What's the difference between reliability and resiliency?

I was asked this question during a recent interview, and the answer I settled on was this:

Reliability is the measure of how well you keep your site up. Resiliency is what happens when it crashes.

Between the two, reliability seems to get all the attention from leadership (and the money), but at the end of the day, your system's health depends on its resilience. No matter how many nines you tacked on to your app's uptime, sooner or later, something will happen to bring it down (just this week, in fact, both Twitter and Slack went down at the same time, now where are you going to go to complain about that outage?). When your system does go down, the systems you have in place to ensure a prompt recovery can spell the difference between a non-issue and an unmitigated nightmare.

The Jewish people are no strangers to resiliency and bouncing back from hard times. Our entire history is a cyclical story of tragedy and triumphs, mourning and celebration.

This week alone, we went through both extremes.

Sunday was the 9th of Menachem Av, the saddest day on the Jewish calendar, the day both Jewish temples in Jerusalem were destroyed, starting the exile and diaspora we still experience today. It is a day of fasting and mourning, a day so sad it gives the whole month of Av a bad name ("When the month of Av comes in, we decrease in joy").

But then, on Friday is the 15th of Av, a day the Gemarah says, "there have been no holidays for the Jewish people greater than the 15th of Av."

It is no coincidence that the 15th of the month in the Jewish calendar coincides with the full moon.

The Medrash compares the Jewish people to the moon. The moon goes through cycles, every month the moon gets smaller and smaller in our view until it completely disappears, only to reappear and grow larger and larger until the next full moon.

Similarly, the Jewish people go through times when we are diminished, only to bounce back stronger than ever, and the greater the diminishment, the greater the comeback afterward. That is why the full moon after the 9th of Av is the day the sages referred to as "there were no holidays greater for the Jewish people."

This is the message of the 15th of Av, a message of resiliency.