"The simplest form of servers are disk servers and file servers. In a file server, the client passes requests for files or file records over a network to the file server. The more advanced form of servers are database servers, transaction server and application servers. In database servers, clients passes SQL request 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. In 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 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, all the time it requires tight controls over the security and integrity of the database. 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."
