F Wide Area DNS Operations

To direct client requests to the entry point of a production site, use DNS resolution. When a site switchover or failover is performed, client requests must be redirected transparently to the new site that is playing the production role. To accomplish this redirection, the wide area DNS that resolves requests to the production site has to be switched over to the standby site. The DNS switchover can be accomplished by either using a global load balancer or manually changing DNS names.

Note:

A hardware load balancer is assumed to be front-ending each site. Check for supported load balancers at:

https://metalink.oracle.com

The following topics are described in this section:

Using a Global Load Balancer
Manually Changing DNS Names

F.1 Using a Global Load Balancer

When a global load balancer is deployed in front of the production and standby sites, it provides fault detection services and performance-based routing redirection for the two sites. Additionally, the load balancer can provide authoritative DNS name server equivalent capabilities.

During normal operations, the global load balancer can be configured with the production site's load balancer name-to-IP mapping. When a DNS switchover is required, this mapping in the global load balancer is changed to map to the standby site's load balancer IP. This allows requests to be directed to the standby site, which now has the production role.

This method of DNS switchover works for both site switchover and failover. One advantage of using a global load balancer is that the time for a new name-to-IP mapping to take effect can be almost immediate. The downside is that an additional investment must be made for the global load balancer.

F.2 Manually Changing DNS Names

This method of DNS switchover involves the manual change of the name-to-IP mapping that is originally mapped to the IP address of the production site's load balancer. The mapping is changed to map to the IP address of the standby site's load balancer. Follow these instructions to perform the switchover:

Make a note the current Time to Live (TTL) value of the production site's load balancer mapping. This mapping is in the DNS cache and it will remain there until the TTL expires. As an example, let's assume that the TTL is 3600 seconds.
Modify the TTL value to a short interval (for example, 60 seconds).
Wait one interval of the original TTL. This is the original TTL of 3600 seconds from Step 1.
Ensure that the standby site is switched over to receive requests.
Modify the DNS mapping to resolve to the standby site's load balancer giving it the appropriate TTL value for normal operation (for example, 3600 seconds).

This method of DNS switchover works for switchover or failover operations. The TTL value set in Step 2 should be a reasonable time period where client requests cannot be fulfilled. The modification of the TTL is effectively modifying the caching semantics of the address resolution from a long period of time to a short period. Due to the shortened caching period, an increase in DNS requests can be observed.