Difference Between Soft, Hard and Firm Real-Time System

A real-time system can be classified into either hard, soft, or firm real-time system based on the consequences of the task missing the deadline. It is not necessary that all tasks of a real-time application belong to the same category but it is possible that different tasks of a real-time system can belong to different categories.

Soft Real-Time System

A soft real-time system is one in which performance is degraded but not destroyed by failure to meet response-time constraints.

If several tasks miss deadline, then the performance of the system is said to have degraded.

An example of a soft real-time task is web browsing. Normally, after an URL (Uniform Resource Locater) is clicked, the corresponding web page is fetched and displayed within a couple of seconds on the average. However, when it takes several minutes to display a requested page, we still do not consider the system to have failed, but merely express that the performance of the system has degraded.

  

Hard Real-Time System

A hard real-time system is one in which failure to meet even a single deadline may lead to complete or catastrophic system failure.

The task deadlines are in the order of micro or milliseconds.

Many hard real-time systems are safety critical

Examples: Industrial Control Applications, On-board computers, Robots

Firm Real-Time System

A firm real-time system is one in which a few missed deadlines will not lead to total failure, but missing more than a few may lead to complete or catastrophic system failure.

Every firm real-time task is associated with some predefined deadline before which it is required to produce its results. However, unlike a hard real-time task, even a firm real-time task does not complete within its deadline, the system doesn’t fail but the late results are merely discarded. In other words, the utility of the results computed by a firm real-time task becomes zero after the deadline. 

Examples: A video conferencing application, Satellite-based surveillance applications

A Sampling of Hard, Firm, and Soft Real-Time Systems

System	Real-Time Classification	Explanation
Avionic weapons delivery  system in which pressing a  button launches air-to-air  missile	Hard	Missing the deadline to launch  the missile within a specified  time after pressing the button  may cause the target to be  missed, which will result in  catastrophe.
Navigation controller for an  autonomous weed-killer robot	Firm	Missing a few navigation  deadlines causes the robot to  veer out from a planned path  and damage some crops.
Console hockey game	Soft	Missing even a several  deadlines will only degrade  performance
The Library of Congress print- manuscript database system	Soft	The database system is being  used for research purpose and  it limits its urgency. It seems it  would be acceptable for  missed deadlines to degrade  the system.
A police database that  provides information about  stolen automobile	Soft	If the system miss the  deadline  to give the result, it  will not  result in system failure and the  late result would only degrade  the performance.
An Automatic Teller Machine	Soft	As long as the ATM is  executing timely even though  it  miss deadline frequently it’s  output will still have value
A coin operated video game	Soft	Missing even a several  deadlines will only degrade  performance
A university grade-processing  system	Hard	Although the system may  have  response times of days or  even  weeks, it must respond within  certain time or there could be  an academic or financial  disaster.
Video Conferencing	Firm	When a certain frame is being  played, if some preceding  frame arrives at the receiver,  then this frame is of no use  and is discarded.
Satellite based tracking of  enemy movements	Firm	When the ground computer is  being overloaded, an new  image may be received even  before an older image is taken  up for processing. In this case,  the older image is of not much  use.