A free GSLB enabled load balancer

The gap DNS failover fills

Say you did everything right. Two load balancers, both healthy, both serving. Your backends are checked and the dead ones get pulled. Then the machine your domain points at loses power.

Every visitor still gets that address, because DNS does not know anything happened. They wait, they time out, and they leave. The second node is sitting there in perfect health with nobody being sent to it.

How it works here

1. You pick a subdomain to hand over, something like gslb.example.com.
2. At your domain host you create the delegation: nameserver records pointing that subdomain at your nodes, plus glue records giving their addresses.
3. In the GUI you make a zone and add the names you want answered, with the nodes that may answer for each.
4. Your real site name becomes a CNAME to a name inside the delegated zone.
5. From then on, every lookup is answered by your own nodes, and a node that is not healthy is left out of the answer.

The walkthrough guide does all of this properly, including how to test it before you depend on it, and there is a worked example for Amazon Route 53 in the how to section.

Choosing a TTL, honestly

TTL is how long a resolver may keep the answer. Short means faster failover and more queries. Long means fewer queries and a longer outage for anybody holding a stale record.

Be realistic about the promise. Some resolvers ignore short TTLs and keep records longer than they were told. Some browsers and operating systems cache on top of the resolver. A sixty second TTL means most visitors recover in about a minute and a few stragglers take longer. Anybody quoting you instant DNS failover is describing a lab.

GSLB is not a CDN

Both steer visitors, so they get confused with each other. A CDN puts copies of your content in many cities and serves from the closest one, which is about speed. GSLB decides which of your own servers to send somebody to, which is about availability.

You can run both. If you want your visitors in another country served from that country, you want a CDN, and this is not one. If you want your site to survive the room your server lives in catching fire, that is this.

What happens when everything is down

There is a setting for it, and it is worth thinking about rather than accepting the default. If every node fails its check, you can either answer with nothing, or answer with all of them anyway.

Answering with all of them is usually right. If every node looks dead, the most likely explanation is that your health checking is broken rather than that your entire fleet died at the same instant. Handing out an address that might work beats handing out nothing, which guarantees failure.