Transaction processing that occurs interactively with the end user is referred to as online transaction processing or OLTP.

One of the main characteristics of a transaction system is that the interactions between the user and the system are very short. The user will perform a complete business transaction through short interactions, with immediate response time required for each interaction. These systems are currently supporting mission-critical applications; therefore, continuous availability, high performance, and data protection and integrity are required.

Online transactions are familiar to most people. Examples include:

• ATM machine transactions such as deposits, withdrawals, inquiries, and transfers
• Supermarket payments with debit or credit cards
• Purchase of merchandise over the Internet

For example, inside a bank branch office or on the Internet, customers are using online services when checking an account balance or directing fund balances.

In fact, an online system performs many of the same functions as an operating system:

• Managing and dispatching tasks
• Controlling user access authority to system resources
• Managing the use of memory
• Managing and controlling simultaneous access to data files
• Providing device independence

Some industry uses of mainframe-based online systems include:

• Banks – ATMs, teller systems for customer service
• Insurance – Agent systems for policy management and claims processing
• Travel and transport – Airline reservation systems
• Manufacturing – Inventory control, production scheduling
• Government – Tax processing, license issuance and management

How might the end users in these industries interact with their mainframe systems? Multiple factors can influence the design of a company's transaction processing system, including:

• Number of users interacting with the system at any one time.
• Number of transactions per second (TPS).
• Availability requirements of the application. For example, must the application be available 24 hours a day, seven days a week, or can it be brought down briefly one night each week?

Before personal computers and intelligent workstations became popular, the most common way to communicate with online mainframe applications was with 3270 terminals. These devices were sometimes known as "dumb" terminals, but they had enough intelligence to collect and display a full screen of data rather than interacting with the computer for each keystroke, saving processor cycles. The characters were green on a black screen, so the mainframe applications were nicknamed "green screen" applications.

Based on these factors, user interactions vary from installation to installation. With applications now being designed, many installations are reworking their existing mainframe applications to include Web browser-based interfaces for users. This work sometimes requires new application development, but can often be done with vendor software purchased to "re-face" the application. Here, the end user often does not realize that there is a mainframe behind the scenes.

In this section, there is no need to describe the process of interacting with the mainframe through a Web browser, as it is exactly the same as any interaction a user would have through the Web. The only difference is the machine at the other end!

Online transactions usually have the following characteristics:

• A small amount of input data, a few stored records accessed and processed, and a small amount of data as output
• Immediate response time, usually less than one second
• Large numbers of users involved in large numbers of transactions
• Round-the-clock availability of the transactional interface to the user
• Assurance of security for transactions and user data

In a bank branch office, for example, customers use online services when checking an account balance or making an investment.

Figure 1. Typical online use