Hai LQ
I am practing ldap in RHEL5. My question is that what is different between backend and database? Both backend and database are using bdb,ldbm,ldap and so on.

frontend and backend are words used to describe a system which consists of 2 or more computers. The frontend is the computer which interacts with the user. The backend is the computer which does most of the processing. So typically a user's request for service is entered into the frontend. The frontend passes the request to the appropriate backend which does the processing necessary to satisfy the request. The backend then signals the frontend that the task is finished and the frontend passes the results back to the user.

The software types used by the frontend and the backend is not relevant to the definition of frontend and backend.

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Steve Stites

More generally, the frontend is the portion of the system that faces the user and the backend is the portion that manipulates the data/does the processing. This is regardless of the number of computers involved; could all be on one computer.