In light of recent Internet attacks, whether a sound HA architecture should withstand a massive (distributed) denial-of-service ([D]DoS) attack or be able to mitigate its effects has become a legitimate question. From my point of view, a state-of-the-art HA architecture should have some inherent self-healing capabilities; HA architects should also add another line of defense to assist in at least crippling or weakening (D)DoS attacks and their progeny. Several, sometimes complementary and orthogonal, lines of defense are crucial to prevent (D)DoS attacks, as they are to overall security architectures.
HA in terms of almost 100 percent service availability within strict service level agreements (SLAs) and monitored key performance indicators (KPIs) represents a significant challenge for today's finest engineers and designers. The problem with any (D)DoS defense is that every system's strength defines its weaknesses, too. For example, handing over control of a firewall ruleset to a network intrusion detection system (NIDS) means that any successful trigger of this defense mechanism (spoofing) effectively locks out legitimate networks from crucial services. Therefore, a system designed to protect or prevent might become the perfect DoS trap.
Recent hostile activities on the Internet have proven to me that, in general, operational staff are overwhelmed by and overburdened with reactive actions because of weak underlying network design and planning.