There are cases where hostdb does not reference a ttl of nameserver.

hostdb is set as follows:
CONFIG proxy.config.hostdb.ttl_mode INT 0
CONFIG proxy.config.hostdb.timeout INT 1

If response of dns the next,ATS queries the DNS every 30 seconds.

$ dig foo.bar.com. A
;; ANSWER SECTION:
foo.bar.com. 30 IN A 10.10.10.1

If ttl is reduced as follow:, ATS will query the DNS every 10 seconds.

$ dig foo.bar.com. A
;; ANSWER SECTION:
foo.bar.com. 300 IN A 10.10.10.1

However, if it increased the value of ttl, ATS has continued to query DNS every 10 seconds.

$ dig foo.bar.com. A
;; ANSWER SECTION:
foo.bar.com. 300 IN A 10.10.10.1

However, is adapted then HUP the traffic_server...
Why??