18.3. JCA resource adapters: rationale and principles
Using EJBs to interact with external resources via socket connections and the like has a number of complications.
The external resource may be limited in the number of clients it can support, for technical or pragmatic reasons. In this example, the quote server may be a subscription service, charged per concurrent connection. Concurrent access to the server may be required by many instances of a number of different EJBs. Some legacy systems may even be single-threaded and support only one connection at a time.
Even if connection concurrency is not, in itself, a problem, setting up the socket connection is a costly operation, particularly if the external resource is not part of the same network ...
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