There used to be only two guarantees in life:  death and taxes.  In today’s complex IT environment, we can add a third:  Your production software systems will fail.  This is an absolute guarantee because the math is stacked against you.

Why?  Because failure rates are cumulative.  Simply put, five integrated systems each with 99.9% uptime yields an overall 99.5% uptime, unless you somehow figure out how to get them to fail at the same time.  Since you can’t do that, you will have a 5 failures for each 1000 runs of your program/service/job/etc.

That’s the math.  Better plan for failure.