• How does Credentials are passed to Weblogic server?
• How do we communicate with external components?
• What is cross cluster replication in weblogic?
• What is a DBPing and multiCast utility do?
• What is the use of Pack Command>\?
• Does weblogic server works as a Web Server?
• What are the available roles in weblogic?
• What is a Client application archive?
• What are the types of deployment methods in Weblogic?
• What are the types of packaging files for deployment?
• Can an application be deployed without any deployment descriptors?
• What are the values tuned in web logic?
• What are the differences between multicast and unicast clustering?
• What are the data source states?
• When does a Data Source goes to Suspend State?
• What is a Overload state in Weblogic?
• What is the sequence of Weblogic Server startup?
• How can the cluster members will communicate?
• What are the difference b/w web server and proxy server?
• Difference between horizontal clustering, vertical clustering and where both are used?
• If data source is configured but then after DB server is not running then what is the state of data source?
• What is the difference between heap dump and thread dump?
• What kind of problems would you expect to encounter when trying to increase a managed server's maximum heap size to 3 GB? If an application was suffering short periodic freezes, and no errors were appearing in the logs, can you suggest a possible cause?
• If a managed server is crashing with a StackOverflow exception, what could be the cause?
• Describe how you might run a WLST script?
• What is a Smart Update in weblogic?
• What is a weblogic version compatibility?
• Can a node manager run with a different version of weblogic version?
• Can weblogic cluster be configured in a mixed platform ?
• What is a Harvester in WLDF?
• What are Watches and Notifications in WLDF?
• What are the types of data sources provided in weblogic?
• Can JDBC datasource be shared across clusters or to other developers?
• What is Cluster-wide JNDI tree?
• What are the types of Session Replication in weblogic?
• When does a Weighted Round Robin algorithm used,Explain a Scenario?
• What is the Diff b/w Xa and Non-Xa Datasource ?
• What are the Deployable Modules and Deployment modes?

Thanks to 

Jagadesh Manchala

July 30 · Edited

Thread Dump

Thread Dump

Thread Dump:

1. What is Thread dump?
2. When we will take Thread dump? (Scenarios)
3. How  Many  ways take Thread Dumps
4. Thread Dump Generating Procedure
5. What can I Analysis with Thread Dump?
6. How can I analysis thread dump?
7. Actions taken for Issue resolving
8. References

 Coming to step by step learning:

--------------------------------

What is Thread dump?

Thread Dump is a textual dump of all active threads and monitors of Java apps running in a Virtual Machine.

When we will take Thread dump? (Scenarios)

1.  Scenario 1: when server is hang Position, i.e. that time server will not respond to coming requests.

2.  Scenario 2: While sever is taking more time to restart

3.  Scenario 3: When we are Getting exception like “java.lang.OutOfMemoryException”

4.  Scenario 4: Process running out of File descriptors. Server cannot accept further requests because sockets cannot be created

5.  Scenario 5: Infinite Looping in the code

How many ways take Thread Dumps?

Many types we have to take a Thread dumps. As per your flexibility you can choose one Procedure. For analyzing take dumps some Intervals (like every 10mins, 10mins etc.).

Generating Dump Talking Procedures

1. Take Thread dump from Console by Using of below command

    $kill -3 PID

   (For Getting PID, Use this Command ps –ef | grep “java”)

Here The Output of the Thread Dump will be generated in the Server STDOUT.

(Note: If a process is not responding to kill -3 then it’s a JVM bug.)

2. Generation Thread Dump via Admin Console

a.  login to Admin Console(with Admin Username/Password)

b.  Click on Server, after choose your server

c.  Goto Monitoring TAB

d.  Goto Threads TAB, after click on “Dump Thread Stack” Button

e.  Now you can view the all the Threads in Same page

f.  Copy and paste in a txt file.

3.  We can Collect Thread Dump Using “WebLogic.Admin” which is deprecated, but still available or may be available in near future as well As i think because it is one of the best debugging utility for Admins.

  java WebLogic.Admin -url t3://hostname: port -username Weblogic -password Weblogic THREAD_DUMP

This Thread Dumps will be generated in Servers STDOUT file

4. Getting Thread Dumps by using Jstack Utility

    a.jstack –m (to connect to a live java process)

    b. jstack –m [server_id@]

                (to connect to a remote debug server)

    (-m Means print both java and native frames (mixed mode)) 

5. By Using WLST Script, can contain extension of (.py)

connect(‘weblogic’,'weblogic’,'t3://hostname:port′) cd (”Servers’) ls()cd (‘AdminServer’) ls() threadDump()

 Execute this Script in console. 

What can I Analysis with Thread Dumps?

We need to analyze the thread dumps for analyzing running threads and their states to identifying.

How can I analysis thread dumps?

For analyze thread dumps we have lots of tools to understand easily thread states

1.  samurai tool :

  (Visit: http://yusuke.homeip.net/samurai/en/index.html)

    In this tool you can identify all the Thread states by     identifying colors. We need to take care about Deadlocks and waiting state threads.

   More Details:

    $ java -jar samurai.jar

     After running we will get a Screen like below

      Goto Thread dump tab

    When Samurai detects a thread dump in your log, a tab named "Thread Dump" will appear.

 You can just click "Thread dumps" tab to see the analysis result. Samurai colors idle threads in gray, blocked threds in red and running threds in green. There are three result views and Samurai shows "Table view" by default. In many case you are just interested in the table view and the sequence view. Use the table view to decide which thread needs be inspected, the sequence view to understand the thread's behavior. You should takecare especially threds always in red.

2.  TDA Tool :

 (Visit: https://tda.dev.java.net/)

Actions taken for Issue resolving

1.  Classic Dead Locks : Look for the threads waiting for monitor entry

For Example :

"ExecuteThread: '95' for queue: 'default'" daemon prio=5 tid=0x411cf8 nid=0x6c waiting for monitor entry [0xd0f80000..0xd0f819d8]

    at weblogic.common.internal.ResourceAllocator.release(ResourceAllocator.java:766)

    at weblogic.jdbc.common.internal.ConnectionEnv.destroy(ConnectionEnv.java:590)

Reason: The above thread is waiting to acquire lock on Resource Allocator object. The next step is to identify the thread that is holding the Resource Allocator object

"ExecuteThread: '0' for queue: '__weblogic_admin_rmi_queue'" daemon prio=5 tid=0x41b978 nid=0x77 waiting for monitor entry [0xd0480000..0xd04819d8]

    at weblogic.jdbc.common.internal.ConnectionEnv.getPrepStmtCacheHits(ConnectionEnv.java:174)

    at weblogic.common.internal.ResourceAllocator.getPrepStmtCacheHitCount   (ResourceAllocator.java:1525)

Reason: This thread is holding lock on source Allocator object, but is waiting for Connection Env object. This is a classic deadlock.

     

2.  Threads in wait() state:

   A sample dump:

"ExecuteThread: '10' for queue: 'SERV_EJB_QUEUE'" daemon prio=5 tid=0x005607f0 nid=0x30 in Object.wait() [83300000..83301998]

  at java.lang.Object.wait(Native Method)

  - waiting on (a weblogic.ejb20.pool.StatelessSessionPool)

at weblogic.ejb20.pool.StatelessSessionPool.waitForBean(StatelessSessionPool.java:222)

Reason: The above thread would come out of wait() under two conditions

 (Depending on application logic)

1) One of the thread available in the execute queue pool would call notify() on this object when an instance is available. (If the wait() is indefinite).

  This can cause the thread to hang for ever if server never does a notify() to this object.

2) If the timeout exceeds, the thread would throw an exception and back to execute queue thread pool.

References

Thanks to Bodepudi Kotesh ..

What is lok file ?how many types of lok files are there?

• 
  
• 

  Lubomír Maliník lok files are for locking some action which should be run or made only by one user or holding process. You can find config.lok invoked for serial update of config.xml, then each server has its own lok file for server and lok file for embedLDAP to avoid start same manged server in time twice. Then you can find edit.lok invoked by Edit actions for one user editing domain configuration in time. I am not sure if other exist.
• 

  Lubomír Maliník It is good to know it, because sometimes when server crash you cant start servers and it writes something like "Instance is already running" Or second issue server begin to start and then after line in log with text "....IIOP..." next step is load embedLDAP but server stuck for long time in this step because ldap is locked or corrupted. Remove of these lok files usually can help you to start server perfectly again

Persistent Store

• 
  
• 

  Moqthayar Ali Shaik It is used to store weblogic resources and weblogic subsystems

  December 25, 2014 at 6:23pm · 
• 

  Raju Sagi persistent store stores all you JMS messages into either file store (which will be saved in .dat format) or else in database store, so even though our weblogic went crash ,our JMS messages still persist in the file or database, in other hand if it is non-persistent our Weblogic intance keep messages in its memory but if instance went down all its memory along with messages will be wiped out.

  December 25, 2014 at 9:46pm · 
• 

  Maadhavamadhausudhan Reddy K it is a physical repository for storing jms server data like queue and topic. There are two types. file and jdbc store
• 
  
• 

  Chandra Varagani If the server crashes when the requests/messages are processing.. those requests will save in one place which calls persistent store. it depends on you that you can configure it or not. As the guys said above, there are 2 types. first, file persistent store can be defined in JMS till 8x version.. guess till 9.1.. From 9.2 onwards, JDBC persistent store have been introduced. so, you can configure it under JDBC.

  
  

Hi, Suppose we have apache webserver and weblogic manage servers in an environment. Client can access an application only from external URL..if so do we need SSL only for Apache or both apache and Weblogic manage servers.?

• 

  Sambi Reddy Apache

  December 30, 2014 at 10:20pm · Like
• 

  Sri Vas What is the need of SSL, keystore sections in weblogic console? if SSL configuration for apache is enough for the environment to protect data?

  December 31, 2014 at 1:01am · Like
• 

  Sambi Reddy Ssl is of multiple types like one way two way.. Pls go through it

  December 31, 2014 at 5:02am · Like
• 

  Anfraz Shaikh Enabling SSL on Weblogic would be an extra level of security. But if you dont have option then it would be enough to enable SSL on Apache as long as application is accessed via Apache URL.

  January 1 at 1:16am · Like · 
• 

  Madhu Katuri It depends on your Infrastructure requirements. 1) You can terminate SSL at Apache or 2) Extend till WebLogic Managed Servers. You need to configure SSL only on Apache in case 1 and on both Apache and WebLogic in case 2 above. Case 1 (Browser to Apache) is one way SSL and Case 2 is Two-Way SSL . Generally between servers is Two-Way SSL

  January 7 at 7:37pm · Like