ach VIP is associated with both primary and failover sites that are accessed through Anycast routing (Metz, 2002). This redi- rection is very fast, taking less than a second to redirect queries to the failover site. In addition to these georedundancy features, each site provides local redundancy through load balancing across Architecting the Network Cloud for High Availability Client DNS queries Network Anycast routing: BGP sends queries to next nearest site vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM vDNS VM Primary Primary VIP Alternate Alternate VIP VIP Site Failover Site VIP Site Failover Site FIGURE 5.14 vDNS architecture and redundancy. many VMs spread across multiple servers within the site. Therefore, if a VM or server fails, queries are quickly diverted to others that are still operational. As a stateless network function in multiple sites, vDNS-R is able to take advantage of a combina- tion of these resiliency features. Figure 5.15 illustrates the impact of each of these features on overall availability (time-based) or service reliability (transaction-based) expressed as DPMs. By employing these resiliency techniques, the vDNS-R service is able to achieve extremely high reliability. 5.5 AREAS FOR FURTHER STUDY As seen in the examples cited in this chapter, the basic principles and mathematics of reliability analytics have not changed with the introduction of virtualized solutions. However, there are new challenges that must be addressed to ensure that the desired reliability for virtual solutions can be achieved. In this section, some topics for further exploration are identified: 1. Software stability for Agile developments. The Agile software development process uses short development cycles and continuous integration and deployment. This makes it very difficult to assess software stability using traditional software reliability tech- niques that involve rigorous and lengthy testing. Netflix developed a tool called Chaos Monke