Throughput Requirements

Definition

A throughput requirement is a performance requirement that specifies a minimum amount of the performance quality subfactor throughput.

Objectives

The typical objectives of a throughput requirement are to:

• Ensure that the application or component successfully supports the necessary frequency of:
  • Use case paths.
  • Transactions.
  • Interactions with externals.

Measurements

Throughput requirements are typically specified in terms of the following measurements:

• Minimum frequency.

Examples

The following are typical examples of throughput requirements:

• “The application shall be able to successfully process a minimum of 150,000 customer orders per day including credit card authorizations under average loads.”
• “For a minimum period of 7 days, the application shall be able to successfully process a minimum of 500,000 customer orders per day including credit card authorizations during periods of peak loading.”
• “The application shall be able to successfully handle 20,000 customer registrations per day.”
• “The missile avionics system shall update the position of the ailerons 20 times a second.”
• “At least 98% of the time, the application shall be able to successfully display the results of a keyword search in no more than 4 seconds.”
• “At least 98% of the time, the application shall be able to successfully scroll one page up or down in a 150 page document in no more than 1 second.”
• “At least 98% of the time, the application shall take no more than 0.2 seconds to change focus from a completed entry field to the next entry field in the same input screen.”
• “At least 98% of the time, the application shall take no more than 1.3 seconds to change focus from a completed entry field to the next entry field in the next input screen.”
• “At least 98% of the time, the application shall take no more than 15 seconds to display any of the required management reports.”

Guidelines

The following guidelines have been found to be useful when producing throughput requirements:

• The scope of a throughput requirement can be:
  • A business enterprise
  • An application
  • A component
  • A center
• Specify the maximum expected initial load (plus a reasonable fudge factor).
• Throughput times increase rapidly as load increases. Therefore, be careful about specifying maximum-permitted throughput requirements for all operations under all conditions (e.g., of load). Be realistic about specifying absolute limits that may be impossible or extremely expensive to achieve under extremely rare circumstances of maximum peak loads. Don't force the developers to sacrifice usability to provide an unusable “solution” that meets the letter (but not the spirit) of the requirement.
• Because different paths through use cases will involve different sets of interactions and processing, throughput may need to be specified in terms of use case paths rather than entire use cases.
• Because of the large number of system operations, use cases (and use case paths), it is impractical to specify separate throughput requirements for each. Only specify the truly significant throughput requirements, and specify them for large groups of functionality whenever practical.
• Ensure that the scope of the requirements is properly specified in cases where part of the throughput times is not under the control of the development organization. For example, the throughput time for an operation involving the Internet depends on Internet communication times. Whereas architects may group transactions in order to minimize the number of Internet messages, Internet communication time should not be specified. You can specify requirements for the application, but you cannot specify requirements for externals such as the Internet, external applications, and external users.
• Use statistical methods (e.g., Markov exponential distributions) to determine appropriate and feasible maximum-permitted average and maximum throughput times.
• Throughput requirements should not be confused with:
  • Capacity requirements, which deal with the storage of information.
  • Scalability requirements, which deal with future capacity and throughput requirements.
• Throughput requirements depend on the type of application as in (hard real-time, soft real-time, and not real-time).
• Some throughput requirements are based on human psychological limits:
  • The average typist can type continuously if movements between fields are less than 0.2 seconds.
  • The average typist takes approximately 1.35 seconds to mentally switch gears when moving from one set of typing to another (which usually occurs when data entry clerks move from one screen to the next).
  • The average person will wait for no more than 20 seconds before looking for something else to do (and less if we are talking about an Internet shopper).