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Most DNS providers will allow you to access an interface to make these changes reactively. If you're planning to use DNS to provide this functionality, it is essential that you set the TTL for your domain name to a suitable value. (The default value for most pro‐ viders is 24 or 48 hours.) Some DNS providers do not allow changes to TTL records or have minimum values. More feature-rich managed DNS providers provide automated fail‐ over functionality that will constantly monitor the availability of sys‐ tems and update DNS records to alternative systems in the event that failure is identified. Support for Geolocation A simple way to mitigate the impact of latency is to serve content from as close to users as possible. This can be achieved by caching content close to user (see "CDNs" on page 20); however, it can also be achieved by hosting multiple systems at different locations around the world. Anycast Anycast is an addressing methodology that allows a "one-to- nearest" transmission of traffic to a target node, usually using BGP to simultaneously advertise the same IP address at multiple loca‐ tions. In practice, this means that traffic to a single IP address can be routed to multiple locations based on the location of the request. Managed DNS providers use anycast networks to allow resolution of DNS records of the most geographically relevant system. This has some interesting potential solutions that can be employed to increase your control over your systems. The obvious implementation of this is to host multiple versions of your system within different regions. DNS resolution will then route the user to the one that is nearest to them. Many cloud providers will provide this functionality as part of their service. However, using a managed DNS service, rather than a cloud pro‐ vider, to manage geolocation allows you to have more granular con‐ trol over the situation under which you will use a cloud provider. For example, if your chosen cloud provider is weak in one area, then you can use a secondary provider in that region. Use a Best-of-Breed DNS Provider | 19

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