Weblogic Interview Questions &FAQ's -1

Weblogic Application Server FAQ's -1

How do I provide user credentials for starting a server?

While creating a domain, the Configuration Wizard prompts you to provide the username and password for an initial administrative user. If you create the domain in development mode, the wizard saves the username and encrypted password in a boot identity file. A WebLogic Server instance can refer to a boot identity file during its startup process. If a server instance does not find such a file, it prompts you to enter credentials. If you create a domain in production mode, or if you want to change user credentials in an existing boot identity file, you can create a new boot identity file.

Can I start a Managed Server if the Administration Server is unavailable?

By default, if a Managed Server is unable to connect to the specified Administration Server during startup, it can retrieve its configuration by reading a configuration file and other files directly. You cannot change the server's configuration until the Administration Server is available. A Managed Server that starts in this way is running in Managed Server Independence mode.

What is the function of T3 in WebLogic Server?

T3 provides a framework for WebLogic Server messages that support for enhancements. These enhancements include abbreviations and features, such as object replacement, that work in the context of WebLogic Server clusters and HTTP and other product tunneling. T3 predates Java Object Serialization and RMI, while closely tracking and leveraging these specifications. T3 is a superset of Java Object. Serialization or RMI; anything you can do in Java Object Serialization and RMI can be done over T3. T3 is mandated between WebLogic Servers and between programmatic clients and a WebLogic Server cluster. HTTP and IIOP are optional protocols that can be used to communicate between other processes and WebLogic Server. It depends on what you want to do. For example, when you want to communicate between a browser and WebLogic Server-use HTTP, or an ORB and WebLogic Server-IIOP.

How do you set the classpath?

WebLogic Server installs the following script that you can use to set the classpath that a server requires:
WL_HOME/server/bin/setWLSEnv.cmd (on Windows)
WL_HOME/server/bin/setWLSEnv.sh (on UNIX)

How do stubs work in a WebLogic Server cluster?

Clients that connect to a WebLogic Server cluster and look up a clustered object obtain a replica-aware stub for the object. This stub contains the list of available server instances that host implementations of the object. The stub also contains the load balancing logic for distributing the load among its host servers.

What happens when a failure occurs and the stub cannot connect to a WebLogic Server instance?

When the failure occurs, the stub removes the failed server instance from its list. If there are no servers left in its list, the stubb uses DNS again to find a running server and obtain a current list of running instances. Also, the stub periodically refreshes its list of available server instances in the cluster; this allows the stub to take advantage of new servers as they are added to the cluster.

How do I limit the number of Oracle database connections generated by WebLogic Server?

You can use connection pools to limit the number of Oracle database connections generated by WebLogic Server in response to client requests. Connection pools allow T3 applications to share a fixed number of database connections.

How does a server know when another server is unavailable?

WebLogic Server uses two mechanisms to determine if a given server instance is unavailable. Each WebLogic Server instance in a cluster uses multicast to broadcast regular "heartbeat" messages that advertise its availability. By monitoring heartbeat messages, server instances in a cluster determine when a server instance has failed. The other server instances will drop a server instance from the cluster, if they do not receive three consecutive heartbeats from that server instance

WebLogic Server also monitors socket errors to determine the availability of a server instance. For example, if server instance A has an open socket to server instance B, and the socket unexpectedly closes, server A assumes that server B is offline.

How are notifications made when a server is added to a cluster?

The WebLogic Server cluster broadcasts the availability of a new server instance each time a new instance joins the cluster. Cluster-aware stubs also periodically update their list of available server instances.

How do clients handle DNS requests to failed servers?

If a server fails and DNS continues to send requests to the unavailable machine, this can waste bandwidth. For a Java client application, this problem occurs only during startup. WebLogic Server caches the DNS entries and removes the unavailable ones, to prevent the client from accessing a failed server twice.

Failed servers can be more of a problem for browser-based clients, because they always use DNS. To avoid unnecessary DNS requests with browser-based clients, use a third-party load-balancer such as Resonate, BigIP, Alteon, and LocalDirector. These products mask multiple DNS addresses as a single address. They also provide more sophisticated load-balancing options than round-robin, and they keep track of failed servers to avoid routing unnecessary requests.

Why do I get an error while trying to retrieve the text for ORA-12705?

This error occurs when you have not set the ORACLE_home environment variable properly — a common mistake. In order to use WebLogic jDriver for Oracle, the Oracle client software needs to be installed and ORACLE_home must be set.
You may also see this error message if you try to use WebLogic jDriver for Oracle's internationalization capabilities with a language/codeset combination that is not installed on your system. If you get the ORA-12705 error with the correct error text, then either you have set NLS_LANG improperly, or you do not have the right codesets installed on your system.

What is Cause for error "ORA-6502?"

The default length of a string bound to an OUTPUT parameter of a CallableStatement is 128 characters. If the value you assign to the bound parameter exceeds that length, you will get this error.
You can adjust the length of the value of the bound parameter by passing an explicit length with the scale argument to the CallableStatement.registerOutputParameter() method.

How can I control on which WebLogic Server(s) my application will run?

A system administrator can specify on which WebLogic Server(s) applications will run by specifying targets when configuring connection factories. Each connection factory can be deployed on multiple WebLogic servers.

Why do I run out of resources during updates with Oracle's database link?

When you use Oracle's database link to update your database, you may get error "maximum number of temporary table locks exceeded" even if you close your result sets and statements when you finish.
The database link is an object in the local database that allows you to access tables, views, and such in a remote database. The database link is controlled by the Oracle server, so the driver has no control over its use of resources. The link appears to perform the commit (since other processes could see the records that were being created), but it doesn't free any resources until the connection is closed. The solution is to remove the database link and use the JDBC driver to do your selects, inserts, and updates.


How can I run multiple instances of the same servlet class in the same WebLogic Server instance?

If you want to run multiple instances, your servlet will have to implement the SingleThreadModel interface. An instance of a class that implements the SingleThreadModel interface is guaranteed not to be invoked by multiple threads simultaneously. Multiple instances of a SingleThreadModel interface are used to service simultaneous requests, each running in a single thread.

When designing your servlet, consider how you use shared resources outside of the servlet class such as file and database access. Because there are multiple instances of servlets that are identical, and may use exactly the same resources, there are still synchronization and sharing issues that must be resolved, even if you do implement the SingleThreadModel interface.


How do I restrict access to servlets and JSPs?

The Java Servlet API Specification v2.2 allows you to declaratively restrict access to specific Servlets and JSPs using the Web Application Deployment descriptor. Section 13.3.2 of the specification has an example deployment descriptor that uses declarative security. For more information, see Programming WebLogic HTTP Servlets.