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Every description on the Message Bus architecture seems to describe a centralised intermediary for routing messages:

Each application has a single connection to the message bus instead of multiple dedicated connections to each of the other applications.

http://msdn.microsoft.com/en-us/library/ff647328.aspx

Structure the connecting middleware between these applications as a Message Bus that enables them to work together using messaging.

http://www.eaipatterns.com/MessageBus.html

Service Bus is messaging infrastructure that sits between applications allowing them to exchange messages in a loosely coupled way for improved scale and resiliency.

http://www.windowsazure.com/en-us/home/features/messaging/

The descriptions seem identical to the Message Broker architecture; leading me to believe the following:

The term 'Message Bus' is effectively the same as 'MoM' - a generic term for referring to a messaging infrastructure without explicitly mentioning a topology. I.e. Message Broker is a type of Message Bus.

This seems like a controversial topic, and whilst there's a few similar questions on SO, none have (IMO) definitive answers.

P.s. I am aware that Enterprise Service Bus is entirely different, and is effectively a 'feature rich' MoM.

Lawrence Wagerfield
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3 Answers3

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You are right, this area of terminology is very messy and there are no definitive answer, but this is my view from my professional life:

The first thing is to do is to think about this in different levels of abstractions - architectural concepts vs products.

A message bus (http://www.eaipatterns.com/MessageBus.html) is often referd to a as an architectural princip to decouple applications often together with canonical data formats (http://www.eaipatterns.com/CanonicalDataModel.html), much regardless of technology. Often implemented using MoM products.

A MoM typically refers to a family of middleware products that implements asynchronous message passing. Often built supporting standards such as JMS and AMQP (typically WebSphere MQ, ActiveMQ, Azure messaging, hornetQ, RabbitMQ, etc.etc.).

A message broker is often a MoM, and both terms are used for the same messaging product. There might be confusion here, since the main ESB product of IBM is called WebSphere Message Broker while it rather an ESB used side by side with the MoM product - WebSphere MQ. Usually most MoM call themselves brokers, such as RabbitMQ, ActiveMQ, QPid etc.

Hope this did not cause more confusion :)

Petter Nordlander
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The differences are mainly different vendors trying to differentiate themselves in the Market and increase their mind-share. Since vendors typically defined their own terms, there will usually be a slight differences in what each vendor means by the terms they use.

However in general they are the same class or type of system. They organise and manage the transport, queueing and persistence (storage) of messages from one software service to another, beyond that, you need to be looking at vendors specific feature lists for their product.

Martin Spamer
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WebSphere Message Broker belongs to the ESB layer of an SOA and it is also MOM. WebSphere MQ is also MOM. I think a lot of the confusion is around abstract architecture concepts and actual software products. SOA and ESB are abstract concepts. The whole enterprise architecture industry IBM, Microsoft, Oracle, TIBCO etc. are always vying for market share. Software companies often pretend they invented something new and better in order to get attention from their customers. They make a lot of wild promises about their products when in reality they create a whole new set of expensive problems (e.g. XML transformations cause serious performance and security issues). Because it is corporate greed driven and "get to market fast" there is always an abundance of confusion and IT problems and much stress for IT workers.

user916115
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