Here at Itomic we’re upgrading some of our hosting servers, which means moving lots of websites from old servers to new servers. Naturally we attempt to minimise any “downtime” for every site we move using a variety of techniques, but inevitably a little downtime, typically only a few minutes, does occur.
Sometimes we can see the site on the new server before the site owner can – and vice versa, i.e. the site looks just fine for us, but still appears to be down (unreachable) for the owner of the website. The primary reason for this is that the updated information about where a website is hosted (stored) takes some time to “propagate” around the planet, i.e. for this new information to be shared amongst all the computers that need to know. Depending on various factors, it might take anywhere from seconds to hours for this to happen.
In the more technical words of Wikipedia: “As a noteworthy consequence of this distributed and caching architecture, changes to DNS records do not propagate throughout the network immediately, but require all caches to expire and refresh after the TTL.”
A website can be “down” for lots of reasons – not just in relation to it moving hosting servers. Regardless of the reason, here’s a handy online (and appropriately named!) tool to help you work out whether your site is down for everyone, or just you:
http://www.downforeveryoneorjustme.com/
So even if your site is down at your computer, it can be a little comforting to know that it’s up and running for lots of other people on the planet.