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Petals ESB inherited from the JBI specification different kind of artifacts.
This page defines and illustrates these artifacts and their relations.
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h1. Components

What we call the Petals kernel is in fact a JBI container.
Like any other container, it can host components, that act as platform extensions.

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The notion of container and component is well-known in Computer Science and Software.

EJBs are JEE components, and need to run in a JEE container (this is the core of a JEE application server).
(OSGi) Bundles are OSGi components, and run in an OSGi runtime / container.
Fractal and SCA define other component models, each one running in a container.

From this point of view, JBI (and so Petals ESB) is completely similar to other component models.

* The [Petals BPEL component|Petals-SE-BPEL] is in charge of executing BPEL processes in Petals ESB.
* The list of the Petals components is available [here|Components User Guide].

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The pictures below try to illustrate the relations between the Petals container and its components.

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In Petals, a component is identified by its name, and configured by a JBI descriptor (a jbi.xml file).
A component can be directly deployed in Petals ESB.

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Petals being a JBI container, it can host any JBI component.
In the same way, all the Petals components are JBI components and can be deployed on any container that respect the JBI specification.

h1. Service-Units

Service-units are ZIP archive used to create services or define service consumers in Petals.
A service-unit always targets one component. This component must be deployed before any service-unit that would target it.
The content of the service-unit will be used to emulate a service (the component handles the messages and uses the service-unit resources and configuration to process the messages).

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In Petals, a service-unit is identified by its name, and configured by a JBI descriptor (a jbi.xml file).
A service-unit cannot be directly deployed in Petals ESB. Its deployment must be done through a service assembly.


h1. Service Assemblies

Service assemblies are ZIP archives that contain one or several service-units.
The service assembly is only a deployment unit. It associates service-units and target components.
All the target components described in a service assembly must be deployed before the assembly is deployed.

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Deploying several service-units within a single service assembly ties their life cycle.
More exactly, deploying one of these service-units means deploying them all. And vice-versa for the undeployment.

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In Petals, a service assembly is identified by its name, and configured by a JBI descriptor (a jbi.xml file).
A service assembly can be directly deployed in Petals ESB.

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Grouping several service-units per service assembly can be a good solution in integration projects.
In SOA projects, the best practice is rather to have only one service-unit per service assembly.
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h1. Shared Libraries

Shared libraries are ZIP archive that contain libraries that can be mutualized in the bus and shared among several components.

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In Petals, a shared library is identified by its name, and configured by a JBI descriptor (a jbi.xml file).
A shared library can be directly deployed in Petals ESB.