Authentication process on databases

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Ex. There may be a number of applications like a OLTP with a front end interface, A warehouse with reporting tool, a document management tool etc each of which has a separate database instance. Now how does a single signon work in a three tier architecture under the above scenario. 

 

Further, I want to understand the pros and cons of using multiple userid / passwords to access database as opposed to authenticating userid / passwords at the OS level or using some third party tool and the application server accessing the database using a single user id only. 

 

 

Please Help on this...

 

Thanks,