Dynamic persistence switching

Question

I have a legacy application with multiple databases with identical schema [one for each customer]. I would like to switch the persitence objects [entity beans] dynamically, depending on the customer context. Is this 1) a bad idea and 2) doable?? This is a Sun Java Application server environmet [Netbeans 5.5 ide] with Toplink persistance engine...

Answer 1

Does each customer mean each user, or are there many users for each customer?
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2) doableIn general you can use a TopLink DatabaseSession for each user/customer. You can use the same TopLink Project but can use a different login for each session you create.
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If you have many users for each customer, you could potentially use a different TopLink ServerSession for each customer and allow the users to benifit from connection pooling and caching.
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If you are using JPA, there is no easy way to do this currently. You could create the ServerSession yourself and then wrap it yourself with the TopLink EnitityManager implementation. You could use the same TopLink Project from your JPA persistent unit ServerSession and use a different login in your new ServerSession. If each customer has only one user then you could use a connection pool with only 1 connection.
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1) a bad idea<p>
This is probably not a good idea. You avoid being able to share resources such as connection pools across your customers, and cannot share data. But it depends on how many customers you have and how distinct they are. Having each customer run on different databases allows you to have a different machine and application server for each customer, which you may need depending on how each customer scales. If you end up having a different server for each customer, then you could just have a single persistence unit in your application and change the persistence.xml or data-source when you deploy a new customer server.
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<p>James Sutherland