java.interview.preparation: JMS Interview questions

JMS is used for asynchronous communication in java. This is reliable medium to communicate between applications. If the application is using any JMS implementation interviewer will like to check if the basics of JMS is clear to applicant. In any case this is one of the easy topic in java where you can score full marks as here numbers of questions are limited and complexity is less. Try to score full marks here as there are very generic questions like PUB/SUB Model, Durable Subscription, JMS Transaction.

1. What is synchronous messaging?

Synchronous messaging involves a client that waits for the server to respond to a message. So if one end is down the entire communication will fail.

2. What is asynchronous messaging?

Asynchronous messaging involves a client that does not wait for a message from the server. An event is used to trigger a message from a server. So even if the client is down , the messaging will complete successfully.

3. What is the difference between queue and topic ?

A topic is typically used for one to many messaging, while queue is used for one-to-one messaging. Topic .e. it supports publish subscribe model of messaging where queue supports Point to Point Messaging.

4. What are the core JMS-related objects required for each JMS-enabled application?
Each JMS-enabled client must establish the following:

A connection object provided by the JMS server (the message broker)
Within a connection, one or more sessions, which provide a context for message sending and receiving
Within a session, either a queue or topic object representing the destination (the message staging area) within the message broker
Within a session, the appropriate sender or publisher or receiver or subscriber object (depending on whether the client is a message producer or consumer and uses a point-to-point or publish/subscribe strategy, respectively). Within a session, a message object (to send or to receive)

5. What is the difference between Byte Message and Stream Message?

Bytes Message stores data in bytes. Thus the message is one contiguous stream of bytes. While the Stream Message maintains a boundary between the different data types stored because it also stores the type information along with the value of the primitive being stored. Bytes Message allows data to be read using any type. Thus even if your payload contains a long value, you can invoke a method to read a short and it will return you something. It will not give you a semantically correct data but the call will succeed in reading the first two bytes of data. This is strictly prohibited in the Stream Message. It maintains the type information of the data being stored and enforces strict conversion rules on the data being read.

6. What are transaction options available in JMS?

An application has myriad transaction options available, including whether or not it wants to participate in transactions. If your application does not use transactions, it can use one of these acknowledgement modes: auto, duplicates okay, and client. You specify the acknowledgement modes when creating a JMS session. If your application uses transactions, it can choose from these transaction options: transacted session, MDB with container-managed transaction demarcation (CMTD), and MDB with bean-managed transaction demarcation (BMTD). The following lists briefly describe these acknowledgement modes and transaction options.

Acknowledgement options:

·Auto mode: When a session uses auto mode, the messages sent or received from the session are automatically acknowledged. This is the simplest mode and expresses JMS's power by enabling once-only message delivery guarantee.

·Duplicates okay mode: When a session uses duplicates okay mode, the messages sent or received from the session are automatically acknowledged just like auto mode, albeit lazily. Under rare circumstances, the messages might be delivered more than once. This mode enables at-least-once message delivery guarantee.

·Client mode: When a session uses client mode, the messages sent or received from the session are not acknowledged automatically. The application must acknowledge the message receipt. This mode gives the application (rather than the JMS provider) complete control over message acknowledgement, at the cost of increased code complexity.

Other types of acknowledgement modes are possible. However, these acknowledgement modes are JMS provider specific, and therefore, compromise the JMS application portability.

Transaction options:

·Transacted session: An application can participate in a transaction by creating a transacted session (or local transaction). The application completely controls the message delivery by either committing or rolling back the session.

·Message-driven beans with CMTD: An MDB can participate in a container transaction by specifying CMTD in the XML deployment descriptor. The transaction commits upon successful message processing or the application can explicitly roll it back.

·Message-driven beans with BMTD: An MDB can choose not to participate in a container transaction by specifying BMTD in the XML deployment descriptor. The MDB programmer has to design and code programmatic transactions.

7. What are Message delivery stages in JMS?

Toward the end of delivery, the message conceptually passes through the following stages: message with JMS provider and message in application processing.

Message with JMS provider   In this stage, the message stays with the JMS provider just before the provider delivers it to the application. Consider a catastrophic situation where the JMS provider fails. What happens to the messages that the provider has not yet delivered to the client? Will the messages be lost?
The messages' fate depends not upon the transaction options outlined earlier, but rather upon the delivery mode. There are two delivery modes: nonpersistent and persistent. Messages with nonpersistent delivery modes are potentially lost if the JMS provider fails. Messages with persistent delivery modes are logged and stored to a stable storage. The JMS provider saves these messages to a stable storage, such as a database or a file system, and eventually delivers them to the application for processing.
   producer.setDeliveryMode(DeliveryMode.NON_PERSISTENT);
   producer.setTimeToLive(60000);

Message in application processing In this stage, the application receives the message from the JMS provider and processes it. Consider a failure occurring during message processing. What happens to the message? Will the message be lost or redelivered for successful processing later? The answers to these questions depend upon the transaction options you choose.

8. What is Client acknowledgement in JMS?

To implement client acknowledgement mode, when you create the receiver's session, specify false as createSession()'s first argument and Session.CLIENT_ACKNOWLEDGE as its second argument. Specifying false creates a nontransacted session. In client mode, invoking the Message class's acknowledge() method explicitly acknowledges the message. In fact, using the acknowledge() method makes sense when only using the client mode.

9. How to create Transacted session?

To implement the transacted session mode, when creating the receiver's session, specify true as createSession()'s first argument. You ignore the createSession() method's second argument; to clearly denote its lack of use, pass in a dummy value such as -1.

The application indicates successful message processing by invoking the Session class's commit() method. The application can reject a message or a message group by invoking Session class's rollback() method. Calling the commit() method commits all the messages the session receives. Similarly, calling the rollback() method rejects all the messages the session receives. Session's commit() and rollback() methods make sense only with the transacted session option. The transacted session uses a chained-transaction model. In a chained-transaction model, an application does not explicitly start a transaction. Upon calling either the commit() or the rollback() methods, the application automatically starts a new transaction. Because a transaction is not explicitly started, it is always present and available.

10. What is Durable subscription?

You must not cache session handles in stateless session beans that operate in transactions started by a client of the bean. Caching handles in this way causes the bean to be returned to the pool while the session is still involved in the transaction. Also, you should not cache non-durable subscribers due to the restriction mentioned above.

A non-durable subscriber can only be used in the same transactional context (for example, a global transaction or an unspecified transaction context) that existed when the subscriber was created. For more information about this context restriction, see The effect of transaction context on non-durable subscribers.

Using durable subscriptions with the default messaging provider. A durable subscription on a JMS topic enables a subscriber to receive a copy of all messages published to that topic, even after periods of time when the subscriber is not connected to the server. Therefore, subscriber applications can operate disconnected from the server for long periods of time, and then reconnect to the server and process messages that were published during their absence. If an application creates a durable subscription, it is added to the runtime list that administrators can display and act on through the administrative console.

Each durable subscription is given a unique identifier, clientID##subName where:

clientID

The client identifier used to associate a connection and its objects with the messages maintained for applications (as clients of the JMS provider). You should use a naming convention that helps you identify the applications, in case you need to relate durable subscriptions to the associated applications for runtime administration.

subName

The subscription name used to uniquely identify a durable subscription within a given client identifier.

For durable subscriptions created by message-driven beans, these values are set on the JMS activationSpec. For other durable subscriptions, the client identifier is set on the JMS connection factory, and the subscription name is set by the application on the createDurableSubscriber operation.

To create a durable subscription to a topic, an application uses the createDurableSubscriber operation defined in the JMS API:

public TopicSubscriber createDurableSubscriber(Topic topic,

java.lang.String subName,

java.lang.String messageSelector,

boolean noLocal)

throws JMSException

Topic

The name of the JMS topic to subscribe to. This is the name of an object supporting the javax.jms.Topic interfaces, such as found by looking up a suitable JNDI entry.

subName

The name used to identify this subscription.

messageSelector

Only messages with properties matching the message selector expression are delivered to consumers. A value of null or an empty string indicates that all messages should be delivered.

noLocal

If set to true, this prevents the delivery of messages published on the same connection as the durable subscriber.

Applications can use a two argument form of createDurableSubscriber that takes only topic and subName parameters. This alternative call directly invokes the four argument version shown above, but sets messageSelector to null (so all messages are delivered) and sets noLocal to false (so messages published on the connection are delivered). For example, to create a durable subscription to the topic called myTopic, with the subscription name of mySubscription.

11. Whats is a subtle difference between a "durable" JMS message and a "persistent" JMS message ?
A "durable" JMS message is applicable only to publish/subscribe paradigm and the "persistent" JMS message is applicable to both "Point-to-Point" & "publish/subcribe" paradigms.
Let's look at both:
A " durable message " is a message where the JMS server will hold on to a message if the subscriber is temporarily unavaliable. So the durability is defined by the relationship between a "Topc Subscriber" and the "JMS Server". Durability is applicable only to publish/Subscribe paradigm. For this to happen subscribers need to register themselves with a unique " client id ".
A " persistent message " is a message that defines the relationship between a "Message Producer" and the "JMS Server". This can be established for both
point-to-point and publish/subscribe. This has to do with the guaranteed once only delivery of the message by persisting the message after it has been recieved from the message producer.

12. How to create Temporary Destinations ?

Normally, you create JMS destinations--queues and topics--administratively rather than programmatically. Your JMS provider includes a tool that you use to create and remove destinations, and it is common for destinations to be long-lasting. The JMS API also enables you to create destinations--TemporaryQueue and TemporaryTopic objects--that last only for the duration of the connection in which they are created. You create these destinations dynamically using the Session.createTemporaryQueue and the Session.createTemporaryTopic methods.

The only message consumers that can consume from a temporary destination are those created by the same connection that created the destination. Any message producer can send to the temporary destination. If you close the connection that a temporary destination belongs to, the destination is closed and its contents are lost.

You can use temporary destinations to implement a simple request/reply mechanism. If you create a temporary destination and specify it as the value of the JMSReplyTo message header field when you send a message, then the consumer of the message can use the value of the JMSReplyTo field as the destination to which it sends a reply. The consumer can also reference the original request by setting the JMSCorrelationID header field of the reply message to the value of the JMSMessageID header field of the request. For example, an onMessage method can create a session so that it can send a reply to the message it receives. It can use code such as the following:

producer = session.createProducer(msg.getJMSReplyTo());

replyMsg = session.createTextMessage("Consumer " +"processed message: " + msg.getText());

replyMsg.setJMSCorrelationID(msg.getJMSMessageID());

producer.send(replyMsg);