Enterprise Service Bus

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ESB
It has a set of service containers, used to adapt a wide variety of IT assets to the ESB.
It has a reliable messaging system, used to allow the service containers to interact.
It has a standard (WSDL) services model is used for inter-container interaction. That is, all adapted assets are modelled using services. An asset can be a provider of services, a consumer of services, or both. The services model is based on message exchange.
It uses messages that are exchanged between containers using standard message exchange patterns.
It uses messages that consist of XML data, and message metadata.
It provides message transformation services
It provides message routing services
It provides security features to control access to services
It is centrally administered, despite being a distributed system.
It allows incremental changes to services without requiring shutdown or other disturbance to system availability.