Following are the key components of persistence API in EJB:
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Entity - A persistent object representing the data-store record. It is good to be serializable.
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EntityManager - Persistence interface to do data operations like add/delete/update/find on persistent object(entity). It also helps to execute queries using Query interface.
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Persistence unit (persistence.xml) - Persistence unit describes the properties of persistence mechanism.
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Data Source (*ds.xml) - Data Source describes the data-store related properties like connection url. user-name,password etc.