OpenEJB was founded on the idea that it would be embedded into third-party environments whom would likely already have three things:
Thus the focus of OpenEJB was to create an EJB implementation that would be easily embeddible, configurable, and customizable.
Part of achieving that is a drive to be as simple as possible as to not over-define and therefore restrict the ability to be embeddible, configurable and customizable. Smaller third-party environments could easily 'downscale' OpenEJB in their integrations by replacing standard components with lighter implementations or removing them all together and larger environments could 'upscale' OpenEJB by replacing and adding heavier implementations of those standard components likely tailored to their systems and infrastructure.
Container and Server are mentioned in the EJB spec as being separate things but are never defined formally. In our world Containers, which implement the basic component contract and lifecycle of a bean are not coupled to any particular Server, which has the job of providing a naming service and providing a way for it's clients to reference and invoke components (beans) hosted in Containers. Because Containers have no dependence at all only Server, you can run OpenEJB without any Server at all in an embedded environment for example without any work or any extra overhead. Similarly you can add as many new Server components as you want without ever having to modify any Containers.
There is a very strong pluggability focus in OpenEJB as it was always intended to be embedded and customized in other environments. As a result all Containers are pluggable, isolated from each other, and no one Container is bound to another Container and therefore removing or adding a Container has no repercussions on the other Containers in the system. TransactionManager, SecurityService and Connector also pluggable and are services exposed to Containers. A Container may not be dependent on specific implementations of those services. Service Providers define what services they are offering (Container, Connector, Security, Transaction, etc.) in a file they place in their jar called service-jar.xml.
The service-jar.xml should be placed not in the META-INF but somewhere in your package hierarchy (ours is in /org/apache/openejb/service-jar.xml) which allows the services in your service-jar.xml to be referenced by name (such as DefaultStatefulContainer) or more specifically by package and id (such as org.apache.openejb#DefaultStatefulContainer).
The same implementation of a service can be declared several times in a service-jar.xml with different ids. This allows for you to setup several several different profiles or pre-configured versions of the services you provide each with a different name and different set of default values for its properties.
In your openejb.conf file when you declare Containers and Connectors, we are actually hooking you up with Service Providers automatically. You get what is in the org/apache/openejb/service-jar.xml by default, but you are able to point specifically to a specific Service Provider by the 'provider' attribute on the Container, Connector, TransactionManager, SecurityService, etc. elements of the openejb.conf file. When you declare a service (Container, Connector, etc.) in your openejb.conf file the properties you supply override the properties supplied by the Service Provider, thus you only need to specify the properties you'd like to change and can have your openejb.conf file as large or as small as you would like it. The act of doing this can be thought of as essentially instantiating the Service Provider and configuring that instance for inclusion in the runtime system.
For example Container(id=NoTimeoutStatefulContainer, provider=DefaultStatefulContainer) could be declared with it's Timeout property set to 0 for never, and a Container(id=ShortTimeoutStatefulContainer, provider=DefaultStatefulContainer) could be declared with it's Timeout property set to 15 minutes. Both would be instances of the DefaultStatefulContainer Service Provider which is a service of type Container.
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