DNS
BIND has long had the behavior of responding in round robin fashion when a DNS zone has multiple CNAME or A records. A query with multiple records is returned as a list of answers. Typically, the application that originally made the query just uses the first one from the list.

If the remote DNS server is a modern BIND installation, it will rotate amongst the list it has cached. This isn't a part of DNS, just a behavior of BIND; since it's the most widely deployed name server it's not a bad bet to depend on it.

Anyway, by using low TTL's [1] on the zone records or just the records that are subject to round robin rotation, we can inform remote DNS servers that they should keep the response they've received for a short time.

1] the amount of time before a query answer should be discarded from the cache, the time-to-live for a record