File servers. -With a file server, the Computer Network client passes requests for computer network files or file records over a computer network to the file server. This form of computer network data service requires large bandwidth and can slow a computer network with many users down considerably. Traditional LAN (Local area Network) computing allows users to share resources, such as data files and peripheral devices, by moving them from standalone PCUs onto a Networked File Server (NFS).
Database servers-In database servers, clients passes SQL (Structured Query Language) requests as messages to the server and the results of the query are returned over the network. The code that processes the SQL request and the data resides on the server allowing it to use its own processing power to find the requested data, rather than pass all the records back to a client and let it find its owndata as was the case for the file server.
Transaction servers- Clients invoke remote procedures that reside on servers which also contains an SQL database engine. There are procedural statements on the server to execute a group of SQL statements (transactions) which either all succeed or fail as a unit. The applications based on transaction servers are called On-line Transaction Processing (OLTP) and tend to be mission-critical applications which require 1-3 second response time, 100% of the time and require tight controls over the security and integrity of the database.
The communication overhead in this approach is kept to a minimum as the exchange typically consists of a single request/reply (as opposed to multiple SQL statements in database servers). Application servers are not necessarily database centered but are used to server user needs, such as. download capabilities from Dow Jones or regulating a electronic mail process. Basing resources on a server allows users to share data, while security and management services, which are also based in the server, ensure data integrity and security.