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Glossary

Here are brief explanations of the most important messaging related terms.

ActiveMQ

ActiveMQ (now known as "ActiveMQ Classic") is an open source messaging software. From its website:

Apache ActiveMQ Classic is a popular and powerful open source messaging and Integration Patterns server.

Despite the addition of Artemis under the ActiveMQ "umbrella", the old classic software is still being developed. The latest major version is 6, released at the end of 2023. See its download page for more information.

AMQP

AMQP (meaning Advanced Message Queuing Protocol) is a messaging protocol.

In fact, there are two very different major versions of AMQP:

  • 0-x (i.e. 0-8, 0-9, 0-10...) used mainly by RabbitMQ
  • 1.0 (officially "ISO/IEC 19464") supported by all major messaging software

Artemis

Artemis (now known as "ActiveMQ Artemis") is an open source messaging software. From its website:

The Next Generation Message Broker by ActiveMQ

In fact, it is a descendant of HornetQ and it is being developed alongside ActiveMQ Classic.

Body

See message body.

Broker

A message broker is an intermediary program that receives messages from senders and forwards them to receivers. It allows for asynchronous message delivery.

Most messaging software use brokers, with the exception of ZeroMQ.

See message header.

JMS

JMS (meaning Java Message Service) is a messaging API, not a wire protocol like AMQP.

It is widely used and its main models (point-to-point and publish/subscribe) are supported by all major messaging software.

Message

A message (as used in Message Oriented Middleware) consists of a header and a body.

The header is a collection of header fields.

Each header field is a key/value pair where the key and the value are text strings. The key is unique within the header so we can use a hash table (Perl) or dictionary (Python) to represent the header of the message.

The body is either a text string or a binary string. This distinction is needed because text may need to be encoded (for instance using UTF-8) before being stored on disk or sent across the network.

When putting data into a message, the data itself is often put in the body while information about the data (also known as meta-data) ends up in the header.

Messaging

From The Java EE Tutorial:

Messaging is a method of communication between software components or applications. A messaging system is a peer-to-peer facility: A messaging client can send messages to, and receive messages from, any other client. Each client connects to a messaging agent that provides facilities for creating, sending, receiving, and reading messages.

Messaging enables distributed communication that is loosely coupled. A component sends a message to a destination, and the recipient can retrieve the message from the destination. What makes the communication loosely coupled is that the destination is all that the sender and receiver have in common. The sender and the receiver do not have to be available at the same time in order to communicate. In fact, the sender does not need to know anything about the receiver; nor does the receiver need to know anything about the sender. The sender and the receiver need to know only which message format and which destination to use.

MQTT

MQTT (meaning Message Queuing Telemetry Transport) is a messaging protocol targeted at IoT devices.

OpenWire

OpenWire is a messaging protocol used only by ActiveMQ.

Point-to-point

From The Java EE Tutorial:

A point-to-point (PTP) product or application is built on the concept of message queues, senders, and receivers. Each message is addressed to a specific queue, and receiving clients extract messages from the queues established to hold their messages. Queues retain all messages sent to them until the messages are consumed or expire.

PTP messaging, has the following characteristics:

  • Each message has only one consumer.
  • The receiver can fetch the message whether or not it was running when the client sent the message.

Use PTP messaging when every message you send must be processed successfully by one consumer.

JMS queue

See also Wikipedia's message queue.

Publish/subscribe

From The Java EE Tutorial:

In a publish/subscribe (pub/sub) product or application, clients address messages to a topic, which functions somewhat like a bulletin board. Publishers and subscribers can dynamically publish or subscribe to the topic. The system takes care of distributing the messages arriving from a topic’s multiple publishers to its multiple subscribers. Topics retain messages only as long as it takes to distribute them to subscribers.

Pub/sub messaging has the following characteristics:

  • Each message can have multiple consumers.
  • A client that subscribes to a topic can consume only messages sent after the client has created a subscription, and the consumer must continue to be active in order for it to consume messages.

Use pub/sub messaging when each message can be processed by any number of consumers (or none).

JMS topic

See also Wikipedia's publish–subscribe pattern.

Queue

A message queue implements JMS' point-to-point model.

RabbitMQ

RabbitMQ is an open source messaging software. From its website:

RabbitMQ is a reliable and mature messaging and streaming broker, which is easy to deploy on cloud environments, on-premises, and on your local machine. It is currently used by millions worldwide.

STOMP

STOMP (meaning Streaming Text Oriented Messaging Protocol) is a lightweight messaging protocol.

Topic

A message topic implements JMS' publish/subscribe model.

Virtual Destinations

ActiveMQ's virtual destinations allow to mix the concepts of queues and topics.

Here is how they are used at CERN.

Producers simply see a topic and send messages to a destination like /topic/foo.

One ore more virtual queues are attached to it, for instance a test queue and a prod queue, corresponding to destinations like /queue/Consumer.test.foo and /queue/Consumer.prod.foo.

Consumers see two different queues and simply consume from any of them, like any other queue.

Each incoming message is duplicated and ends up in both queues.

WebSocket

WebSocket allows web applications to maintain bidirectional communications with server-side processes.

It is not a messaging protocol but it can wrap protocols like STOMP so that they can be used from a web browser.

See for instance ActiveMQ's WebSockets page for more practical information.

ZeroMQ

ZeroMQ (aka 0MQ) is a brokerless open source messaging software. From Wikipedia:

ZeroMQ is a high-performance asynchronous messaging library, aimed at use in distributed or concurrent applications. It provides a message queue, but unlike message-oriented middleware, a ZeroMQ system can run without a dedicated message broker.