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The current implementation of the Persistence Service SPI is provided in 'cps-ri' which is specific to relational DB. If we want to use another type of DB later, the Persistence and Repository layer needs to be re-written. Therefore, to achieve the original intent (use a different DB type later), it is important to keep the business logic in the 'Service' layer and have only database-specific logic in the 'Persistence' layer.
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