Network Applications

There are three categories of applications with regard to networks:

1. Stand alone applications – Includes editors

2. Network versions of stand alone applications – May be licensed for multiple users.

3. Applications only for a network include databases, mail, group scheduling, groupware. Models for network applications

1. Client-server – Processing is split between the client which interacts with the user and the server performing back end processing.

2. Shared file systems – The server is used for file storage and the processing of the file is done on the client computer.

3. Applications that are centralized – An example is a Telnet session. The data and the program run on the central computer and the user uses an interface such as the Telnet client or X server to send commands to the central computer and to see the results.

E-mail Systems

• Novell GroupWise – Also called Windows Messaging
• Microsoft Mail
• Microsoft Exchange – This is for the Microsoft Exchange Server. There is a Microsoft Exchange client for the Microsoft Exchange server and a client for an internet mail account only.
• Lotus Notes
• cc:Mail – From Lotus and IBM

There are several types of programs used in the process of sending and receiving mail. They are:

• MUA – Mail users agent. This is the program a user will use to type e-mail. It usually incorporates an editor for support. The user types the mail and it is passed to the sending MTA. This may also be called the user agent (UA).
• MTA – Message transfer agent is used to pass mail from the sending machine to the receiving machine. There is a MTA program running on both the sending and receiving machine. Sendmail is a MTA.
• MS – Message Store is a storage area for messages that can’t be delivered immediately when the recipient is off-line.
• AU – Access Unit provides access to resources like fax, telex, and teletex.
• LDA – Local delivery agent on the receiving machine receives the mail from its MTA. This program is usually procmail.
• Mail notifier – This program notifies the recipient that they have mail. Normally this requires two programs, biff and comsat. Biff allows the administrator or user to turn on comsat service.

Other components of mail service include:

• Directory services – A list of users on a system. Microsoft provides a Global Address List and a Personal Address Book.
• Post Office – This is where the messages are stored.

Database Management Systems (DBMS)

They are used to share data on a network. DBMS standards for distributed databases:

• SQL – Structured Query Language is a database access language. It is used by most client/server database applications.
• ODBC – Open Database Connectivity (ODBC) from Microsoft lets application developers integrate database connections in applications. It is an application programming interface (API). ODBC drivers convert an application’s query int SQL and send it to the database engine program.
• DRDA – Distributed Relational Database Architecture is from IBM.

When information is processed in a distributed database, it is called a transaction. The two phases of a transaction are:

1. Write or Update – The data is temporarily updated. An abort can cancel what this phase did by removing the changed data from a temporary storage area.

2. Commit – The changed data is made permanent in the database.

Databases store multiple copies of the data which is called replication. They must be sure the various copies of the database on various servers is accurate with identical data. Data is also partitioned into smaller blocks of data.