As Tessitura RAMP access is down for the second time this month and fourth time since we put tickets on sale in January, I'd like to hear what if any contingency plans others have set up for system downtime. We've only had one instance of being down during our actual Festival, but we did have a few hours during our Donor Presale that we were down and it was a nightmare.
That's where the TTL values come into play. If you have a DNS record for www.myorganization.org that points to your website, and that record has a TTL of 15 minutes any client that has that record cached will do a DNS lookup the next time the client attempts to access the site if more than 15 minutes has transpired since the last lookup.
It's true that other DNS servers may cache the records as well. But... they should be respecting the TTL also.
There was a time when there were quite a few poorly configured and managed DNS servers out there which could cause problems. However, DNS hacks and other issues over the years have done a lot to clean this up. We've found that a 15 minute TTL will be very well propagated within that time frame.