{"id":218,"date":"2017-05-05T20:22:17","date_gmt":"2017-05-05T20:22:17","guid":{"rendered":"http:\/\/www.morsello.com\/?p=218"},"modified":"2017-05-05T20:22:17","modified_gmt":"2017-05-05T20:22:17","slug":"remembering-fail-safe","status":"publish","type":"post","link":"https:\/\/www.morsello.com\/index.php\/2017\/05\/05\/remembering-fail-safe\/","title":{"rendered":"Remembering Fail-Safe"},"content":{"rendered":"

The description \u201cfail safe\u201d is commonly used to mean something foolproof, or a system with backup systems to prevent failure.\u00a0 <\/span>In other words, \u201csafe from failure\u201d.<\/span><\/p>\n

That\u2019s a shame, since we have plenty of words that already mean that. \u00a0 <\/span>My dictionary defines fail-safe as \u2026 a system \u2026 that insures safety if the system fails to operate\u00a0 <\/span>properly.\u00a0 <\/span><\/i>The original meaning meant \u201csafe in case<\/i> of failure\u201d.\u00a0 <\/span>Things break.\u00a0 <\/span>How do we head off catastrophe?<\/span><\/p>\n

Real World examples<\/b><\/span><\/p>\n

The TCP network protocol \u201cguarantees\u201d delivery, but it\u2019s fail-safe.\u00a0 <\/span>If a packet can\u2019t be delivered, as happens, the connection is dropped rather than either accepting partial or corrupted data.<\/span><\/p>\n

In the movie Die Hard<\/i>, the engineers of Nakatomi Plaza decided that safety meant that in the event of a power failure all the security systems of the building would be dis<\/i>-abled.\u00a0 <\/span>In the movie that meant the bad guys could get into the vault.\u00a0 <\/span>In the real world, that decision would prevent people from being locked into the building.<\/span><\/p>\n

After thousands of deaths resulting from train accidents, train car brakes are now engaged<\/i> by default.\u00a0 <\/span>A pressure line powered by the locomotive pulls the brake pads away from the wheels.\u00a0 <\/span>In the event that any of the braking system (the non-braking system?) fails, the brakes are pressed against the wheels.<\/span><\/p>\n

Airplanes use positive indicators for the status of important functions such as the landing gear being down.\u00a0 <\/span>Instead of an error light if the gear has failed, there\u2019s a no-error light if the gear is locked.\u00a0 <\/span>Should the sensor, wiring, or bulb fail, the indication is that gear is not<\/i> down.\u00a0 <\/span>Better to have gear down and think it\u2019s not than think it is when it isn\u2019t.<\/span><\/p>\n

Value in software<\/b><\/span><\/p>\n

This idea that we should expect failure isn\u2019t novel, it\u2019s called testing<\/i>.\u00a0 <\/span>But arguably the primary purpose of testing is to identify defects in the software to avoid failure in production.\u00a0 <\/span>Is there value in assuming that we won\u2019t be successful at preventing every possible anomalous condition, including that our code does what we expect?\u00a0 <\/span>Consider the questions that fail safe<\/i> raises?<\/span><\/p>\n

What can fail?<\/span><\/p>\n

Your software has bugs in it.\u00a0 <\/span>Networks go down.\u00a0 <\/span>You may get broken input.\u00a0 <\/span>You may get correct input that breaks your system because you didn\u2019t know the correct format.\u00a0 <\/span>You may get data in the wrong order.\u00a0 <\/span>Software you didn\u2019t write but you\u2019re counting on may fail.<\/span><\/p>\n

What is \u201csafe\u201d?<\/span><\/p>\n

What\u2019s the best result when failure happens?\u00a0 <\/span>Roll back a transaction?\u00a0 <\/span>Immediately kill a system?\u00a0 <\/span>Display an error?\u00a0 <\/span>Throw an exception?<\/span><\/p>\n

How we get back from \u201csafe\u201d to operational again?<\/span><\/p>\n

Once having decided what failure means and how to entire a safe mode, we may not have asked ourselves before how to get things going again.\u00a0 <\/span>If we reject entry of a file that contains erroneous data, how do we notify someone to deal with that?\u00a0 <\/span>How do we get it out of a queue to be processed again?<\/span><\/p>\n

\n","protected":false},"excerpt":{"rendered":"

The description \u201cfail safe\u201d is commonly used to mean something foolproof, or a system with backup systems to prevent failure.\u00a0 In other words, \u201csafe from failure\u201d. That\u2019s a shame, since we have plenty of words that already mean that. \u00a0 My dictionary defines fail-safe as \u2026 a system \u2026 that insures safety if the system…<\/p>\n

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