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# Glossary

This glossary provides information about terms and concepts you might encounter while using Message Queue.

acknowledgement

Control messages exchanged between clients and broker to ensure reliable delivery. There are two general types of acknowledgement: client acknowledgements and broker acknowledgements.

administered objects

A pre-configured object—a connection factory or a destination—that encapsulates provider-specific implementation details, and is created by an administrator for use by one or more JMS clients. The use of administered objects allows JMS clients to be provider-independent. Administered objects are placed in a JNDI name space by and are accessed by JMS clients using JNDI lookups.

asynchronous messaging

An exchange of messages in which the sending of a message does not depend upon the readiness of the consumer to receive it. In other words, the sender of a message need not wait for the sending method to return before it continues with other work. If a message consumer is busy or offline, the message is sent and subsequently received when the consumer is ready.

authentication

The process by which only verified users are allowed to set up a connection to a broker.

authorization

The process by which a message service determines whether a user can access message service resources, such as connection services or destinations, to perform specific operations supported by the message service.

broker

The Message Queue entity that manages message routing, delivery, persistence, security, and logging, and that provides an interface for monitoring and tuning performance and resource use.

client

An application (or software component) that interacts with other clients using a message service to exchange messages. The client can be a producing client, a consuming client, or both.

client identifier

An identifier that associates a connection and its objects with a state maintained by the Message Queue broker on behalf of the client.

client runtime

Message Queue software that provides messaging clients with an interface to the Message Queue message service. The client runtime supports all operations needed for clients to send messages to destinations and to receive messages from destinations.

cluster

Two or more interconnected brokers that work in concert to provide scalable messaging services. In the event of failover and reconnection, conventional clusters provide service availability; enhanced clusters provide service and data availability.

cluster connection service

A private protocol that enables brokers in a cluster to provide reliable, synchronized service.

connection

A communication channel between a client and a broker used to pass both payload messages and control messages.

connection factory

The administered object the client uses to create a connection to a broker. This can be a `ConnectionFactory` object, a `QueueConnectionFactory` object or a `TopicConnectionFactory` object.

consumer

An object (`MessageConsumer`) created by a session that is used for receiving messages sent from a destination. In the point-to-point delivery model, the consumer is a receiver or browser (`QueueReceiver` or `QueueBrowser`); in the publish/subscribe delivery model, the consumer is a subscriber (`TopicSubscriber`).

data store

A database where information (durable subscriptions, data about destinations, persistent messages, auditing data) needed by the broker is permanently stored.

dead message

A message that is removed from the system for a reason other than normal processing or explicit administrator action. A message might be considered dead because it has expired, because it has been removed from a destination due to memory limit overruns, or because of failed delivery attempts. You can choose to store dead messages on the dead message queue.

dead message queue

A specialized destination created automatically at broker startup that is used to store dead messages for diagnostic purposes.

delivery mode

An indicator of the reliability of messaging: whether messages are guaranteed to be delivered and successfully consumed once and only once (persistent delivery mode) or guaranteed to be delivered at most once (non-persistent delivery mode).

delivery model

The model by which messages are delivered: either point-to-point or publish/subscribe. In JMS there are separate programming domains for each, using specific client runtime objects and specific destination types (queue or topic), as well as a unified programming domain.

destination

The physical destination in a Message Queue broker to which produced messages are delivered for routing and subsequent delivery to consumers. This physical destination is identified and encapsulated by an administered object that a client uses to specify the destination for which it is producing messages and/or from which it is consuming messages.

domain

A set of objects used by JMS clients to program JMS messaging operations. There are two programming domains: one for the point-to-point delivery model and one for the publish/subscribe delivery model.

encryption

A mechanism for protecting messages from being tampered with during delivery over a connection.

group

The group to which the user of a Message Queue client belongs for purposes of authorizing access to connections, destinations, and specific operations.

JMS provider

A product that implements the JMS interfaces for a messaging system and adds the administrative and control functions needed to configure and manage that system.

message service

A middleware service that provides asynchronous, reliable exchange of messages between distributed components or applications. It includes a broker, the client runtime, the several data stores needed by the broker to carry out its functions, and the administrative tools needed to configure and monitor the broker and to tune performance.

messages

Asynchronous requests, reports, or events that are consumed by messaging clients. A message has a header (to which additional fields can be added) and a body. The message header specifies standard fields and optional properties. The message body contains the data that is being transmitted.

messaging

A system of asynchronous requests, reports, or events used by enterprise applications that allows loosely coupled applications to transfer information reliably and securely.

producer

An object `(MessageProducer)` created by a session that is used for sending messages to a destination. In the point-to-point delivery model, a producer is a sender (`QueueSender`); in the publish/subscribe delivery model, a producer is a publisher (`TopicPublisher`).

queue

An object created by an administrator to implement the point-to-point delivery model. A queue is always available to hold messages even when the client that consumes its messages is inactive. A queue is used as an intermediary holding place between producers and consumers.

selector

A message header property used to sort and route messages. A message service performs message filtering and routing based on criteria placed in message selectors.

session

A single threaded context for sending and receiving messages. This can be a queue session or a topic session.

topic

An object created by an administrator to implement the publish/subscribe delivery model. A topic may be viewed as node in a content hierarchy that is responsible for gathering and distributing messages addressed to it. By using a topic as an intermediary, message publishers are kept separate from message subscribers.

transaction

An atomic unit of work that must either be completed or entirely rolled back.