Differences between hard real-time, soft real-time, and firm real-time?

Question

I have read the definitions for the different notions of real-time, and the examples provided for hard and soft real-time systems make sense to me. But, there is no real explanation or example of a firm real-time system. According to the link above:

Firm: Infrequent deadline misses are tolerable, but may degrade the system's quality of service. The usefulness of a result is zero after its deadline.

Is there a clear distinction between firm real-time vs. hard or soft real-time, and is there a good example that illustrates that distinction?

In comments, Charles asked that I submit tag wikis for the new tags. The example of a "firm real-time system" I provided for the firm-real-time tag was a milk serving system. If the system delivers milk after its expiration time, then the milk is considered "not useful". One can tolerate eating cereal without milk, but the quality of the experience is degraded.

This is just the idea I formed in my head when I initially read the definition. I am looking for a much better example, and perhaps a better definition of firm real-time that will improve my notion of it.

Answer 1

Hard Real-Time

The hard real-time definition considers any missed deadline to be a system failure. This scheduling is used extensively in mission critical systems where failure to conform to timing constraints results in a loss of life or property.

Examples:

• Air France Flight 447 crashed into the ocean after a sensor malfunction caused a series of system errors. The pilots stalled the aircraft while responding to outdated instrument readings. All 12 crew and 216 passengers were killed.

• Mars Pathfinder spacecraft was nearly lost when a priority inversion caused system restarts. A higher priority task was not completed on time due to being blocked by a lower priority task. The problem was corrected and the spacecraft landed successfully.

• An Inkjet printer has a print head with control software for depositing the correct amount of ink onto a specific part of the paper. If a deadline is missed then the print job is ruined.

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Firm Real-Time

The firm real-time definition allows for infrequently missed deadlines. In these applications the system can survive task failures so long as they are adequately spaced, however the value of the task's completion drops to zero or becomes impossible.

Examples:

• Manufacturing systems with robot assembly lines where missing a deadline results in improperly assembling a part. As long as ruined parts are infrequent enough to be caught by quality control and not too costly, then production continues.

• A digital cable set-top box decodes time stamps for when frames must appear on the screen. Since the frames are time order sensitive a missed deadline causes jitter, diminishing quality of service. If the missed frame later becomes available it will only cause more jitter to display it, so it's useless. The viewer can still enjoy the program if jitter doesn't occur too often.

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Soft Real-Time

The soft real-time definition allows for frequently missed deadlines, and as long as tasks are timely executed their results continue to have value. Completed tasks may have increasing value up to the deadline and decreasing value past it.

Examples:

• Weather stations have many sensors for reading temperature, humidity, wind speed, etc. The readings should be taken and transmitted at regular intervals, however the sensors are not synchronized. Even though a sensor reading may be early or late compared with the others it can still be relevant as long as it is close enough.

• A video game console runs software for a game engine. There are many resources that must be shared between its tasks. At the same time tasks need to be completed according to the schedule for the game to play correctly. As long as tasks are being completely relatively on time the game will be enjoyable, and if not it may only lag a little.

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^{Siewert: Real-Time Embedded Systems and Components.
Liu & Layland: Scheduling Algorithms for Multiprogramming in a Hard Real-Time Environment.
Marchand & Silly-Chetto: Dynamic Scheduling of Soft Aperiodic Tasks and Periodic Tasks with Skips.}